Hello,

Can power supply filter chokes be used beyond their stated current
ratings? Are they typically rated for continuous duty? If so, if used
in ICAS service can I draw more current than rated?

For example, I have a choke rated for 5 Henries at 400 MA (25 Ohms).
I'd like to use it in a circuit drawing up to 650 MA peak. This would
be for a class C amp -- CW only.

Thanks for any input.

73,
-HZ

"Hank Zoeller" wrote in message
...
Hello,

Can power supply filter chokes be used beyond their stated current
ratings? Are they typically rated for continuous duty? If so, if used in
ICAS service can I draw more current than rated?

For example, I have a choke rated for 5 Henries at 400 MA (25 Ohms). I'd
like to use it in a circuit drawing up to 650 MA peak. This would be for
a class C amp -- CW only.

Thanks for any input.

73,
-HZ


You're proposing to run the choke at more than 150% of its current rating.
That will have at least two very adverse effects.
First, you could overheat the windings to the point of burning insulation on
the windings, burning the wire open, causing a fire, etc.
Second, you're running the very possible risk of saturating the core,
causing major loss of inductance. The effect this would have on your
circuit depends on its actual component values and how well the rest of the
circuit can forgive the loss of inductance.
My recommendation would be to get a choke of the appropriate ratings.

That said, you should download a copy of Duncan's power supply designer from
http://www.duncanamps.com/psud2/index.html and input your component values,
load it with a stepped constant current load and see how it performs. This
program will show you the current and voltage waveforms and values, but it
won't show the core saturation effects. You'll need more sophisticated
software for that (Spice).

Cheers
--
HAM AND EGGS -- A day's work for a chicken, a lifetime commitment for a pig.

Dave M

Dave M wrote:
You're proposing to run the choke at more than 150% of its current rating.
That will have at least two very adverse effects.
...edited for brevity..

Thanks for the reality check. It was wishful thinking on my part --
based on some hazy recollection of running transformers in ICAS service
beyond their CCS ratings.


That said, you should download a copy of Duncan's power supply designer from
http://www.duncanamps.com/psud2/index.html
...edited for brevity..

That's a great resource. Thank you for that link!

--
73,
HZ

My comments are below....

On Thu, 18 Jun 2009, Dave M wrote:

Date: Thu, 18 Jun 2009 18:03:21 -0400
From: Dave M
Newsgroups: rec.radio.amateur.homebrew
Subject: Filter choke specs

"Hank Zoeller" wrote in message
...
Hello,

Can power supply filter chokes be used beyond their stated current
ratings? Are they typically rated for continuous duty? If so, if used in
ICAS service can I draw more current than rated?

Good questions.

See below.

For example, I have a choke rated for 5 Henries at 400 MA (25 Ohms). I'd
like to use it in a circuit drawing up to 650 MA peak. This would be for
a class C amp -- CW only.

Thanks for any input.

73,
-HZ


You're proposing to run the choke at more than 150% of its current rating.
That will have at least two very adverse effects.
First, you could overheat the windings to the point of burning insulation on
the windings, burning the wire open, causing a fire, etc.

People need to think about the duty cycle of CW. The 400 ma rating of the
choke _should_ be for CCS and that _should_ mean 100% duty cycle. Figure
out yourself what the duty cycle of CW would be: a string of dots would be
50% duty cycle. A string of dashes would be 75% duty cycle. The other
question would be, for CW, what is his key up resting current going to be
compared to 650 ma (at peak)?

You have to think how much a choke heats up under its rated CCS current.

Hes got a choke with 25 ohms, so put a DC voltage accross that to get a
current of 650 ma and feel the choke with your fingers every 4-5 minutes.
Or get a thermometer and touch the bulb to the case of the choke and watch
it to see if the temperature comes up to a plateau (it should).

Quite frankly, I think its a _go_ project.

More below.

Second, you're running the very possible risk of saturating the core,
causing major loss of inductance.

That would depend entirely on whether the core had an anti-saturation air
gap somewhere. As far as I know (and there was one curve in QST decades
ago on this), all chokes have an inductance which has some relationship to
current. When I was a kid I built a magnetic amplifier (and it worked)
based on variable inductance in one winding controlled by variable current
in another winding.

If this choke is going into a power supply and you are worried about
whether such currents are going to hurt your voltage regulation, then you
can hook up an oscilloscope (through an appropriate dropping voltage
divider) and go ahead and key the current.

There are a lot of high power amplifiers that don't even use chokes in
their power supplies, but put in extra capacitance.

Of course, if you are a purist-perfectionist, then go and plunk out the
money for the bigger choke.

