Rebuilding an Electronic Load


Several years ago I built a simple 5A electronic load. It was very basic just an NPN power transistor and some components around it. It was built into the case of a PC
power supply using the original heatsinks. To operate it two external power supplies and one external DMM were needed. One variable power supply to supply base
current to the transistor and the other a 12V supply to power the fan and activate the relay to enable the transistor. The DMM was needed to monitor the current via the
voltage across the 1 ohm emitter resistor. It had not been used a lot since it was built but when I recently had to use it to verify a DC supply I realized how user unfriendly
it was with all the external devices needed and one thing I had not thought about before was the obvious that the monitored current contained also the base current which
could be over 500 mA. So a major rebuild had to be done. This time using a big MOSFET power transistor and a 12V powered control unit with a ten turn pot to set
current and two digital metetrs for DUT voltage and current.

The parts for the Main Unit . They are only the MOSFET, current sensing resistor and a fan.



The MOSFET and heatsink.



The 0.1 ohm resistor and heatsink.


These are the parts for the Control Unit. It will be attached on top of the Main Unit with screws.
The silicone mess on the rear of the case is because it is reused from another old project which left the large holes there.  A piece of aluminum sheet has
been glued to the back with silicone to cover all the holes.


The finished Control Unit from the front ....


.... and the inside.

In operation and loading the power supply with 5A @ 15V.


Schematic here


Email me with comments. /Håkan


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