Help with a current loop circuit

jenifer85 said:

Hi,

thanks for showing me that circuit!

I tried it and it's true, it is working I get 4mA to 20mA.

The transistor and the op-amp are not heating! But it's R11 that is heating now. I used a 1/4W.

Correct me if Im wrong but. In R11 there's 24V with a maximum of 20mA. So 20mA*24V = 0,48mW
If I take a resistor of 1W I should be ok?

Click to expand...

hi J,
If the dissipation is 0.48W , use a 1W

For 20mA the power dissipated in the 1k resistor is I²R or 0.02² x 1000 = 400mW. The worst-case dissipation if the transistor were fully on into a short circuit would be V²/R or 24²/1000 = 576mW. For either case a 1W resistor will be fine.

Edit: If you are thinking about designing any other circuits, you should consider simulating with a free Spice program such as LTSpice. A simulator allows you to verify that the circuit is likely to work before you build the breadboard. It often saves a lot of redesign and reworking of the breadboard circuit.

Yeah good idea to simulate a circuit before making it

I pluged 4 resistors of 4K 0.25W in parallel and they are heating I don't understand!
It should do the same thing as a 1K 1W resistor no?

I mesured that 8.5mA is going through the 4 resistors, so 8.5/4 is the current in each resistor. It should not heat normally! It's something wrong with the transistor...

jenifer85 said:

I pluged 4 resistors of 4K 0.25W in parallel and they are heating I don't understand!
It should do the same thing as a 1K 1W resistor no?

Click to expand...

Yes, but a 1W resistor dissipating 567mW is going to be pretty hot - the rating doesn't mean it runs cool, it just means it survives.