If it were me, I'd consider the criteria I mentioned above, do the DC
current experiment (one at 400 ma and one at 650 ma) and see how hot the
choke got. When chokes and transformers get a little overheated, they may
smoke or give off fumes that you can smell, too. If you let the thing keep
smoking, then after some time you will start to damage the insulation and
wax impregnated paper layers.

A 25 ohm choke would need a little under 12 volts to get the 400 ma, and
maybe somewhere around 16-19 volts to get up to around 2/3 of an amp. That
will end up (without doing the actual arithmetic) being about ten watts of
heat to dissipate. Don't forget that the actual duty cycle will be 50-75%,
on CW, so your choke at 650 ma intermittent service is probably going to
be dissipating heat as if it were close to the 400 ma CCS operating
conditions.

I still think the project is a _go_.


The effect this would have on your
circuit depends on its actual component values and how well the rest of the
circuit can forgive the loss of inductance.
My recommendation would be to get a choke of the appropriate ratings.

That said, you should download a copy of Duncan's power supply designer from
http://www.duncanamps.com/psud2/index.html and input your component values,
load it with a stepped constant current load and see how it performs. This
program will show you the current and voltage waveforms and values, but it
won't show the core saturation effects. You'll need more sophisticated
software for that (Spice).

Cheers
--
HAM AND EGGS -- A day's work for a chicken, a lifetime commitment for a pig.

Dave M

"Hank Zoeller" wrote in message
...
Hello,

Can power supply filter chokes be used beyond their stated current
ratings? Are they typically rated for continuous duty? If so, if used in
ICAS service can I draw more current than rated?

For example, I have a choke rated for 5 Henries at 400 MA (25 Ohms). I'd
like to use it in a circuit drawing up to 650 MA peak. This would be for
a class C amp -- CW only.

Thanks for any input.

73,

If the choke ratings are actually CCS (and not ICAS as many were/are), I
would go for it. If it doesn't quite work out, and the smoke bothers your
eyes, there is another easy solution. Since this is about homebrew, it is a
relatively simple project to rewind a choke (except, horror of horrors, a
potted choke). Use a little heavier wire. Since there probably won't be
enough
room for as many turns, you'll lose a little inductance, but who will know
the
difference? A little hum on a CW signal is quite distinguished anyway.

Old Chief Lynn, W7LTQ

"Hank Zoeller" wrote in message
...
Hello,

Can power supply filter chokes be used beyond their stated current
ratings? Are they typically rated for continuous duty? If so, if used in
ICAS service can I draw more current than rated?

For example, I have a choke rated for 5 Henries at 400 MA (25 Ohms). I'd
like to use it in a circuit drawing up to 650 MA peak. This would be for
a class C amp -- CW only.

Thanks for any input.

73,
-HZ


I think most chokes today are rated for continuous duty. However, check with
the manufacturer to be sure.

With respect to heating the choke, you can run higher current if you keep
the rms value down to the choke's rating. To do this, you should have a duty
factor as follows:

400 mA = 650 mA x sqrt(D) where D is the duty factor you must not exceed.

This gives (.4/.65)^2 or a duty factor of about 38%. (Can CW be sent at this
duty?)

Expanding a bit on Dave M's comments:

Inductance at the higher current might be more problematic. As the iron gets
closer to saturation, inductance takes a nose dive. This will cause higher
peak currents which invalidate the above calculations because the higher
peak currents mean higher RMS. This higher RMS current might also be bad for
your filter capacitors. The only way to know for sure is to measure the true
RMS choke and capacitor currents at the desired load current. You could then
adjust your duty factor to not exceed the choke's rating. Then check the
filter capacitor's RMS rating to see if it is exceeded. High RMS capacitor
currents cause heating which dries out the electrolyte thus reducing the
capacitor's life. Also check your ripple voltage to see if it is tolerable.
Do all this without overheating the choke.

Good luck with your project.

John - KD5YI

On Jun 21, 1:01*pm, "John KD5YI" wrote:
"Hank Zoeller" wrote in message

...

Hello,

Can power supply filter chokes be used beyond their stated current
ratings? *Are they typically rated for continuous duty? *If so, if used in
ICAS service can I draw more current than rated?

For example, I have a choke rated for 5 Henries at 400 MA (25 Ohms). I'd
like to use it in a circuit drawing up to 650 MA peak. *This would be for
a class C amp -- CW only.

Thanks for any input.

73,
-HZ

I think most chokes today are rated for continuous duty. However, check with
the manufacturer to be sure.

With respect to heating the choke, you can run higher current if you keep
the rms value down to the choke's rating. To do this, you should have a duty
factor as follows:

400 mA = 650 mA x sqrt(D) where D is the duty factor you must not exceed.

This gives (.4/.65)^2 or a duty factor of about 38%. (Can CW be sent at this
duty?)

