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'[EE]: Snubber network for Triac'
2001\02\27@035440 by Robert A. LaBudde

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I would like recommendations from the list for a snubber network to use
with a 400 V Triac on a 120 VAC power line.

Requirements:

1) Load inductance ~ 200 uH (lines to resistive heater).
2) 400 V 8 A triac, expected full 8 A load possible.
3) 120 VAC, 60 Hz power line.
4) Ignore load resistance for the estimation (i.e., assume pure 200 uH
inductive load).

Questions:

1) What R and what C are ideal for this application?
2) What voltage requirement for C?
3) What voltage and wattage requirement for R?

Thanks.
================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: spam_OUTralTakeThisOuTspamlcfltd.com
Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
824 Timberlake Drive                     Tel: 757-467-0954
Virginia Beach, VA 23464-3239            Fax: 757-467-2947

"Vere scire est per causas scire"
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2001\02\27@042928 by Jeszs

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I have several triacs at home (220V, 50 Hz) working with transformers for
halogen lights with the following parallel network:
- 100 Ohm / 600V
- 1 uF / 600V

I performed strage calculations (look for 'snubber network' in Altavista) to
obtain these figures but my experience is that they are valid for most of
the home appliances.

ps: do not forget to use quick fuses; better safe...

--------------------
Jeszs Gonzalo
Lesn (SPAIN)
--------------------

{Original Message removed}

2001\02\27@044259 by Jinx

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I have a few circuits (some heaters) here that all show
100R 1W and 100n 630V. This is for 240VAC 50Hz
is that's a start

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2001\02\27@125309 by Dan Michaels

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At 10:12 PM 2/27/01 +1300, you wrote:
>I have a few circuits (some heaters) here that all show
>100R 1W and 100n 630V. This is for 240VAC 50Hz
>is that's a start
>

And it is fairly important to use capacitors which are
especially made for this type of app - eg, Orange Drop/etc
- because the wrong types will vaporize nicely.

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2001\02\27@130949 by Spehro Pefhany

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At 03:54 AM 2/27/01 -0500, you wrote:
>I would like recommendations from the list for a snubber network to use
>with a 400 V Triac on a 120 VAC power line.

I suggest you calculate it. You can use the resonant method, see
SCR manuals for the (simple) equations. You'll also need the maximum
(off) dv/dt for the particular triac you have in mind.

>Questions:
>
>1) What R and what C are ideal for this application?

See above.

>2) What voltage requirement for C?

Use a UL approved type (X2, IIRC).

>3) What voltage and wattage requirement for R?

Use a surge proof type, 1/2W Metal oxide film is usually fine.

Best regards,

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2001\02\27@133253 by Chris Carr

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Although not specifically dealing with capacitors in snubber applications,
this tech note may be of help

http://www.evox-rifa.com/technote_pdf/rfi_fact.pdf

It does a least explain why you should be using a type X capacitor.

Other tech notes on capacitors on that site are listed at
www.evox-rifa.com/europe/technotes_filmcaps.htm
Note the integrated RC snubber capacitors close to the bottom

See also
http://www.evox-rifa.com/europe/pulsecad.htm

which describes their free program PulseCAD which you may find of use

regards
Chris



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'[EE]: Snubber network for triac'
2001\03\01@171549 by Robert A. LaBudde
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Here are my conclusions based on the responses received to my question and
to investigations and modeling I've done:

1. The optimum series R is about 0.5 x sqrt (L / C), where R = snubber
resistor, C = snubber capacitor and L = load inductance.

2. The capacitor should be as large a value as possible without forcing too
large a wattage resistor. In any event, C should be < 7e-8 / L.

3. The resistor wattage should be > 60 C Vp ^ 2, where Vp = 170 the peak
voltage on a 120 VAC, 60 Hz line.

4. The triac voltage rating should be > I R, where I is the max load current.

5. The resistor voltage rating should exceed Vp or I R, whichever is larger.

6. The capacitor voltage rating should be > Vp.

7. The type of capacitor is relatively unimportant unless large line
voltage transients are significant, in which case a varistor would be useful.

8. The triac dV/dt requirement is probably not an issue in practice, or can
be satisfied by a low value (100 pF) capacitor across the triac.

