disabling cap draining resistors

[Jamie Morken]

Hi,

I have a power supply with some big 400Volt electrolytic caps with
100kOhm resistors in parallel to drain them for safety, I was wondering
if a d-fet could be used in series with the resistor so that once the
power supply is online, the d-fet could be turned off, to save power
by not having 1.8watts going through this resistor (100kOhm at 400VDC).

D-fets (ie. DN3765) are normally on switches, so once the power supply
goes offline, the caps should drain for safety. Is this a good way to
save power, also are there other ways to do this?

cheers,
Jamie

 

[neon]

those resistora are there for one thing and one thing only is to provide a min load when there is no loadi t is a neccessary waste. when you turn off power the power will decay as a RC time decay. I have no idea where can anybody live that the small waste of 1.6 watts will be a bother. can't be the money though the implementation you propose will cost a hell of a lot more.

 

[Joerg]

Sure. Just pull the gate negative somehow when AC is on. You could do
the equivalent with an enhancement mosfet circuit, except that it
would have to leave a few volts in the caps, ultimately.

There are some normally-on SSR's, which encapsulate a depletion-mode
fet and a photo-isolated driver. That might be handy.

Or use a real relay!

Especially if this is for a product I'd be careful because the
TUEV/UL/CSA or whatever blessing will be gone afterwards.

 

[Joerg]

LEDs in series with the bleeders is fun, and serves as a warning too.

Some of my bigger amps have a manually-operated bleeder switch, and a
biggish resistor+led thing always connected. Modern led's are visible
at very low currents.

Talking about bigger amps, why is it that EMC labs in your neck of the
woods don't always have a huge RF amp on the shelf? After resigning to
the fact that we can't get a tractor-trailer rig cert'ed in the Bay Area
on account of its weight and size at least we wanted to have a mock
chassis blasted. Whenever I asked about >>20V/m the guys at the other
side began to choke a bit. That was a lot easier in Europe. Most places
I used there were geared for mil-level.

 

[Joerg]

The EMC labs are usually out in the boonies, not around here. The
background fields here are outrageous. The AM, FM, TV, and certain
mysterious bands are pretty much saturated. I get -20 dBm on a couple
of FM frequencies with a clip lead hanging out the front of my
spectrum analyzer.

That's why I was surprised that there are any labs in the Bay Area. It
used to be that we had to schlepp everything to Mariposa.

Besides, FCC only checks for radiated emissions, not susceptibility.
So nobody here needs amps unless they're doing CE too.

Well, most stuff that I design or help to design is eventually marketed
world-wide so we have to. Plus automotive where you can't get anything
on the road without some serious susceptibility testing. You don't want
the electronic brake assist to come on hard just because the trucker
next to you keyed his CB radio mike.