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4-20mA to adjustable set point relay

ACT56

New Member
Why does this not work?
I can adjust the pot and the relay will energize. The 4-20mA will not energize the relay even if I set the pot very close to the energize point.
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gophert

Well-Known Member
Most Helpful Member
You will see 5v across Ron's 250 ohm resistor at 20mA input. So that give 2.5v into the op amp. If you eliminate the wto 100k voltage divider, and connect the (+) op amp input to the 4-20mA source, then you will get a 5v dynamic range.

if you change Ron's resistor to 300 ohms, you will get 6v range at 20mA to match the 6.2v zener on the (-) op amp input.

Also, R6 is way too big. That can be cranked down to 10 to 1k ohms. No current flows through that once the gate is charged (and a 100k resistor will take a relatively long time to charge the gate.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Also, R6 is way too big. That can be cranked down to 10 to 1k ohms. No current flows through that once the gate is charged (and a 100k resistor will take a relatively long time to charge the gate.
I think the intention of that is a voltage divider; 12V is a reasonable gate voltage, 24V is excessive and rather above the limit for most devices.

Also, without the high value input resistors, there would be no positive feedback to give hysteresis. A single 47K from input to pin 3 in place of the two 100k resistors would retain that while giving the larger input range.
 

SPDCHK

Member
Just my 2 cents here ...

24V is excessive and rather above the limit for most devices.
24V is normal for control and instrumentation in the industry. That's the de facto standard for all control systems, together with 4..20mA.
I agree for normal business and domestic use, 24V is not quite the standard, vs. 3V3, 5V0, 9V0 or 12V0.

I assume the OP needs this done in some control environment, hence the 24V
 

rjenkinsgb

Well-Known Member
Most Helpful Member
I'm not referring to the 24V supply.

I was commenting on the idea of changing the FET gate resistor R6 to a low value, which would have put excessive voltage on the gate.
Most FETS have a gate-source limit around 18 to 20 volts, sometimes rather lower.
 

gophert

Well-Known Member
Most Helpful Member
I'm not referring to the 24V supply.

I was commenting on the idea of changing the FET gate resistor R6 to a low value, which would have put excessive voltage on the gate.
Most FETS have a gate-source limit around 18 to 20 volts, sometimes rather lower.

Or he could just put a voltage regulator on his references and op amp (or just his op amp).

Also, why is he using 6v coil on his 24v circuit?
 

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