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'230VAC->5Vdc without transformer'
1998\10\03@215639 by

I would like to build a a 5V d.c. power supply for a PIC derived from
the mains supply of 230V A.C. with out using a transformer.

Currently my only idea is to use a couple of 5W ceramic resistors in
series across the mains supply as a voltage divider and use this to
supply a bridge rectifier / three terminal regulator circuit.

Is there a better or more efficient way of obtaining 5V d.c. with a
small compact circuit?

Seth Fischer

Seth Fischer wrote:

> Is there a better or more efficient way of obtaining 5V d.c. with a
> small compact circuit?

Yes, use a series capacitor (mains rated) with a 1/8W (fusible)
resistor in series and a bridge rectifier if the application is not
linked to one mains terminal, or a "voltage doubler"-style pair of
rectifiers if it is.

Note that this arrangement will not allow you to detect the mains
zero-crossing; if you want that you can use a low-power resistor *as
well*.

You tell me how much current you need, what ripple you will tolerate,
and I'll give you the (rough) values.
--
Cheers,
Paul B.

Seth Fischer wrote:
> Is there a better or more efficient way of obtaining 5V d.c. with a
> small compact circuit?

Depends on the current you want.  ;-)  A PIC doesn't take much, but are
you driving anything with it?

I've never done it, but I've seen described a simple high voltage
capacitor, then a diode forward biased, then into a 5V2 zener with an
electrolytic capacitor across it.

+5V          0V
O           O
|           |
+----| |----+
240VAC                              |           |
| |                   |           |
O-----------| |-----------|>|-----+----]<|----+-----O
| |

active        high vcap       diode        zener       neutral

I'm not qualified to tell you how to do this safely.
Perhaps others can fill it in with component values and limits.

--
James Cameron                                    (cameronstl.dec.com)
Digital Equipment Corporation (Australia) Pty. Ltd. A.C.N. 000 446 800

Look at TB008 from Mocrochip's web page.  They describe a transformerless
power supply derived from 110VAC.  The circuit can be easily changed to work
from 230VAC.  Note that this type of supply is only good for currents around
20mA, after that the series capacitor gets to be too large.  I have used
this type of supply to power a PIC with good results.  If the current draw
is very low, a resistor can be used in place of the 1uF capacitor (in my
application the capacitor took up too much space).

This supply is NOT isolated from the mains.  Care should be taken to protect
the user from touching any part of the circuit (even 5V) in case of a fault.

Alan
{Original Message removed}
James Cameron wrote:

> I've never done it, but I've seen described a simple high voltage
> capacitor, then a diode forward biased, then into a 5V2 zener with an
> electrolytic capacitor across it.

Oh dear, dear.  Let«s see if I can fix this:

+5V          0V
O           O
|    +      |
+----] [----+  Electrolytic
240VAC             | |              |           |
O-----VVVVV------| |---+--|>|-----+----]<|----+-----O
| |   |                      |
+---|<|----------------+
active  resistor  high vcap  diodes       zener       neutral

The resistor is to limit the inrush current and burn up if the
capacitor fails.
--
Cheers,
Paul B.

On Sun, 4 Oct 1998, James Cameron wrote:

- snip -

This will work, BUT 2 all-important parts are missing: R1 and R2, also
the zener is misplaced:

>
>                                       +5V          0V
>                                        O           O
>                                        |    C2     |
>                                        *----| |----*
>    240VAC                              |           |
>    hot    R1     C1                    |           |     neutral
>    O-*--/\/\/\---| |-------*---|>|-----+           *-----O
>      |                     |                       |
>      |                  K  -                       |
|                     A   Zener               |
|                  A  -                       |
|    R2               |                       |
+--/\/\/\-------------*-----------------------+

> I'm not qualified to tell you how to do this safely.

Neither am I but I did use it ;)

> Perhaps others can fill it in with component values and limits.

The key is: Zc = 1 / ( 2 * PI * C ) [ohm, Farad], where Zc is considered
the current limiter for the Zener current. Currents above 30 mA are not a
good idea. C1 must be capable to operate at that current and peak mains
voltage for a long time.

R1 is dimensioned to limit inrush at Vpp = 1.42 * Vmains to the limit
admissible zener current, and should be a fuse resistor or at least
flameproof, and R2 drains the charge in C1 if it retains any when the
power cord is removed, and prevents zapping the operator. It's typically
something like 470 kOhms, 0.5-1 Watt (for higher voltage withstanding
capability).

