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Input protection, (C-B current of transistor)

I=92ve been tasked with adding over-voltage input protection to an existing instrument amp circuit (AD620) The Inamp looks at a signal from a parallel tuned LC circuit. R of about 10 ohms and Q ~100, so a source impedance of ~100k ohm (R*Q^2). There is 5 Meg ohm to ground on each input line to the Inamp for bias current.

I was going to use the C-B junction of a transistor (2n3094) from the inputs to power rails, as suggested by Analog Devices. (Apparently this will have less leakage current than a 1N4148.) They suggest a maximum current for the 2N3904 C-B junction of 20mA. Any idea where this number comes from?

I=92m protecting against ~60 volts of overvoltage and so will use a 3k ohm series resistor.

Thanks, George H

Reply to
George Herold
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It's got to be a base current limit. The one spec that I checked didn't have one, though someone might have extrapolated the 200mA Ic limit with the standard

1/10th Ib/Ic. Yeah, a WAG.
Reply to
cassiope

a

Thanks, I figured something like that. I'm 'allowed' to push as much current through the BC junction as through the BE?

George H.

Reply to
George Herold

I'd say the 20mA is just a safe ball park figure, not a precisely calculated limit. My datasheet for the 2N3904 does not give an absolute maximum base current rating. It's probably around

50-100mA.
Reply to
Pimpom

If you connect BE together, it should take the full 200 mA that the datasheet suggests for Ic(continuous); that would be the safe limit of the connection wiring. Base current goes through Rbb, so there might be some non-ideal-diode behavior in using it for a clamp.

Reply to
whit3rd

Reply to
Nemo

The BAV199 series of dual diodes leak at pA levels, several orders of mag below regular signal diodes. BAV199DW has two series pairs in one package.

A nice current limiter is back-to-back LND250 depletion mosfets.

This can stand +-250 volts:

ftp://jjlarkin.lmi.net/22S490B_ch12.pdf

John

Reply to
John Larkin

Oh, +/- 15V supply rails. not enough reverse bias voltage in the Vbe junction.

George H.

Reply to
George Herold

e

Opps sorry I misunderstood your post. BE together? I'll have to try it. Thanks,

George H.

Reply to
George Herold

shure, revers mode. This is well known. The chfe isn't big in this configuration but most of the current will flow through emitter to collector as it should be.

Marte

Reply to
Marte Schwarz

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