Example:

8 A, 400 V triac operating on 120 VAC (Vp=170 V).

L = 200 uH (mostly wiring)

C < 10000/L = 350 uF

Choose C = 0.1 uF, 250 V

R = 0.5 x sqrt (L / C) = 22 ohm

P(R) > 60 C Vp ^ 2 = 0.17 W, so a 1/4 W resistor would work.

V(R) = I R = 8 x 22 = 176 V, so a normal 250 - 350 V resistor will work.

=> Snubber network consists of a series 22 ohm, 0.25 W resistor with a 0.1
uF 250 V capacitor. This will near-optimally damp EMI and and limit
back-emf to 176 volts.

Any comments as to the adequacy of this analysis or set of design rules?

Thanks.
================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ralspamKILLspamlcfltd.com
Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
824 Timberlake Drive                     Tel: 757-467-0954
Virginia Beach, VA 23464-3239            Fax: 757-467-2947

"Vere scire est per causas scire"
================================================================

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2001\03\01@174550 by Chris Carr

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.
>
> 7. The type of capacitor is relatively unimportant


NO NO NO

It MUST be an X Type Capacitor.

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2001\03\02@005632 by Robert A. LaBudde

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At 10:42 PM 3/1/01 +0000, you wrote:
>.
> >
> > 7. The type of capacitor is relatively unimportant
>
>
>NO NO NO
>
>It MUST be an X Type Capacitor.

Why is this? The test procedure indicates kilovolts. This is more likely
due to high-voltage spikes on the line rather than the triac switching.

If this is expected, as I said, the proper solution would seem to be a
varistor to limit the voltage excursions rather than try to find a
capacitor which could survive them.

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ralspamspam_OUTlcfltd.com
Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
824 Timberlake Drive                     Tel: 757-467-0954
Virginia Beach, VA 23464-3239            Fax: 757-467-2947

"Vere scire est per causas scire"
================================================================

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2001\03\02@030804 by Chris Carr

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> > > 7. The type of capacitor is relatively unimportant
> >
> >
> >NO NO NO
> >
> >It MUST be an X Type Capacitor.
>
> Why is this? The test procedure indicates kilovolts. This is more likely
> due to high-voltage spikes on the line rather than the triac switching.
>
> If this is expected, as I said, the proper solution would seem to be a
> varistor to limit the voltage excursions rather than try to find a
> capacitor which could survive them.
>
>
Its all to do with Failure Modes not normal operation. X type capacitors are
self healing in the event of a flashover, ordinary capacitors tend to fail
to a short circuit until they burst into flames.

Regards

Chris

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2001\03\02@035242 by Spehro Pefhany

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At 05:16 PM 3/1/01 -0500, you wrote:
>
>=> Snubber network consists of a series 22 ohm, 0.25 W resistor with a 0.1
>uF 250 V capacitor. This will near-optimally damp EMI and and limit
>back-emf to 176 volts.
>
>Any comments as to the adequacy of this analysis or set of design rules?

Neither the triac rating or the ratings of either of the snubber
parts is adequate to meet minimum standards of safety and reliability.

Suggest you read up on expected line transients and fusing/heat sinking
requirements. Also, you might want to calculate the peak power in your
resistor and compare that to the data sheet rating. Also consider the
failure mode of carbon-film resistors.

Best regards,


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2001\03\02@044510 by Roman Black

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Robert A. LaBudde wrote:
>

> => Snubber network consists of a series 22 ohm, 0.25 W resistor with a 0.1
> uF 250 V capacitor. This will near-optimally damp EMI and and limit
> back-emf to 176 volts.
>
> Any comments as to the adequacy of this analysis or set of design rules?


>From years of inspecting commercial equipment I
feel you have underated the parts. From gueswork
I would go to a 1w or 2w resistor, mounted 1cm
above the board to keep heat away from the board,
and for the capacitor you will often find a 630v
poly cap is a similar size and cost to the 250v
poly cap. Many suppliers only stock the 630v caps.
Sourcing them may be easier, even cheaper.
And they are a much better part if it will be
exposed to mains spikes and lightning etc.

Your values sound good, I am always seeing 22ohm,
27ohm snubbers with 0.1 and 0.15 uF caps in TV
and VCR mains switching supplies. Most manufacturers
are using the 2w round resistors, these are a
high temp device with the matt red/green paint,
usually mounted above the board and on better
units mounted on ceramic lifters.

