Adjustable DC load

The Adjustable DC load is on the left, I use it to test power supply circuits such as the one on the right. The question with any power supply is always: 1) what is the internal resistance, 2) what is the ripple voltage, 3) what happens when we gradually increase the current, i.e. what part gets warm in the power supply 4) does the power supply limit the current at some point, or, will it trip a fuse, and finally, 5) how does a power supply react to a short circuit?

The adjustable load design is inspired by The idea is that you set the pot-meter, the first part of the opamp is a voltage follower, next you take half of that voltage and it goes in the second opamp which has a negative feedback of the voltage over a 1 ohm resistor at the source of a N channel power MOSFET. Effectively the both opamps will control the N channel MOSFET such that the current running from Drain to Source is proportional to the pot. meter setting.

Any IRF N channel power MOSFET will do, the used IRF840 can take up to 8 Amps  as long as you keep the junction temperature under 150C. Temperature is more you problem than Amps. With the IRF840 and the used heatsink you can continuously dissipate 50W, in that case the heatsink becomes something like 90C. If you want to you can dissipate 100W shortly, but at some point the MOSFET will give up. You should not dissipate more than 125W into a IRF840 at 25C. The alternative is to also put a separate resistor above the drain so that you can use the adjustable DC load for higher voltages. A drain resistor of 15 Ohm could be handy for voltages up to 60V, 5 Ohm would be helpful up to 30V, etc.

A separate PC fan  will increase the efficiency of the heatsink and the range for the DC load, you can put several MOSFETs and resistors and PC fans on different dummy load to even further increase the range of the adjustable load. Imagine what it will look like when you try to dissipate 1KW at 60 Volt, it should be a lot of plumbing.

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