BE AWARE that the circuit must be assembled to meet IP2 class or better,
that is no metallic parts can be touch-able by any person and because of
the hot chassis no connectors leading out from the circuit are safe. USE
THIS AT YOUR OWN RISK. And I'm also not qualified to tell you how to do it
safely ;)

Peter

At 02:31 1998-10-04 +0000, you wrote:
>Seth Fischer wrote:
>> Is there a better or more efficient way of obtaining 5V d.c. with a
>> small compact circuit?
>
>Depends on the current you want.  ;-)  A PIC doesn't take much, but are
>you driving anything with it?
>
>I've never done it, but I've seen described a simple high voltage
>capacitor, then a diode forward biased, then into a 5V2 zener with an
>electrolytic capacitor across it.

- The posted schematic censored, because : -
You cant have a diode in series with the cap, then it will only give power
on the first cycle.

X2 cap
O--[R1]--+--||---.
Line      |       |  1N4001
'-[R2]--X----AK----+-----O +5V
|          |
230VAC        5V6 K         --- +
Zener A         --- Elytic
Neutral           |          |
O----------------+----------+-----O 0V

In the power cycle, as the voltage rises positive current flow thru X2 cap,
voltage is limited by the zener, and fed to the elytic; as voltage go down
the zener works in forward and charges X2 cap with mains negative peak
voltage.

You need an series resistor R1 to limit inrush current; Think about when
this device is connected to mains during 230VAC peak voltage which is 320V.
Or if shortly disconnected during positive peak, and reconnected during
negative peak, (connecting using bad connectors, or switch) the cap has to
change voltage by 640V. Huge peak!

R1 should be wire wound or carbon composite to survive peaks well.  Also
check the stated max voltage peak for the resistor,  it should stand the
very high peaks that can occour in the mains net!

I use to dimension R1 resistance and power for 20 to 50 V RMS drop, and
select at least double power rating to survive peaks better.

Also, if disconnected during cycle high voltage, the wall plug will be
dangerous to touch.  Therefor a resistor R2 should discharge the X2 cap
from 300V to 50V in half a second (my opinion).  The peak voltage is
limited by the cap so a class 350V resistor is probably enough.  Measure
the RMS voltage you get, and calculate the power the resistor has to
withstand.

The cap should be X2 type.  Theese can take lot of over-voltage.
To calculate current:
it is charged 50 times per second from -320 to +320V minus the peak
viltages across the limiting resistor, minus the putput voltage, minus two
diode forward drops, approximate total 570V charge.
I= 50/s * C * 570V

Also the current through R2 contributes, but not with all currens it
supplies, as it is not in phase with the cap current.

The zener should be at least 1,3W type to handle the rush current.

To cheaply double current, use the current through X2 cap both ways:

X2 cap
O--[R1]--+--||---.  _____
Line      |       | |     |
'-[R2]--+-|AC  +|--------+-------+-----O +5V
|     |        |       |
|     |   5V1  K      --- +
|     |  Zener A      --- Elytic
Neutral             |     |        |       |
O------------------|AC  -|--------+-------+-----O 0V
|_____|
Full wave rectifier

But here you can«t as easily control a TRIAC as neither PIV Vdd nor Vcc ar
commont to any mains line.  Possible to couple the signal around though,
but better solution is the discrete capacitive pump described below,
applied on the first cirquit.

However, with bothe the designs above there might be undesirable high noise
and spikes on the output.  Also, specifically with the first one, the
voltage will drop some between charge pulses.  Two solutions:

Select higher voltage for the above zener, and
1)      add a 70L05 or better.  (78L05 consumes pretty much power itself)
2)      add a second shunt regulator ( R and Zener, and probably poutput cap ).
In the case 2 you can omit hte first zener after selecting the elytic to
withstand highest possible voltage.

To get more power there ar lot of more tricks.

For instance use a capacitive charge pump.
Instead build a discrete, that operates directly on the input, after the X2
cap and input resistor.

For a current doubler:
Design it to during the charge cycle charge two small elytics in series,
and during no charge, connect them parallel (one of them is the output cap
from which you get continuous power out.  It only takes one transistor, two
1N4001 and two 1N4148 diodes and a resistor, if i remember correct.
I leave the rest as a practice for the interested ;)
(I have not seen the design anywhere else, but designed it for a very
peculiar application where i had to get the supplu power from very weak
pulses.)

