Help selecting transistor and fet for a 4..20mA transmitter

Thread Starter

Axel_sr

Joined Feb 28, 2022
47
Hello,

I am wondering good candidates for Q1 and Q2.

1696927061066.png

Fet linearity in mind, I know that at least IRFZ24 is a good option in terms of linearity.
For Q2 I think that "any" transistor would work if the only purpose is to make sure that the fet is shut down. Probably 2N3394 fits the purpose.
Any recommendations?

br,

A
 
Last edited:

Sensacell

Joined Jun 19, 2012
3,400
The closed-loop, opamp configuration takes care of linearity, so that's not important.

Q2's job is to fail-safe clamp the current to ~ 25 mA max, you could omit this portion and it would still work.
The FET just needs to:

1) turn on enough with the available gate voltage (~ 3V?)
2) withstand the 40 Volts max supply voltage (use a 100V part)
3) withstand the max power dissipation. (0.8 watts - worst case?)
 

Thread Starter

Axel_sr

Joined Feb 28, 2022
47
The FET just needs to:
1) turn on enough with the available gate voltage (~ 3V?)
2) withstand the 40 Volts max supply voltage (use a 100V part)
3) withstand the max power dissipation. (0.8 watts - worst case?)
Hi. Thanks for the reply. IRL540 + 3394 should work well...

br,
A
 
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