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'[EE]: Does anyone have a circuit to power a PIC fr'
2001\10\22@070518 by Mathew cohen

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I have seen circuit on the web to do this though it uses a current regulator
that I can not source nor find the specs for. Does anyone have a circuit
that might be a bit more relevant.

Thanks in advance...

Mathew Cohen

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2001\10\22@072337 by Gerhard Fiedler

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At 19:07 10/22/2001 +1000, Mathew cohen wrote:
>I have seen circuit on the web to do this though it uses a current regulator
>that I can not source nor find the specs for. Does anyone have a circuit
>that might be a bit more relevant.

If you don't need much current, you can use a capacitor to get the voltage
down without eating up too much power. Microchip has an app note about
this, and I've run a few circuits in similar ways. Try to use an isolation
transformer while you're working on the circuit, because it is connected
directly to mains voltage.

ge

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2001\10\22@080256 by Quentin

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Check out Microchip's note: TB008

Quentin

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2001\10\22@085445 by Mike Blakey

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Depends what your circuit is doing, if you DONT need the isolation and are
switching mains you can 'attach' Vcc direct to LIVE and float the 0v at LIVE -5v
with a zenner. The problem is that the whole circuit and any LED's, switches etc.
will be at mains live, but you do get a nice zero crossing detector too.



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2001\10\22@091403 by Byron A Jeff
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On Mon, Oct 22, 2001 at 07:07:51PM +1000, Mathew cohen wrote:
> I have seen circuit on the web to do this though it uses a current regulator
> that I can not source nor find the specs for. Does anyone have a circuit
> that might be a bit more relevant.

I know where you are going with this. However here's my few compnents list:

- Fuse
- Transformer (240V->8V is ideal, 240V->9V is acceptable)
- Full Bridge Rectifer
- Smoothing Cap (4700 uF should be sufficient)
- 7805 linear regulator
- Anti-osciallation caps as required.

Not isolating the circuit is bad news. One slip up and your circuit and
any operators of that circuit will be connected directly to the mains lines.
It's simply not worth it not to have the transformer unless you have no
other alternative.

Now I'll throw out a related question: What is the smallest transformer one
can get that converts 120/240V to 9V or so?. I'm looking for something small
enough to fit into a single switch box with a circuit.

BAJ

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2001\10\22@092404 by Spehro Pefhany

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At 09:09 AM 10/22/01 -0400, you wrote:
>
>Now I'll throw out a related question: What is the smallest transformer one
>can get that converts 120/240V to 9V or so?. I'm looking for something small
>enough to fit into a single switch box with a circuit.

If height isn't too much of an issue, the 2.4VA ones are pretty reasonably
priced in quantity and the footprint is reasonable (you can put stuff under
them, to some degree) . I've seen much smaller ones in Japanese equipment
(but maybe not approved), and I have a piece of industrial equipment here
that has a UL/CSA transformer that is 1.5" x 1.2" but only 0.9" high (I'd
estimate 1VA). The 2.4VA (and similar 1.2VA types) are 1.4" x 1.2" x 1.2"
high.

Best regards,

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2001\10\22@172459 by Russell McMahon

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Subject: [EE]: Does anyone have a circuit to power a PIC from 240v AC with
only a few components.


> I have seen circuit on the web to do this though it uses a current
regulator
> that I can not source nor find the specs for. Does anyone have a circuit
> that might be a bit more relevant.


This is irregularly repeating question - it will be in the archives.
Also, the recent (and also frequently repeated) "how do I drive a LED from
the mains" thread provides general pointers.

Note that by far the safest method is (probably) to use a small mains
transformer. This will be of minimal size for your application and very
small units are available. This gives you mains isolation and safety whereas
most of the other solutions leave your circuit potentially at mains voltage.
Unless you are ABSOLUTELY certain that the circuit will always be isolated
and that ALL parts of it are always treated as being at mains voltage then
you should not use the unisolated solutions (unless the life of the user is
unimportant to you :-( ).




           Russell McMahon

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2001\10\23@202433 by Germain Morbe

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part 1 900 bytes content-type:text/plain; (decoded 7bit)

Hello Mathew,

for low currents e.g. a PIC i would suggest my favorite circuit as attached.
(pvcc2.gif)
It deliveres up to 3 mA at 5V and has some advantages over other approaches.

At the 1st glance it is small lightwight and really cheap.
At the second, due to dividing the series resistor into two parts, you get
an excellent line filter at no extra costs which is burst proof up to 4kV.
Given todays EMC requirements, the capacitor approach as mentioned earlier
by someone else is no longer recommended since the caps impedance drops at
higher frequencys and therefore prefer burst and surge pulses over line
voltage to feed your circuit.
Finally, the resistors in both wires limit the max. current to about 4.5 mA
AC which is well below a dangerous level. It may hurt you if you touch the
circuit, but it will definately not kill you.

regards Germain Morbe



part 2 3014 bytes content-type:image/gif; (decode)


part 3 105 bytes
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2001\10\23@203715 by Fritz Braun Jr.

