Electronic load

project: , published: by Maarten Tromp.

In EEVblog episode 102, Dave explained and built a programmable electronic load. This seemed handy and easy enoug to build one myself as well.

An electronic load is one of the handy devices to have in your electronics lab. You apply a DC voltage on the input terminals, dial in the current you want, and the electronic load mimics the correct resistor to make this current happen. When the voltage changes, the resistance is changed accordingly, so the current remains on the set value.

Shortly after this episode, several people started building similar designs. There are various threads on the EEVblog forum about their builds. After a while I decided to join them and build an electronic load as well. It seemed useful to have one around, without being too much of a hassle to make one.

The design of my electronic load is almost identical to all the other electronics loads on the forum. The power section consists of an 1Ω shunt resistor and a N-channel fet. I settled for modest specs and used a single IRF620 power mosfet. This results in a maximum current of 2A, maximum voltage of 200V and maximum power dissipation of 50W. The fet is mounted to an old CPU heatsink that sits on top of the enclosure. This should be more than enough to dissipate the generated heat.

The control circuit consists of a 10-turn potmeter to set the current, an opamp to compare the set current with the measured current, an RC filter to stop oscillation from happening and a little power supply. On the front panel are input terminals for the load, a buffered output of the shunt voltage and the potmeter to set the current. As you can see on the picture there are another terminal and a switch on the front panel as well. The plan was to have a control voltage input, but I never connected this.

As it turns out, it is indeed handy to have an electronic load around. Especially when testing power supply designs. I have 2 multimeters permanently hooked up to the electronic load; one to measure input voltage and one to measure shunt voltage. The whole system is not ground referenced, so easy to hook up. There are plenty of ideas for improvements (higher voltage, higher current, higher power, dual channels to match a dual rail power supply, panel meters on the front, constant power mode, etc), but I'm glad I built this one.