Current Sink Keeps The Smoke In

One of the most versatile tools on anyone’s work bench, at least as far as electrical projects are concerned, is a power supply. Often we build our own, but after we’ve cobbled together some banana jacks with a computer’s PSU or dead-bug soldered a LM317 voltage regulator to a wall wart, how will that power supply perform? Since it’s not desirable to use a power supply that’ll let the smoke out of everything it powers (or itself, for that matter) a constant current sink, or load, can help determine the operating limits of the power supply.

[electrobob] built this particular current sink from parts he had lying around. The theory of a constant current sink is relatively straightforward so it’s easily possible to build one from parts out of the junk drawer, provided you can find a few transistors, fuses, an op amp, and some heat sinks. The full set of schematics that [electrobob] designed can be found on his main project page. He’s also gone a step further with this build as well, since he shorted out his first prototype and destroyed some of the transistors. But, using a few extra transistors in his design also improves the safety and performance of the load, so it’s a win-win.

This constant current load also has the added feature of being able to interface with a waveform generator (an Analog Discovery, specifically) and as a result can connect and disconnect the load quickly. If you aren’t in need of an industrial-grade constant current sink and you have some spare parts lying around, this would be a great one to have around the work bench.

11 thoughts on “Current Sink Keeps The Smoke In

  1. Without selecting a proper “Linear-Qualified” current-pass MOSFET, this design in (some cases) may actually let the “Smoke OUT”. At first-glance I do not think the Vishay IRLZ24 N-MOS pass transistor used in this person’s design is “Linear Qualified”,

    Remember, most higher current capable MOSFETs are designed for switch-mode operation (they are NOT “Linear-Qualified”). In a simple example, non-linear qualified MOSFETs consist of multiple MOSFETs on the component’s internal die in parallel. So there’s little to nothing preventing one of the multiple on-die transistors from conducting before the others do as Vgs changes linearly, especially when Vgs changes slowly over time, like this person’s application.

    That said, there are switch-mode/current-mode load designs possible that would eliminate most of the parts used in this person’s design (including multiple transistors, heat-sinks, fans etc.) From memory, these switch-mode designs typically fall into topologies with coupled inductive charge-storage elements (e.g. SEPIC and/or Cuk?) Some designs even work bi-directional. But switch-mode loads are typically “noisy” compared with linear designs – so it depends on your specific application which approach works best for you.

    1. Even if you ignore that, 99% of DIY dummy loads fail pretty horribly when it comes to loading outputs driven in PWM, and those are extremely common nowadays…

    2. IRLZ24 is pretty good for linear operation. You can tell from the datasheet details (SOA, “Ease of Paralleling”)
      Hotspot is not that much of a problem since they are thermally coupled and MOSFETS tend to conduct less when they get hotter.

  2. An alternative current sink with even less parts: solder some “large” resistors in parallel and/or series, to get the needed resistance. Solder on two thick wires, and dump the resistors in a bucket of water. (only for relatively low voltages, but can sink a lot of heat for a long time, depending on the size of the bucket.)

    1. I have a large resistor bank for this purpose, but note that a resistor is not a constant current load. When testing batteries, the voltage drops, and then so does the current…

  3. The article missed the point – this is not necessarily designed to simply load a supply with some current. It is designed to be controlled by the analog discovery and monitored at the same time – so you can f**k with the supply you are testing (like loading it with a 1% duty cycle at 1 MHz) and observing the effects.
    So you can measure the dynamic characteristics of your power supply and test things like stability and noise.
    I tested a power supply with it here http://www.electrobob.com/peaktech-6225a-review/

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