"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