Using charge pumps and other tricks we can
1)  Use smaller X2, R1, and R2 = cheaper, smaller, less heat
2)  If heat is not a big concern, maybe omit the expensive components X2
and R1, just using R2 (with higher voltage capability needs)

It is of course also pretty easy to extend the designs to generate negative
and/or multiple voltages.

When testing: remember that you are working with mains power!
A isolation transformer is very good for laborating.

Theese cirquits are even more exciting than assembler! :)
Have fun
/Morgan
/  Morgan Olsson, MORGANS REGLERTEKNIK, SE-277 35 KIVIK, SWEDEN \
\  mrtiname.com     ph +46(0)414 70741     fax +46(0)414 70331 /

At 02:41 PM 10/4/1998 +1300, you wrote:
>I would like to build a a 5V d.c. power supply for a PIC derived from
>the mains supply of 230V A.C. with out using a transformer.
>
>Currently my only idea is to use a couple of 5W ceramic resistors in
>series across the mains supply as a voltage divider and use this to
>supply a bridge rectifier / three terminal regulator circuit.
>
>Is there a better or more efficient way of obtaining 5V d.c. with a
>small compact circuit?
>
>
>Seth Fischer
>

Hi Seth,

Harris has some integrated circuits which can take 50-280V A.C. and will
convert it to 5 V, without the use of a transformer.  The current is
relatively low (30 to 50 mA), however, but it should be able to power a
PIC.  Check out the HIP5600 and HV-2405E from Harris, http://www.semi.harris.com.
Also check out the article "Transformerless Power Conversion", by Jeff
Bachiochi, in the Aug 98 (Issue #97) of Circuit Cellar Ink, pp 78-80.

Regards,
Gary Pepper

e-mail: gpeppercapitalnet.com

On Sun, 4 Oct 1998 12:32:18 +1000 "Paul B. Webster VK2BZC"
<paulbmidcoast.com.au> writes:
>Seth Fischer wrote:
>
>> Is there a better or more efficient way of obtaining 5V d.c. with a
>> small compact circuit?
>
>  Yes, use a series capacitor (mains rated) with a 1/8W (fusible)
>resistor in series and a bridge rectifier if the application is not
>linked to one mains terminal, or a "voltage doubler"-style pair of
>rectifiers if it is.
>
>  Note that this arrangement will not allow you to detect the mains
>zero-crossing; if you want that you can use a low-power resistor *as
>well*.
>
>  You tell me how much current you need, what ripple you will
>tolerate,
>and I'll give you the (rough) values.

Maxim has a shunt regulator chip just for this purpose.  Their chip
includes the diodes (I think there are two part numbers, one half wave,
the other bridge), a zener shunt regulator, and perhaps a series linear
regulator.  Check out the Maxim web site.

Harold

___________________________________________________________________
Get completely free e-mail from Juno at http://www.juno.com
Or call Juno at (800) 654-JUNO [654-5866]

Lucent (AT&T) makes a part, few resistors and caps, 8pin DIP, takes
230VAC, 5VDC, non-isolated.

Morgan Olsson wrote:

> R1 should be wire wound or carbon composite to survive peaks well.
> Also check the stated max voltage peak for the resistor,  it should
> stand the very high peaks that can occur in the mains net!

I look at it from a different perspective.  R1 is in my eyes, a fuse
resistor as well as an inrush limiter.  Presuming we have selected a
capacitor to pass 30mA (corresponding to a 15mA DC draw by the circuit),
then a resistor value of 330 ohms would limit inrush to 1 amp; a quite
survivable figure for most resistors even if the instantaneous but brief
dissipation is 330W.  (Extrapolating, if the capacitor charges in 10mS
(one half mains cycle) at 30mA, then at 1 amp it would charge in 100nS;
this is a rather short time in thermal terms and a correspondingly *very
small* amount of energy.)

This situation is many orders of magnitude less severe than for the
one ohm resistor and some hundreds of microfarads involved in my little
tale at http://www.midcoast.com.au/~paulb/faq_rsis.html ...

> I use to dimension R1 resistance and power for 20 to 50 V RMS drop,
> and select at least double power rating to survive peaks better.

