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