If in doubt, build it and measure the temp-rise of
the parts when run testing. I always tune snubbers
this way. :o)
-Roman

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2001\03\02@080942 by Alan B. Pearce

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>Why is this? The test procedure indicates kilovolts. This is more likely
>due to high-voltage spikes on the line rather than the triac switching.

>If this is expected, as I said, the proper solution would seem to be a
>varistor to limit the voltage excursions rather than try to find a
>capacitor which could survive them.

I am not familiar with type X capacitors as I have not had to get involved with
this sort of network for a long time, but the capacitor type can be important as
the dV/dt rating can be important, even with the resistor in circuit. This is
affected by the type of dielectric used in the capacitor.

An example of this was about 25 years ago I was involved in developing a
capacitor discharge ignition system along with some work colleagues. We punched
through several of the Philips "lollipop" polycarbonate capacitors because the
losses in the capacitor generated hot spots that caused the capacitor to short
through. we replaced these with a capacitor designed for TV Line Output stages
that had an infinite dV/dt rating and had no more failures.

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2001\03\02@110646 by Roman Black

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Alan B. Pearce wrote:
>
> >Why is this? The test procedure indicates kilovolts. This is more likely
> >due to high-voltage spikes on the line rather than the triac switching.
>
> >If this is expected, as I said, the proper solution would seem to be a
> >varistor to limit the voltage excursions rather than try to find a
> >capacitor which could survive them.
>
> I am not familiar with type X capacitors as I have not had to get involved with
> this sort of network for a long time, but the capacitor type can be important as
> the dV/dt rating can be important, even with the resistor in circuit. This is
> affected by the type of dielectric used in the capacitor.
>
> An example of this was about 25 years ago I was involved in developing a
> capacitor discharge ignition system along with some work colleagues. We punched
> through several of the Philips "lollipop" polycarbonate capacitors because the
> losses in the capacitor generated hot spots that caused the capacitor to short
> through. we replaced these with a capacitor designed for TV Line Output stages
> that had an infinite dV/dt rating and had no more failures.

I have buckets of HV polyester caps from TVs.
They are cool. I also like the 2kv and 6kv blue
coated ceramics, these are just about indestructable.
Anytime I need a bulletproof cap under 2nF I go for
one of those. They are about $1 each but you get
a few in every TV, and it's good for the apprentices
to strip dead TVs!! Good soldering and finger burning
practice.:o)
-Roman

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2001\03\05@152002 by Robert A. LaBudde

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At 03:06 AM 3/3/01 +1100, Roman wrote:
{Quote hidden}

I have investigated this issue and have come to the following conclusions:

1. The X and Y capacitor issues arises from the problem with fires started
by TV sites that have EMI filters and are always connected to the power lines.

2. The degradation of the capacitor results from high-voltage spikes which
occur sporadically on all power lines. This can end up shorting the
capacitor over time.

3. The MTBF is dependent on the "on" time, and is worst for something like
a TV set, which may remain connected for 10 years or more.

4. The issue is of little importance for a power control triac system which
will be on only a small portion of the time and will generally be under
operator supervision.

5. The wattage of the resistor is irrelevant, so long as the basic power
requirement is met. If the capacitor holds, the resistor will always be
operating within its rating. If the capacitor blows (shorts), 120 VAC
through a 22 ohm resistor is 650 W, so a 2W resistor is not going to
survive either. Mounting it on stilts probably just makes a safer fuse out
of it.

6. A fuse is a better solution for my application than trying to get the
capacitor and resistor to survive lightning strikes.

7. Varistors are apparently an acceptable substitute for special or
high-voltage capacitors in the EMI continuous line-filter application.

8. The triac will act as a varistor across the snubber network, since when
the voltage gets well above its rating, the triac will fire. The whole
point of the snubber network was to prevent this from happening (viz., to
prevent false triggering of the triac).

In conclusion, it appears a 0.25 W resistor and a 250 VAC polyester
capacitor of the correct voltage rating and values will undoubtedly work fine.

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: EraseMEralspamlcfltd.com
Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
824 Timberlake Drive                     Tel: 757-467-0954
Virginia Beach, VA 23464-3239            Fax: 757-467-2947

"Vere scire est per causas scire"
================================================================

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2001\03\06@030441 by Roman Black

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Robert A. LaBudde wrote:

> 5. The wattage of the resistor is irrelevant, so long as the basic power
> requirement is met. If the capacitor holds, the resistor will always be
> operating within its rating. If the capacitor blows (shorts), 120 VAC
> through a 22 ohm resistor is 650 W, so a 2W resistor is not going to
> survive either. Mounting it on stilts probably just makes a safer fuse out
> of it.
>
> In conclusion, it appears a 0.25 W resistor and a 250 VAC polyester
> capacitor of the correct voltage rating and values will undoubtedly work fine.


