Author Topic: FIXED : Keithley 199 ohms readings never settle  (Read 2730 times)

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Online Kleinstein

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Re: Keithley 199 ohms readings never settle
« Reply #50 on: March 20, 2019, 06:44:57 am »
PN4392 are discontinued and hard to find, but Digikey had PN4391s in stock for a dollar apiece.

Specs are quite similar, except for IDss and Vgs(off) which are doubled.
I figured the higher Vgs(off) would be OK since the LM339 (U2 pin13) sends -14.8V when the transistor should be off, well above worst case Vgs(off) of -10 V.


The gate voltage would be relative to the signal voltage. However it looks like there are no voltages outside +-5 V expected here - so the PM4391 should be OK. Also the -10 V V_of are only the extreme values.

One might still consider replacing the 2N3904 with something a little more powerful: this transistor is for protection and probably already failed the first time causing the broken Q11. AFAIR the original transistor was a little high current. Still nothing to do in a hurry.
 
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