Now in routine operation, the 330 ohm resistor would develop about ten
volts drop and dissipate about 1/3 watt.  I can see no point at all in
dropping a higher voltage or dissipating more power.  In fact, the only
reason I see for using this value is to actually load the resistor near
to its rating so that less of an overload is required to fuze it.  A ¸
watt resistor should serve quite well, of if you wish to be conservative
on the voltage rating, a pair of 180 ohm ¹ watt resistors in series.

This is more a matter of impulse protection, as the "ballast" resistor
R2 which I omitted to suggest earlier tends to prevent the capacitor
retaining voltage from previous switch-off to be added to a following
switch-on transient.

> Also, if disconnected during cycle high voltage, the wall plug will be
> dangerous to touch.  Therefore a resistor R2 should discharge the X2
> cap from 300V to 50V in half a second (my opinion).

Note by the way, that R2 can be used between the active proper and a
second diode pair across the DC supply capacitor, the voltage at this
point to provide a zero-crossing detection.

> The cap should be X2 type.  These can take lot of over-voltage.

I tend to say it should be one specifically rated for AC mains voltage
use (phase compensation or interference suppression).

> Also the current through R2 contributes, but not with all current it
> supplies, as it is not in phase with the cap current.

With my suggestion above, it would contribute notwithstanding phase
shift.

> To get more power there are lot of more tricks.  For instance use a
> capacitive charge pump.

Best approach is to develop say, 50V with the series capacitor and
rectifier, then drop this using a switching regulator which provides
more currrent (through the commutating diode).
--
Cheers,
Paul B.

At 23:45 1998-10-05 +1000, Paul B. Webster wrote:
>Morgan Olsson wrote:
>
>> R1 should be wire wound or carbon composite to survive peaks well.
>> Also check the stated max voltage peak for the resistor,  it should
>> stand the very high peaks that can occur in the mains net!
>
>  I look at it from a different perspective.  R1 is in my eyes, a fuse
>resistor as well as an inrush limiter.  Presuming we have selected a
>capacitor to pass 30mA (corresponding to a 15mA DC draw by the circuit),
>then a resistor value of 330 ohms would limit inrush to 1 amp; a quite
>survivable figure for most resistors even if the instantaneous but brief
>dissipation is 330W.  (Extrapolating, if the capacitor charges in 10mS
>(one half mains cycle) at 30mA, then at 1 amp it would charge in 100nS;
>this is a rather short time in thermal terms and a correspondingly *very
>small* amount of energy.)

Yes, but as time is very short, most of that that energy has to be absorbed
by the resistive material iteslf, like the thin carbon/metal layer on
normal resistor.  The high and quick temperature rise on the resistor
surface is like if it have been hit by a short laser puls.  I have seen
them literally explode.  As i could not find any reliable spec for that
high pulse rating i selected the other method.

I hate inrush currents.  Lower R demands higher current handling on diode,
zener, and elytic, and don«t suppress power line peaks as much to the PIC
supply, thereby demanding a second stage regulation, all making higher cost.
But, yes, we ought to fuse it too...

When using theese designs almost ten years ago i could not find well
defined fusible resistors, so i used separate fuse.  Maybe i did not search
good enough then, or there might be better specs now.

Higher resistance minimizes the peak voltage handling requirement of the
cap, so technically it don«t have to be X2 type (thereby much cheaper).
I believe however the CE-lab might have something to say about the cirquit
if not X2 type?  Or require more thourough testing = more expensive.

BTW, there are nice compact RC filters made of X2 + integrated R (carbon on
part of the foil!) in suitable values.
But this type of R vill *not* fuse easily.

{Quote hidden}

Very good point.
I believe that R2 can be directly connected to an PIC input; clamped by the
integrated protection diodes, as long as it is only about a mA.
(dont use a input that id not clamped! (some are))
Is there an effect of the PIC drawing more power in input stage?

{Quote hidden}

Yes, for relatively high power.
However all cirquits for inductive switching i have seen consume a lot more
quiscent supply than the capacitive ones.
But the inductive ones can have much more corrent boost as the input
voltage can be high.  They will need a zener for input voltage limit, as
when input voltage go up, they use *less* power.

The discrete current doubler or tripler charge pump is pretty simple to
construct (as we already have a pulsing input) and is cheapest, also
counting  mounting cost (few more components).