I must argue this one... What wattage *overhead* have
you allowed for the resistor, and are you familiar with
its temperature rise per wattage??

If you have not allowed enough overhead the part will
run too hot and with a smaller resistor this heat is
primarily conducted out through the legs into the solder
joints, causing bad dry joints.

With a larger size resistor, especially mounted higher
away from the board, the primary escape of dissipated
heat is to the air from the larger body of the resistor.
These 2w resistors have a special matte paint which helps
with this heat dissipation, and even much larger metal
end caps which also help. That's why i mentioned the
paint colour.

I am curious how much overhead you think is appropriate
for your resistor?? In my experience most engineers
are not adequately trained re the overheads needed for
reliable real world usage. How often do you see a chart
of temp rise vs dissipation for your resistors? :o)

Making things worse is that Asian manufacturers are
constantly skimping with parts sizes and material costs,
and the parts are getting smaller but watts are not.
The result is that many resistors are totally unsuitable
for running continuously at their rated dissipation.
I have seen smoke come off some 1w resistors when
run at 1w. A good engineer understands the diference
between real specs and marketing specs.

I assume you are not exposing the 1/4 resistor to
voltages higher than its max voltage spec? I would not
expose a 1/4 resistor to more than 100v, maybe less if
it is AC or high dv/dt.
-Roman

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2001\03\06@042107 by Bob

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You may get by with the 1/4 watt resistor (I'd recommend using 1/2 watt "metal
film" instead).  Don't know what this kick Roman has about resistors, but I've
never (ever) had a snubber resistor blow out on me (or even get slightly warm
for that matter), and I've manufactured 1000's of analog triac circuits that are
using a .01uf 400v poly cap, and a 330ohm 1/2 watt resistor for the snubber
circuit.

Unless your gonna use something like a large 1+uf cap, a 1/2 watt metal film
(typically rated at 350 volts) resistor is more than adequite.  If your worried
that a line spike might kill it, there are companies that do make resistors
rated to withstand upwards of 10kv spikes (although the rest of your circuit
would be toast any ways).  Take a look at any fan speed controller sold in
hardware stores, as they typically only have a 1/4 or 1/2 watt resistor in the
snubber circuit (and they are UL listed).

But you absolutely NEED to use "at least" a 400V Poly Cap on 120VAC lines, and
at least a 600V poly cap on 240VAC lines (800V if you can find them).  Look at
any Teccor or ST triac circuit data sheets, and you'll never see them use
anything under those ratings for a snubber cap.

Remember, that on a nominal 120VAC line (which could easily be upwards of
145VAC), your voltage peak is 1.414 times the RMS value (like upwards of a 205
voltage peak, on a 145VAC line).  45 volts is not much of a safety margin,
especially with a capacitor.  These caps are rated in "absolute MAXIMUM DC
voltage" that they can withstand, not AC RMS/Average voltages.  i.e., you don't
want to be running them at, or even near, their rated voltage.

If you use a 250V cap, and listen to the new circuit run in a quite room, you'll
hear it crackle and snap a bit if you listen closely.  I suspect that is that
self healing thing that someone else mentioned earlier about X caps, and it
means that you are close to exceeding its voltage rating.  Yep, tried that with
a 400v cap running on 240VAC once, it does tend to not last very long.

A varistor won't necessarily catch every line spike fast enough to keep a 250V
cap from eventually frying itself (besides, a 400V cap is only a few cents more
than a 250V one, which is much cheaper than a varistor).

A 250V poly cap may work initially, but it won't last too long....


{Quote hidden}

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2001\03\06@081024 by Roman Black

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Bob wrote:
>
> You may get by with the 1/4 watt resistor (I'd recommend using 1/2 watt "metal
> film" instead).  Don't know what this kick Roman has about resistors, but I've
> never (ever) had a snubber resistor blow out on me (or even get slightly warm
> for that matter), and I've manufactured 1000's of analog triac circuits that are
> using a .01uf 400v poly cap, and a 330ohm 1/2 watt resistor for the snubber
> circuit.