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How would you modify this circuit to work with 120VAC?

----- Original Message -----
From: "Germain Morbe" <spamBeGoneGMorbespamBeGonespamCDSGMBH.COM>
To: <TakeThisOuTPICLISTEraseMEspamspam_OUTMITVMA.MIT.EDU>
Sent: Tuesday, October 23, 2001 20:12
Subject: [EE]: Does anyone have a circuit to power a PIC from 240v AC with
only a


> Hello Mathew,
>
> for low currents e.g. a PIC i would suggest my favorite circuit as
attached.
> (pvcc2.gif)
> It deliveres up to 3 mA at 5V and has some advantages over other
approaches.
>
> At the 1st glance it is small lightwight and really cheap.
> At the second, due to dividing the series resistor into two parts, you get
> an excellent line filter at no extra costs which is burst proof up to 4kV.
> Given todays EMC requirements, the capacitor approach as mentioned earlier
> by someone else is no longer recommended since the caps impedance drops at
> higher frequencys and therefore prefer burst and surge pulses over line
> voltage to feed your circuit.
> Finally, the resistors in both wires limit the max. current to about 4.5
mA
> AC which is well below a dangerous level. It may hurt you if you touch the
> circuit, but it will definately not kill you.
>
> regards Germain Morbe
>
>

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2001\10\24@045659 by Germain Morbe

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Hi Mathew,

the component values are:
zeners : 5.6 V /500mW (Uout  5.6 -0.7 = 4.9V)
diodes: 1N4148
caps: 1n/630VDC or 250VAC, 100n/50V, 100u/16V
resistors: 27 kOhms/1W (can be changed for different VAC or currents)

> How would you modify this circuit to work with 120VAC?

by calculating new resistor values as follows:

the zeners need about 1 mA for proper stabilization, if we need 3.5 mA for
our circuit the max. needed current through the resistors is 4.5 mA.
(if human security is not a concern we could supply up to 20 mA for our
circuit)

As allways the series resistance is U / I = 120VAC / 4.5mA = 26.66 kOhms

This gives us 13.33 kOhms for each resistor. We choose 12 k as the next
lower standard part, leaving us room for potential line undervoltage.

The max. current raises now to 120VAC / (2 x 12k) = 5 mA.

(The only concern about the resistors is their load resp. temperature)

We find the max. power to be 120VAC x 5 mA = 600 mW
Since each resistor is loaded with 300 mW we need at least 0.5W types.

The cutoff frequency of the line filter also changes with the resistor
values but we do not worry about that because the 100 nF is already
calculated for resistors down to 4.7 kOhms.

Germain Morbe

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2001\10\24@052103 by Gerhard Fiedler

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At 02:12 10/24/2001 +0200, Germain Morbe wrote:
>It deliveres up to 3 mA at 5V and has some advantages over other approaches.
>
>At the 1st glance it is small lightwight and really cheap.
>At the second, due to dividing the series resistor into two parts, you get
>an excellent line filter at no extra costs which is burst proof up to 4kV.
>Given todays EMC requirements, the capacitor approach as mentioned earlier
>by someone else is no longer recommended since the caps impedance drops at
>higher frequencys and therefore prefer burst and surge pulses over line
>voltage to feed your circuit.

These are indeed some advantages, but there is also a disadvantage: power
consumption. Even at only 3 mA, you get close to 1 W. If you need more
current, like 20 mA, this is close to 5 W, which in many applications is
too much. Avoiding excessive power consumption is the main reason for the
use of caps instead of resistors.

Has anybody had to abandon such a capacitor-based power supply because it
didn't fulfill EMC requirements?

ge

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2001\10\25@055159 by Germain Morbe

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>These are indeed some advantages, but there is also a disadvantage: power
>consumption. Even at only 3 mA, you get close to 1 W. If you need more
>current, like 20 mA, this is close to 5 W, which in many applications is
>too much. Avoiding excessive power consumption is the main reason for the
>use of caps instead of resistors.

Yes "ge"  - sorry dont know who you actually are -

you are right, power loss is the big disadvantage. But as mosttime in life,
you cant have everything at the same time. I myself would not use the
circuit for more than 5 to 10 mA.

>Has anybody had to abandon such a capacitor-based power supply because it
>didn't fulfill EMC requirements?

Yes, we had and the reason is quite clear.

