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clamp diodes

Assume we have a 1-ohm 2512 current shunt resistor that we don't want to fry. My thought is to put a pair of big diodes across it to limit the current. The associated ADC will be maybe +-250 mV full scale, so we don't want the diodes to conduct much current there.

A 3 amp PN power diode might work, maybe starting from 0.6v at 1 mA and declining 60 mV per decade of current. A pair of TVS zeners, used in their forward directions might work; we have lots of them in stock.

So, I wonder if some doping magic makes a zener diode have higher forward drop if it's a higher voltage zener?

Easier to ask than to pull a bunch of parts from stock and test them. I'm feeling lazy today.

A shorted bridge rectifier would give me two diode drops. The ones in stock are gigantic.

Reply to
john larkin
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Depending on the accuracy required, you might want either a pair of antiparallel PN diodes, or else a bridge with a little bit of bias applied to keep the diodes back-biased.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

Or perhaps BJT wired as transdiode (E to C+B) should have low leakage at low voltage? Two inverse parallel for both directions.

Reply to
piglet

low leakage in reverse, but in forward, maybe

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Reply to
Lasse Langwadt

The current is limited by maximum collector current rating not base current.

E-B junction is far better than many PN diodes.

Reply to
piglet

I guess the question is, given a hunky diode, rated 2 amps or so, does it behave, forward-direction, exponentially all the way down to nanoamps?

Reply to
john larkin

the curves show a few 1A diodes down to 1nA

Reply to
Lasse Langwadt

I'm setting up to test some power diodes and TVS diodes, at 2 amps and

10 uA. 10 uA at 0.3v and about 1 volt at 2 amps would be good enough to protect my 1R shunt resistor and ADC if some yahoo applies 2 amps to my gadget.

Temperature matters too, of course, but I can adjust for that.

The exponential thing doesn't work at high currents, when diodes get ohmic.

Reply to
john larkin

If you want really quite low leakage, you could put two pairs of antiparallel diodes in series, and use an op-amp to drive the voltage across one of the pairs of diodes to about zero, by driving the midpoint of the diode string to the same voltage as one of the ends.

Reply to
Chris Jones

Neat idea! Thanks

Reply to
piglet

I tested a bunch of diodes.

A big conventional PN rectifier diode, SMB package, rated 3 amps, 200 volts, measures 0.76v at 2 amps and 0.37 at 10 uA.

The 600v version is 0.94 and 0.37 respectively.

A 28 volt TVS is 0.91v at 2 amps and 0.45 at 10 uA. The TVS is either a very small junction, or doped such as to conduct less. It does get pretty hot at 2 amps.

I should X-ray them.

A pair of antiparallel diodes, any of these, will protect my 1-ohm shunt and have no serious affect on my 250 mV ADC.

Reply to
john larkin

Sorry if I created confusion, I first heard the term “transdiode “ for the E to CB connection back in the 1960s and assumed it was well known. There are other possible permutations of using a bipolar transistor as a diode each with different characteristics.

Reply to
piglet

Too wimpy. Fault currents of 2A were mentioned.

I hadn’t thought about transdiode forward current being super low—that’s a good candidate for the bag of tricks, thanks.

Of course the beta tanks at very low V_CE, so you’d basically just get the forward conduction of the B-E diode.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

The big diode is hogging the current.

Reply to
john larkin

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