Hmmm... I believe the suggestion was to use a 27ohm
resistor with about a 0.1uF cap?? I see similar every
day and not one had a 1/4 watt resistor. And it's very
rare I see a 1/4 watt resistor exposed to near-mains
voltages either. But most of the circuits I see
are for 220/240v mains. :o)
-Roman

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2001\03\06@122332 by Dan Michaels

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Roman wrote:
>Bob wrote:
>>
>> You may get by with the 1/4 watt resistor (I'd recommend using 1/2 watt
"metal
>> film" instead).  Don't know what this kick Roman has about resistors, but
I've
>> never (ever) had a snubber resistor blow out on me (or even get slightly warm
>> for that matter), and I've manufactured 1000's of analog triac circuits
that are
{Quote hidden}

I am looking at an industrial controller here, designed by a
major manufacturer [forget who] a few years ago for a local
company that I used to consult with. The board was UL approved,
and the company had at least 10,000 in the field.

120VAC board, relays are 10A, snubber caps are 0.1uF, 250VAC as
large as the relays and icons on them from about 6 stds agencies
- UL, CSA, VDE, etc.

The resistors are little 1/4W jobbers with values 100 and 220 ohms.
I just scraped the insulation off one, and it is metal film.

In general, the mains voltage doesn't actually appear across the
snubber R's, but largely across the reactance of the capacitors,
which is quite large at 60hz, so the mains current flow thru the
R's is tiny:

Xc = 1/[2*pi*60hz*0.1uF] = 26K

If I understand it correctly, the resistors are just in there to
limit impulse current levels through the snubber caps when the
relays switch. In this case, I would imagine that the wattage values
are not as important as how much voltage you can safely put across
the resistors - related to insulation, construction material, and
how far apart the leads are.

Regarding R values, they have to be small enough that enough
current will flow in the cap to effectively squelch the switching
transients, but large enough to limit instantaneous currents thru
the relay contacts to non-destructive levels. The 100 ohm R was used
in the switching ckt to a larger load than the 220 ohm R's.

- dan michaels
http://www.oricomtech.com
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2001\03\06@131915 by Robert A. LaBudde

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At 09:20 AM 3/6/01 -0800, Dan wrote:
>The resistors are little 1/4W jobbers with values 100 and 220 ohms.
>I just scraped the insulation off one, and it is metal film.

These typically have a voltage rating of 350-500 V.

>In general, the mains voltage doesn't actually appear across the
>snubber R's, but largely across the reactance of the capacitors,
>which is quite large at 60hz, so the mains current flow thru the
>R's is tiny:
>
>Xc = 1/[2*pi*60hz*0.1uF] = 26K

The capacitor is there to block 60 Hz current from flowing through the
resistor and to move the resonant frequency of the inductance >> 60 Hz.

>If I understand it correctly, the resistors are just in there to
>limit impulse current levels through the snubber caps when the
>relays switch. In this case, I would imagine that the wattage values
>are not as important as how much voltage you can safely put across
>the resistors - related to insulation, construction material, and
>how far apart the leads are.

The resistor is there to damp the back-emf resonance between the inductor
and the capacitor when the triac turns off. The energy flows back and forth
between the capacitor and inductor and the resistor guarantee a rate of decay.

The wattage rating of the resistor is based on its conduction of the
charged capacitor energy (at peak voltage). This energy is simply 0.5 C V^2
in 1/120 sec, or 60 C V^2. The larger the capacitance, the larger the
resistor wattage required.

>Regarding R values, they have to be small enough that enough
>current will flow in the cap to effectively squelch the switching
>transients, but large enough to limit instantaneous currents thru
>the relay contacts to non-destructive levels. The 100 ohm R was used
>in the switching ckt to a larger load than the 220 ohm R's.

The optimum damping R is ~ 0.5 sqrt (L/C).

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ralSTOPspamspamspam_OUTlcfltd.com
Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
824 Timberlake Drive                     Tel: 757-467-0954
Virginia Beach, VA 23464-3239            Fax: 757-467-2947

"Vere scire est per causas scire"
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2001\03\07@032043 by Vasile Surducan

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come on guys ! You'll get the same result with resistors in
50...220 ohm range and capacitor between 47n and 100nF
Vasile


On Wed, 7 Mar 2001, Roman Black wrote:

{Quote hidden}

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