Recently I made a 16uA to 30uA micropower current doubler for a phantom fed
sensing device.

Regards
/Morgan
>--
>  Cheers,
>        Paul B.
>
>
/  Morgan Olsson, MORGANS REGLERTEKNIK, SE-277 35 KIVIK, SWEDEN \
\  mrtiname.com     ph +46(0)414 70741     fax +46(0)414 70331 /

Morgan Olsson wrote:

> Yes, but as time is very short, most of that that energy has to be
> absorbed by the resistive material iteslf, like the thin carbon/metal
> layer on normal resistor.  The high and quick temperature rise on the
> resistor surface is like if it have been hit by a short laser puls.  I
> have seen them literally explode.

I think you're looking at the problem with the wrong mindset here.  If
I were talking about dumping a certain amount of *energy* in a variable
amount of time, then it would be true that the concentration (power)
would be greater the shorter the pulse.  In this case however, the
situation is the opposite; the power level is certainly dependent on the
resistance, but the fact that it will only be for an *extremely* short
time means that the integrated energy "dose" will be correspondingly
minute.

For example, a half microfarad capacitor charged to 340V (peak of 240V
sinewave) contains three hundredths of a joule.  At 50Hz, this capacitor
has a reactance of about 6400 ohms and passes about 37mA which is close
to my presumption of a 30mA source.  Three hundredths of a joule seems
to my "gut feeling" to be a not unreasonable amount to dump into a small
resistor from time to time, even if only takes about 350µs to do so.

> As I could not find any reliable spec for the high pulse rating I
> selected the other method.

I note your caution, but am reasonably sure small resistors *can*
handle such.  The problem I quoted by reference to my website is *two
hundred times* the energy level; i.e., two 220µF capacitors in series.

> I hate inrush currents.  Lower R demands higher current handling on
> diode, zener, and elytic, and don«t suppress power line peaks as much
> to the PIC supply,

330 ohms limits inrush to one amp, which is the *continuous* rating of
common small mains rectifiers, and well within impulse ratings for a 5V
or even a 15V ¸W Zener, or a 100µF electrolytic.

> thereby demanding a second stage regulation, all making higher cost.

I doubt this alone would require a second regulator.  Easier to use a
one or two-watt Zener instead if you were worried.

> When using these designs almost ten years ago I could not find well
> defined fusible resistors,

You can't have it both ways!  If you think the resistor will burn out
if (massively) overloaded, then you must think it's a good fuse.  Close
fusing tolerance (as for example in a TV) is *not* being asked for.  Do
the (literal) "smoke test"; buy a few, stick them across the mains and
watch what happens (with protective glasses)!  If they shatter, you may
wish to shield them (Fiberglass spaghetti seems to be "the go") *just in
case*.  If they set the house on fire, something is badly wrong.

The one thing you *don't* want for this function, as I meant to point
out before, is a wirewound.

> I believe however the CE-lab might have something to say about the
> circuit if not X2 type?

Yes.  Definitely use one designed for the job.

> BTW, there are nice compact RC filters made of X2 + integrated R
> (carbon on part of the foil!) in suitable values.  But this type of R
> will *not* fuse easily.

Why not?  Part of the rating is the specification that the capacitor
not melt down if it shorts.  When directly across the mains, this is
implemented by fusing of the foil itself adjacent to the failure ("self-
healing").  Since integral resistor capacitors will not do this, they
must to obtain approval, fuse the resistor element!

> I believe that R2 can be directly connected to an PIC input; clamped
> by the integrated protection diodes, as long as it is only about a mA.

Theoretically, but I for one won't trust it!  Theoretically, and some
people claim to have done it, the PIC can be *powered* through these
diodes.
--
Cheers,
Paul B.

At 19:09 1998-10-06 +1000, Paul B. Webster wrote:
-snip-
but the fact that it will only be for an *extremely* short
>time means that the integrated energy "dose" will be correspondingly
>minute.