Suppose you have set up a capacitor that gets you 12VAC out of 240V at 50Hz.
This is a division ratio of 20.

Burst pulses are defined to be from 4 to 8 kV at frequencys from 5kHz
upward.
I think you will agree that the impedance of your cap at 5kHz is 100 times
lower than at 50Hz.
It is easy to understand that the voltage division ratio drops in a similar
fashion.
Therefore a 4kV ! impulse at 5kHz will reach your PIC or whatever undamped
with nearly full amplitude and energy.
Sure your zeners will compensate for that but imagine the current spike.
Several Amps!
The EMC problem is not your emission but simply the resistance.

greetings Germain Morbe

p.s.
for those who plan to use the circuit. 100nF for the filter capacitor is not
needed at very low currents. I have setup the following formula for a
capacitor value that gives you a cutoff frequency of 200 Hz which is
sufficient. The presized formula gives the cap in nF for given resistors.

Cf = 1.000.000/ (2 x R)

In my earlier example we used 12k resistors. A sufficient cap would then be:

1000000 / 24000 = 41,66 nF and i would choose 39nF in this case.

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2001\10\25@072529 by Gerhard Fiedler

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At 11:50 10/25/2001 +0200, Germain Morbe wrote:
>Yes "ge"  - sorry dont know who you actually are -

If you look at the "From" header of my message, you should see my name
(Gerhard Fiedler) -- at least I see all the names in there. "Ge" is my
nickname, and since I don't like big signature files, I just don't use any.
BTW, who I'm actually are, is something I've been trying to figure out for
quite some time. Any pointers? :)

>The EMC problem is not your emission but simply the resistance.

You're sure right here. Did you try to use a protection network with
chokes, parallel Cs, (smaller) Rs and TVS, connected between mains and the
C? I haven't done any real measurements, but this is how I'd like to do it.
In this app, a transformer is not an option because of space requirements,
resistors are not an option because of power consumption requirements, and
an offline switcher is a cost the I'd like to avoid.

ge

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2001\10\26@045717 by Germain Morbe

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Hello Gerhard,

i did not fully try to filter the line voltage in front of the cap because
it seemed too costy for our application that time. Also the dimensions of a
cap for 20 mA nearly reaches that of a tiny EI20 transformer.
If you urged to use the cap, it might be possible with filtering for low
currents.
I would use a calculated LC filter in front of the cap and a TVS behind it.

Look at http://www.nuhertz.com for filter design software, they offer a feature
reduced free version of their very powerful Filter Solutions Package.

greetings Germain Morbe

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2001\10\26@084324 by Gerhard Fiedler

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At 10:27 10/26/2001 +0200, Germain Morbe wrote:
>Also the dimensions of a
>cap for 20 mA nearly reaches that of a tiny EI20 transformer.

Where do you get such small transformers? The dimensions for (even smaller)
EI3 or EI5 transformers I know are approx. 40x40x35 mm, and EI20 are
approx. 60x60x50. Smaller types (e.g. 1 VA) get down to 37x31x25 mm (and
$5) -- still a big step to a capacitor solution, seems to me.

ge

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2001\10\27@075512 by Germain Morbe

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> Where do you get such small transformers? The dimensions for (even
smaller)
> EI3 or EI5 transformers I know are approx. 40x40x35 mm, and EI20 are
> approx. 60x60x50. Smaller types (e.g. 1 VA) get down to 37x31x25 mm (and
> $5) -- still a big step to a capacitor solution, seems to me.

Gerhard,
if you speak of mm here then your informations are wrong.

Look here: http://www.era.de/ie4_fs_produkte_e.htm

Their EE20 transformers offer 80 mW and 350 mW at footprints of 22x23 mm and
profiles as low as 12 mm. We use these types a lot. They are below 2$ in
quantities from 50 upward, have a short cirquit protection an are UL listed.

greetings Germain Morbe

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2001\10\27@083325 by Gerhard Fiedler

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At 13:51 10/27/2001 +0200, Germain Morbe wrote:
>Look here: http://www.era.de/ie4_fs_produkte_e.htm
>
>Their EE20 transformers offer 80 mW and 350 mW at footprints of 22x23 mm and
>profiles as low as 12 mm. We use these types a lot. They are below 2$ in
>quantities from 50 upward, have a short cirquit protection an are UL listed.

Thanks a lot for the pointer. I'll have to check it out and it may or not
be an option for my current problem, but it definitely is something I'll
keep in mind.

ge

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2001\10\30@041138 by Michael C. Reid

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I'm joining this discussion late as I have been traveling, but here is a
possible source for a really cool "universal" power supply.
http://www.biascompany.com

Let me know if this module they make works for you!  It is prices under
$3.00 in quantities.

Mike

{Original Message removed}

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