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CIRCUIT NOTEBOOK Interesting circuit ideas which we have checked but not built and tested. Contributions will be paid for at standard rates. All submissions should include full name, address & phone number. Very simple adjustable electronic load There are many occasions when finding the correct load resistor is difficult, especially when the load will dissipate more than 10W. It is also helpful to have a variable load to gradually increase the current such as when testing a switchmode regulator, to see how it behaves across its full load range, or if you are trying to find the trip point of a fuse, PTC thermistor or circuit breaker. This elementary circuit is based on an N-channel Mosfet with a controllable gate voltage. Almost any N-channel Mosfet will work as it is not necessary to have ultra-low drainto-source resistance Rds(on). Arguably, a linear Mosfet like the BUZ11 will work best if very gradual and stable adjustments are desired. The circuit as presented uses an IRF1405, a very rugged switching Mosfet, and it works well. One critical component is the multi-turn potentiometer, which needs a high enough resistance to not exceed its power rating at the maximum expected voltage. This circuit uses a 10kW pot, but 5kW or even lower may be a better choice as long as it has a sufficient power rating. A good heatsink for the Mosfet is essential, especially if you intend to load the unit for more than a minute or so. I used a Jaycar fan type heatsink (Cat HH8570) which fits neatly across the back of a small sloping-front instrument case holding the ammeter and potentiometer. The Mosfet should be mounted directly to the heatsink with some thermal paste (not insulated) to improve heat transfer. Note that the heatsink will be at drain potential (ie, the load voltage). I included a ‘kill switch’ to close down the gate if there is any sign of Circuit Ideas Wanted siliconchip.com.au thermal runaway. In practice, this unit is very stable as an increasing temperature tends to lower the load current. The low-value resistor connected in series with the Mosfet’s source terminal is critical for stability. For a maximum current of 5A, a 5W wirewound resistor works well there. Still, for higher amperages, a higher wattage will be necessary (or use three 0.33W 5W resistors in parallel with plenty of air around them). The fuse needs to be rated no higher than the meter range, which can be as high as you wish, subject to adequate heatsinking and being careful to stop increasing the load when approaching full-scale. My need for this unit arose from having some apparently faulty plugpacks and other power supplies that I needed to test. Unless you are very sure the device under test (DUT) can handle an overload, you should gradually increase the current while watching the voltage supplied by the DUT. I received considerable help from John Clarke in designing and building this circuit. It is simple but effective. Jon Kirkwood, Castlecrag, NSW. ($80) The heatsink (above) and finished electronic load (below). Got an interesting original circuit that you have cleverly devised? We will pay good money to feature it in Circuit Notebook. We can pay you by electronic funds transfer, cheque or direct to your PayPal account. Or you can use the funds to purchase anything from the SILICON CHIP Online Store, including PCBs and components, back issues, subscriptions or whatever. Email your circuit and descriptive text to editor<at>siliconchip.com.au Australia's electronics magazine April 2022  89