Well, you could tell that to my resistor that by time blew up... ;)
(If I remember correct that was an ordinary 1/4W carbon film low ohm
like 22ohm or so, and 1uF cap)

>  For example, a half microfarad capacitor charged to 340V (peak of 240V
>sinewave) contains three hundredths of a joule.  At 50Hz, this capacitor
>has a reactance of about 6400 ohms and passes about 37mA which is close
>to my presumption of a 30mA source.  Three hundredths of a joule seems
>to my "gut feeling" to be a not unreasonable amount to dump into a small
>resistor from time to time, even if only takes about 350µs to do so.
350us 0.03J means during theese 350us the mean power is 85W

Peak power for a 22ohm resistor at 340V connection time is 5.6kW!
In this very short charge time the heat can not travel very far, so alomost
all power will be dissipated in the resistive mass.
So i believe, for thin film resistors, the film (which has very little
mass) will transiently get *very* hot, and then cool very rapidly, thereby
causing high thermomechanic stress in the ceramic substrate, that
eventually will crack in the surface.
The heating is so short I can't think of a method to measure it.

Also it should withstand some spikes on the net:

Consider it is connected to an wall outlet that is fused by the same fuse
as the outlet where you have a ectromagnetic machine (i.e vacuum-cleaner).
If the fuse blow at mains peak voltage +340V, the electromagnetic
inductance machine will probably pull it to more negative than the mains
negative peak voltage.

The same applies if they are connected to a distrubution cable that is
unjacked or switched off.

So, there might be pulses of a kilovolt across the resistor in certain cases!
In this case, however, for not-too-large electromagnetic machines, the peak
power in our resistor will be less when the resistor is sufficiently low,
as the machine pulls with a *current* defined as less then their operating
current.

I don«t want customers to complain that the gizmo died even if it happens
"only" once a yer per customer...

>> As I could not find any reliable spec for the high pulse rating I
>> selected the other method.
>
>  I note your caution, but am reasonably sure small resistors *can*
>handle such.

We might best select thick film resistors?
Both fusible (i think) and more heat mass in teh resistive compound.

I have seen thick film large area resistors designed for large snubbers
(with values like them we use here), as alternative to carbon mass
resistors, with the addition to fuseability.  They are expensive, however.
When I need, I will take a good look at standard thick film SMT resistors.

-good points snipped-

>  You can't have it both ways!  If you think the resistor will burn out
>if (massively) overloaded, then you must think it's a good fuse.  Close
>fusing tolerance (as for example in a TV) is *not* being asked for.

>  The one thing you *don't* want for this function, as I meant to point
>out before, is a wirewound.

I agree, that was a little stupid. The idea was to make it alst so long as
the fuse would safely blow.  But the cost of expensive resistor + fuse is
too large for series production, i believe.

Do anybody have experience of how good carbon mass resistors are for fusing?
(Wold be very type dependant though)

Hmm.. I just got an simple idea I so far think is good,
please try to find problems in the following:

A slow-blow 5x20mm 50mA fuse has an cold resistance of 25mA.
(The one in my hand, that is; they are probably very type/manufacturer
dependant)
This one's fusing "string" is made of a fine wire wound about hundred turns
around some type of (glass?) fibre, total a half millimetre diameter, so
the wire thermal mass is pretty large.

This fuse would act both as a safe and rigid resistor with low drop, and a
safe fusing, IMHO.
Anyone tested?

This might also be able to supply enough power for a self healing process
in the cap (in case if such failure)  Nah, but maybe.

{Quote hidden}

Please rephrase the last sentence; do you think we should have external
fuse as well?

>> I believe that R2 can be directly connected to an PIC input; clamped
>> by the integrated protection diodes, as long as it is only about a mA.
>
>  Theoretically, but I for one won't trust it!  Theoretically, and some
>people claim to have done it, the PIC can be *powered* through these
>diodes.

I have successfully used that method for another CMOS device.
Just check the spec for input overvoltage and current tolerance, and make
very sure not even for a glitch surpass it, as it might set the cirquit in
latch-up, so always use a more than sufficiently high resistor value, and i
think there can be no problem.

When using external protection diodes to protect from heavy spikes, i think
we should also use a series resistor from the diode common point to the
input pin.  If we don't, the current would only be *shared* between
external and internal protection; better have it all outside.
/Morgan
/  Morgan Olsson, MORGANS REGLERTEKNIK, SE-277 35 KIVIK, SWEDEN \
\  mrtiname.com     ph +46(0)414 70741     fax +46(0)414 70331 /

> various interesting guesstimations re: component values

Referring to my ASCII schematic, R1 = 220R 2W ceramic body wire wound FR.
Zener rated 1.5 A peak repetitive current (I think it was a 1 Watt Zener,
5V6). R2 = 470K 0.5W FR. The rectifier diode and the capacitor chosen to
suit the circuit. If a clock is required it can be derived directly with a
resistor from the Cathode of the Zener and injected into the circuit. It
is guaranteed to swing ABOVE and BELOW the Vcc and GND by exactly 0.6 V
;).

