# Constant Current Load op-amp ref voltage limit

I am trying to build a constant current load circuit described in DIY-Constant-Current-load

I wanted a 20mA CC load, so I chose 4V at R2-pin2 and 200ohm for R1. After constructing the circuit, i found out Vref (LM358-pin 2) is at maximum of 1.8v instead of 4v, what is the reason behind this?

• We recommend that you embed the image in your post so we don't all have to follow links to understand your question and so that the question makes sense if the link dies. The Instructables articles are generally way too long for most of us to trawl through trying to find the relevant details. May 2 '18 at 21:43

I wanted a 20mA CC load, so I chose 4V at R2-pin2 and 200ohm for R2

This is a sensible start but your circuit shows R2 as the potentiometer. So ignoring that, the real problem you have is the LM358 - it can only work with an input range from 0 volts to Vcc - 2 volts so immediately you have a problem with the design. The LM358 won't operate with the parameters you have chosen.

The next potential problem is the MOSFET. If you want 4 volts at the source (across the 200 ohm resistor) then, to activate the MOSFET to any reasonable degree you need to gate to be at least 3 volts higher and both your power supply and LM358 are incapable of acheiving that (7 volts on the gate).

The MOSFET data sheet says that its VGS(th) is 2 volts and this means that if gate to source was 2 volts it would guarantee at least 250 uA and clearly this isn't enough.

If you chose R2 to be 100 ohms and therefore designed for 2 volts across it, the LM358 would have to produce an output level of 5 volts and, on a 5 volt supply this won't begin to happen.

If you read the data sheet for the LM358 it might be able to scrape an output level of 3 volts and that leaves you very little headroom for anything else. Consider using either a rail-to-rail opamp or a BJT instead of the MOSFET.

• Thank you for the response. I made a typo, it was 200ohm for R1. I ended up setting Vref at 1V and chose 50ohm for R1 and the circuit works as intended. One question though, how can I tell from the datasheet that LM358 only operates with input range 0 to 2V? From the example datasheet from TI, I am not able to find that reference: ti.com/lit/ds/symlink/lm158-n.pdf May 2 '18 at 19:51
• Look for input common mode voltage range. May 2 '18 at 20:21