Depletion Mode Power MOSFET?

Augment depletion mode FET by germanium BJT ?

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

I was looking for the exact same thing in a non-motor app last month and came up dry. Nothing in the sub-ohm range. You can parallel them but that becomes expensive.

How about some light bulbs as an additional load on the rail? When power is off their cold filaments provide a nice low resistance. Not quite a short but maybe it is good enough. Of course you'll always have to make sure nothing dangerous will happen should filaments break. And the really cool retro thing would be to use tube filaments here :-)

How about putting a lithium battery in series with the gate of an enhancement MOSFET?

Cheers,

Phil Hobbs

A bunch of these in parallel?

Hmm.. if shorting the power rail works okay, then a couple of body diodes in series are not a problem, so maybe just a beefy NPN transistor with a small depletion MOSFET between C and B (similar to what someone else suggested).

Best regards, Spehro Pefhany

A silicon BJT will probably do, and will be much easier to source. The extra half volt or so of drop shouldn't be a deal-killer (it works nicely into a diode, just not into an open circuit).

Thanks.

Thanks. See my reply to Vladimir -- I stalled out on "germanium" when I read his; having you reiterate it without specifying the material got me to consider it.

On a sunny day (Sat, 15 Nov 2008 10:52:47 -0800) it happened Tim Wescott wrote in :

Thyristor with resistor to anode?

AFAIK that'll have the same 1 volt-ish drop, and though it can probably be turned off reliably with enough current being pulled out of the gate it'll be a harder climb to get it through a design review.

But it'd be more compact.

Hmm....

Mr. Joerg, I like your non-trivial thinking. And if the motor produces enough power to warm up the PTC, then the voltage is sufficient to do clipping by conventional means...

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Seriously, in the tube days people did such things to provide the proper bias. They figured that since there isn't any current it'll last forever. A decade later the batteries were all but forgotten about, began to not like all that heat anymore, leaked, bias dropped, plate current rose ... *KABLAM*

Well, you aren't supposed to use this scheme to slow down a large cruise ship :-)

There are also other tricks such as a non-linear electronic load. A few ten milliohms that get switched out when the power supply comes on line and wants to sink some serious current into it. Might be an option for Tim since he wants something that can be retrofitted in the field.

Polyfuse ?

They won't probably endure such abuses but that's amusing.

-- Thanks, Fred

Interesting thought. But personally I don't use fancy auto-reset fuses. Seen too much grief there, including thick smoke.

I've been known to use an SCR, then paralleled by a relay (which resets the SCR.

...Jim Thompson

I see those in my old amateur radio handbooks and I shiver.

And I've never even had real experience with that -- just the notion of a big power tube losing it's bias because some battery wore out is scary.

I've seen one that literally sucked its glass body snug onto the plate. In the other cases they imploded. On a big final amplifier a busted tube can be more expensive to replace than a fried plate voltage transformer. Ok, confession here, I rarely used plate transformers and chose a multiplier cascade with hot chassis a lot. As a student you try the utmost not to have to borrow from the beer budget ...

Understood. On the other hand, batteries now last 20 years, the damping feature is needed only by service folk, not the end user, and the service human can just give the shaft a twist to verify that the battery is still OK. That puts it more in the CMOS backup SRAM class, I'd say.

Cheers,

Phil Hobbs

And having verified that the very expensive piece of equipment cannot be disassembled to reach the battery, which needs to be replaced in order to disassemble the equipment, the service guy hunts me down and kills me.

No Capes! (err -- batteries).

I used to have a Triumph whose cylinder head was held on with studs that went into the block at a 30 degree angle. To take the head off, you had to *extract the studs*. If they'd put the studs in straight, or used bolts instead, life would have been much easier. That designer

There's no logical requirement for the battery to be inaccessible either. What if the FET blew?

But fortunately I'm not the one designing that particular piece of equipment--I have enough troubles of my own trying to rectify light.

Cheers,

Phil Hobbs