R1 dissipates ~0.2 W in normal operation. Note WIRE WOUND AND FR. Use
other types at your own risk. Such resistors are made and available for
switching mode PSU's and other applications, in the usual white
rectangular ceramic case (short).

R2 dissipates ~0.1 W in normal operation. NOTE FR type.

C1 is calculated for 30 mA through the Zener.

hope this helps, and USE AT YOUR OWN RISK,

Peter

As I recall, Philips published pulse duration versus power tolerance
curves for their resistors (or some of them). Based on experience,
Philips resistors are rather superior in reliability to unknown
source oriental units in real life equipment. (Philips resistors are
probably known source oriental units :-)).
.
I can't recall exactly where I saw this - probably in one of the
Philips monthly technical documents that they issue - It may be worth
.
.regards

Russell McMahon.
.
-----Original Message-----
From: Morgan Olsson <    >
....
Do anybody have experience of how good carbon mass resistors are for
fusing?
(Wold be very type dependant though)

Morgan Olsson wrote:

>> the integrated energy "dose" will be correspondingly minute.
> Well, you could tell that to my resistor that by time blew up... ;)
> (If I remember correct that was an ordinary 1/4W carbon film low ohm
> like 22ohm or so, and 1uF cap) ...
> Peak power for a 22ohm resistor at 340V connection time is 5.6kW!

Which is fifteen times that for a 330 ohm resistor.  I'd think that a
significant difference.

> In this very short charge time the heat can not travel very far, so
> almost all power will be dissipated in the resistive mass.

No contest on this theme.

> The heating is so short I can't think of a method to measure it.

I'd just try a number of the resistors you propose to use, with a four
times over-rating; that is the resistor value you want (I still suggest
330 ohm) with a capacitor four times the size you propose to use (2µF).
If they survive a hundred or so impulses, then they should survive a
long time at one fourth the stress.

> Also it should withstand some spikes on the net:

The *vast* majority of power line surges are several milliseconds in
duration, and thus are ten times my quoted time constant, allowing the
capacitor reasonable time to charge (and subsequently, discharge).  I
doubt you expect it to survive lightning hits.

> If the fuse blow at mains peak voltage +340V, the electromagnetic
> inductance machine will probably pull it to more negative than the
> mains negative peak voltage.

But with what time constant?

> We might best select thick film resistors?  Both fusible (I think) and
> more heat mass in teh resistive compound.

Sounds eminently sensible.

> A slow-blow 5x20mm 50mA fuse has an cold resistance of 25mA. (Ohms?)
> (The one in my hand, that is; they are probably very type/manufacturer
> dependant)  This one's fusing "string" is made of a fine wire wound
> about hundred turns around some type of (glass?) fibre, total a half
> millimetre diameter, so the wire thermal mass is pretty large.
> This fuse would act both as a safe and rigid resistor with low drop,
> and a safe fusing, IMHO.

It would presumably take the stress (that *is* its job!).  I
originally suggested 330 ohms because that limits the impulse through
the diodes (and cap) to their nominal rating to be conservative.

> This might also be able to supply enough power for a self healing
> process in the cap (in case if such failure)  Nah, but maybe.

Indeed, it might.  My "gut feeling" says the metallisation is thinner
than the fuse wire, thus will burn first.

> Please rephrase the last sentence; do you think we should have
> external fuse as well?

I presume that to be CE rated, the integral-resistor suppressor must
have a specified and tested failure behaviour if the capacitor shorts
out.  If the resistance limits "self-healing", presumably the failure
mode is for the resistor to vapourise.

Well, that's my bit for this morning - must to work!
--
Cheers,
Paul B.

> As I recall, Philips published pulse duration versus power tolerance
> curves for their resistors (or some of them). Based on experience,
> Philips resistors are rather superior in reliability to unknown
> source oriental units in real life equipment. (Philips resistors are
> probably known source oriental units :-)).
> .
> I can't recall exactly where I saw this - probably in one of the
> Philips monthly technical documents that they issue - It may be worth