Expanding a bit on Dave M's comments:

Inductance at the higher current might be more problematic. As the iron gets
closer to saturation, inductance takes a nose dive. This will cause higher
peak currents which invalidate the above calculations because the higher
peak currents mean higher RMS. This higher RMS current might also be bad for
your filter capacitors. The only way to know for sure is to measure the true
RMS choke and capacitor currents at the desired load current. You could then
adjust your duty factor to not exceed the choke's rating. Then check the
filter capacitor's RMS rating to see if it is exceeded. High RMS capacitor
currents cause heating which dries out the electrolyte thus reducing the
capacitor's life. Also check your ripple voltage to see if it is tolerable.
Do all this without overheating the choke.

Good luck with your project.

John - KD5YI

Hey OM:

I would steal a page from the japanesse, they came out with the best
regulators by swamping components down.

In this case they would swamp the inductor with a power resistor. I've
never seen this tech used by anybody else, except in the orient.

So to gain the extra current all you have to do is parallel a resistor
across the choke.

Even resistors will regulate, smart darn japanesse.

So all the resistor has to do is soak up the other 250ma with a power
rating that it can handle the current and voltage.

You could even make it adjustable slider type so it could be tweaked.

73 OM

de n8zu

On Tue, 23 Jun 2009 13:44:24 -0700 (PDT), raypsi
wrote:

I would steal a page from the japanesse, they came out with the best
regulators by swamping components down.

In this case they would swamp the inductor with a power resistor. I've
never seen this tech used by anybody else, except in the orient.

So to gain the extra current all you have to do is parallel a resistor
across the choke.

Even resistors will regulate, smart darn japanesse.

So all the resistor has to do is soak up the other 250ma with a power
rating that it can handle the current and voltage.

You could even make it adjustable slider type so it could be tweaked.

In the OP's case, the swamping resistor would have to about twice the
DC resistance of the choke to get about 2:1 current share between the
choke and the resistor. These resistances are typically in the order
of a few ohms.

Looking at the ripple frequencies 100/120 Hz and their harmonics, the
inductive reactance for 5 H is in the order of kilo-ohms. A swamping
resistor of a few ohms would be practically a short circuit across the
choke.

Paul OH3LWR

On Jun 24, 1:03*am, Paul Keinanen wrote:
On Tue, 23 Jun 2009 13:44:24 -0700 (PDT), raypsi
wrote:

I would steal a page from the japanesse, they came out with the best
regulators by swamping components down.

In this case they would swamp the inductor with a power resistor. I've
never seen this tech used by anybody else, except in the orient.

So to gain the extra current all you have to do is parallel a resistor
across the choke.

Even resistors will regulate, smart darn japanesse.

So all the resistor has to do is soak up the other 250ma with a power
rating that it can handle the current and voltage.

You could even make it adjustable slider type so it could be tweaked.

In the OP's case, the swamping resistor would have to about twice the
DC resistance of the choke to get about 2:1 current share between the
choke and the resistor. These resistances are typically in the order
of a few ohms.

Looking at the ripple frequencies 100/120 Hz and their harmonics, the
inductive reactance for 5 H is in the order of kilo-ohms. A swamping
resistor of a few ohms would be practically a short circuit across the
choke.

Paul OH3LWR

On Jun 24, 1:03*am, Paul Keinanen wrote:
On Tue, 23 Jun 2009 13:44:24 -0700 (PDT), raypsi
wrote:

I would steal a page from the japanesse, they came out with the best
regulators by swamping components down.

In this case they would swamp the inductor with a power resistor. I've
never seen this tech used by anybody else, except in the orient.

So to gain the extra current all you have to do is parallel a resistor
across the choke.

Even resistors will regulate, smart darn japanesse.

So all the resistor has to do is soak up the other 250ma with a power
rating that it can handle the current and voltage.

You could even make it adjustable slider type so it could be tweaked.

In the OP's case, the swamping resistor would have to about twice the
DC resistance of the choke to get about 2:1 current share between the
choke and the resistor. These resistances are typically in the order
of a few ohms.

Looking at the ripple frequencies 100/120 Hz and their harmonics, the
inductive reactance for 5 H is in the order of kilo-ohms. A swamping
resistor of a few ohms would be practically a short circuit across the
choke.

Paul OH3LWR

Hey OM:

Now for the rest of the story: use a high voltage FET 2Amp at 800V
across the choke and regulate it with a LM431, programmable Zener.
I has a EICO Triband HF rig model 753, it's power supply uses 2
chokes, I regulate the outputs: 790 Volts to the final and 275 Volts
to the rest of the rig. Solid as a brick outhouse. The chokes take
most of the load off the FET's. So if the chokes saturate the FET's
take over. With all the mods I done to the EICO 753's regulating the
DC outputs is best.

73 OM

de n8zu