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DC Electronic Load
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KW:
Hi...

I am trying to construct a DC electronic load. The circuit diagram is attached. The opamp in the circuit is configured as a voltage follower.

In this post, the resistor (HSA50R10J) is denoted as RM, whilst the voltage across the resistor is VM. Ideally, VM should track and be identical to Vset. Hence, the load current (ILoad) is given by Vset/RM.

When TL081 is used, the current increases linearly with Vset (see first graph in "EEV Blog 2.png"). However, when Vset is greater than 0.45V, the load current starts to saturate.

When the opamp in the circuit is replaced with TL071, the current also increases linearly with Vset (see 2nd graph in "EEV Blog 2.png"). In this case, the current starts to saturate at a higher Vset, i.e. 0.6V.

Ideally, the voltage at the non-inverting terminal of a negative feedback opamp circuit should be identical to that at the inverting terminal. However, as can be deduced from the experimental results ("EEV Blog 2.png"), the voltage at the inverting terminal can no longer follow up the increasing voltage at the non-inverting terminal after Vset is greater than 0.45V (in the case of TL081) or 0.6V (in the case of TL071). Consequently, the load current starts to saturate after Vset = 0.45V or Vset = 0.6V. Kindly advise why these anomalous behavior (e.g. voltage at opamp input terminals becomes different, current starts to saturate, etc.) happened.
t1d:
Search this forum for "Jay-Diddy-B"'s electronic load threads. He does a good job of explaining e-load operations and he offers some great designs.

I have just finished building an adaptation of his design. I supplied the schematics and KiCad files.

There are other e-load designs, on this forum, if you want to search them.

https://www.eevblog.com/forum/projects/electronic-load-project-ltv171-irfp250-with-kicad-files/
ArthurDent:
I found this chart on the spec sheet for your MOSFET, which looks a lot like your graph. Try putting another IRF620 in parallel and see if that fixes the problem.

Zero999:

--- Quote from: KW on November 06, 2018, 04:05:10 pm ---Hi, I am trying to design a DC electronic load for my project. The circuit diagram is at the attachment below. Since the voltage follower configuration was used, I found that the measuring results is not as theoretical results which the gain of voltage follower is as 1. By using TL081 operational amplifier, the load current become constant at 0.45A regardless increasing of Vset. When TL081 replace by TL071, the load current become constant at 0.60A regardless increasing of Vset.  Why the current will become constant regardless increasing of Vset and the gain of voltage follower is not as close as 1? Can anyone explain it? Thank in advance.

--- End quote ---
The figures you've given in your post are 1/10 those on the graphs.

If you're getting between 4A and 5A, then it's because MOSFET isn't turning on enough. Look at figures 1 & 2 on the data sheet. The op-amp won't be able to drive the gate with the full 15V supply and you shouldn't expect more than around 4A with 12V of gate drive and drain voltage of 12V, especially at higher temperatures.
http://www.vishay.com/docs/91027/sihf620.pdf

Use a MOSFET with a lower on resistance and higher current rating or several in parallel, as mentioned above.

Are graphs are the simulated values and the figures given in your post the measured values?

If so, then it's probably oscillating and your meter is not giving you the correct readings. An additional RC circuit will be required to stabilise the circuit.

See the thread linked below for an example.
https://www.eevblog.com/forum/projects/linear-led-driver-with-mosfet/msg1796558/#msg1796558
HB9EVI:
my guess'd be oscillations as well, if the opamp is not broken.

- I don't see any decoupling caps on both opamp rails
- I don't see a compensation network

so almost certainly oscillations
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