Hello friends,

I’m planning to build an adjustable constant current electronic load. For that, I’m following the circuit diagram shown below. To build this circuit, I’m testing it part by part to check the output. But the problem is — I can’t figure out or calculate what the output voltage of this circuit should be. When I built the circuit practically, I’m getting an output voltage of around 0.666 volts. Can anyone help me calculate what the correct output voltage should be?


This is the circuit that I’m using as a reference to build mine.

 

There is the 2.5V reference voltage at the cathode of the TL431. You have a simple voltage divider R2,R3 with 2.5V across it, so Vout will be 2.5*R3/(R2+R3) providing insignificant current is drawn from the Vout terminal.

 

when TL431 has cathode shorted to Vadj input like your circuit shows, voltage across TL431 is 2.5V. this is the lowest voltage you can get from it.

Ir1=(12V-2.5V)/R1 = 9.5V/10k = 0.95mA and this is more than 0.5mA which is minimum current for TL431 to regulate so R1 value is acceptable if load (R2 and R3) does not draw much. current through those is 2.5V/(22k+4.7k)= 0.093mA
so TL431 current is really 0.95mA - 0.093mA = 0.857mA this is more than minimum 0.5mA so circuit is ok as TL431 is in within working limits.

now that we know voltage is 2.5V you can apply voltage divider equation to find out output voltage.

 

Below is the LTspice sim of you circuit showing the voltages (assuming the resistor squiggle mark in front of the Ω mark in your drawing is a k).

Your measured voltage suggests there's an error in your circuit.

 

potentiometer is 4.7k so it cannot be adjusted to 16k. with component values as shown in schematics with opamp, wiper of the 4.7k can be only in range 0V-0.44V. if the potentiometer is damaged (burned spot) then resistance will appear larger and voltage like 1.052V are possible.

since the potentiometer voltage is 0-0.44V and same voltage is across 0.1Ohm sense resistor, current range for the output is 0-4.4Amp.

btw. for this to work OpAmp must have inputs that are rail to rail ... or... negative supply for OpAmp needs the be negative (not 0V).
note, check your OpAmp datasheet to see if it is sensitive to ESD. it may be blown if not handled correctly.

 

This is the adjustable constant current load I want to build. My question is whether the ground connected to the MOSFET's source is different from the ground used by the op-amp and the TL431 IC?

 

Below is the LTspice sim of you circuit showing the voltages (assuming the resistor squiggle mark in front of the Ω mark in your drawing is a k).

Your measured voltage suggests there's an error in your circuit.

View attachment 354055

Thanks Everyone. The problem has been solved. You were right—Previously I had built the circuit on a Veroboard, and maybe there was a loose connection, so I wasn’t getting the correct output voltage. Now I have made the circuit on a perf board, and I am getting the calculated output you provided.
BTW! What version of LTspice are you using?

 

My question is whether the ground connected to the MOSFET's source is different from the ground used by the op-amp and the TL431 IC?

they need to be the same. OpAmp is checking voltage across R9 (input+) and comparing it with setpoint (R3).
but since R3 can go to zero, input of the OpAmp also need to be able to go to zero. Some OpAmps cannot do that. In that case negative supply terminal of the OpAmp would need to be negative such as Vss = -5V.

 

What version of LTspice are you using?

I'm using the old IV version.
The new version has some changed features that I don't like.
(For example with the old version I could switch back and forth between the AC and Transient analysis, which is handy when checking feedback loop stability, and it would remember my settings. The new version doesn't.)

 

Hello friends,






I’ve almost completed the soldering work for my Adjustable Constant Current Load project. But now, while testing the circuit, I’m facing a few issues.
When I measure the voltage with respect to ground, the output pin (pin-1) of the OP-AMP is showing 10.25V. The non-inverting input (pin-3) is showing 0.34V, which I think is fine.



But the problem is — even though the OP-AMP output (pin-1) is giving 10.25V to the gate of the MOSFET, when I measure the voltage between gate and ground, I’m getting only about 0.19V.
Here’s what I’m observing:

• Across R7 (1kΩ resistor), there is a voltage drop of around 10.06V
• Across R8 (4.7kΩ resistor), the voltage drop is only 0.19V,
  which is also the voltage reaching the gate of the MOSFET.

As a result, when I apply voltage between the Drain of the MOSFET and Ground, no current flows through the MOSFET. So, across the R9 (0.1Ω current sense resistor), there is no voltage drop either.
My question is: what could be the problem here?How to solve this problem?
And if possible, can someone simulate this circuit in LTspice and show the result?

 

Hello,

Your measurements show a short between gate and source of the mosfet.
Look for solder spots on the components involved.
Also check the gate source of the mosfet.

Bertus