Lately I had to repair a power supply and when I was done I wanted to stress test it to see whether it could provide the rated current. I used some automotive light bulbs as power resistors but it would be nice to have an electronic load.
However these things are expensive. Even the entry level 100-150W ones cost more than $300. Ok, I won’t have to remortgage the house to buy one, but I don’t want to spend 300 bucks just to test the occasional PSU. And I am not interested in battery drain curves either.
So what other uses for an electronic load are there? Should I spend the money on the better multimeter or some components instead?
I always used some big ass wirewound power resistors - I had about six 8 ohm, 100w ones that were good for various combinations, especially for audio loads. If I really needed more capacity, I’d put them in a pan of water…which would sometimes boil. They were aluminum core and very low inductance.
Not sure you need anything more complex unless you must have infinite and precise adjustability of load.
Me too. If you look around a lot you can find some old surplus ones nice and cheap. They might look like something out of a 1940’s Frankenstein movie but they’ll work. Get a variety and you can parallel them to make different loads.
There’s nothing wrong with automobile headlights either. I’ve used those before too.
The heating element for an electric hot water heater can make a good dummy load for testing high power audio amplifiers as well as other devices where you need to sink a lot of power.
Thanks for the replies. I should then spend my money on a better multimeter. I am embarrased to say that, but I have only one multimeter and it is the kind they sell for 10 bucks at hardware stores.
I like that bucket of oil dummy load. I will make a similar one to test car batteries.
In general, I think things like an electronic load are very low on the list for electronics hobbyists and experimenters. In a lab or industrial development situation, it might be critical to be able to apply a very specific, controlled load to a power output. Short of that (no pun intended) bulbs, heating elements and power resistors will probably be adequate.
A good meter, and a good adjustable power supply, are far more important, IMHO.
You’re lucky that a pretty good multimeter, with things like transistor testing and capacitance measurement and a plug-in heat probe, can be had for $50-100. My first “real” meter was a Fluke 8000-something, which handled a basic range of DCV, ACV and resistance, for about $300.
There’s no doubt Fluke meters are built like a brick outhouse but I am not convinced they worth their money for the hobbyist. There are cheaper meters out there with far more features than the entry level flukes.
At the time, it was Fluke, Simpson (which still focused on analog meters) or nothing. It was a huge jump in my development abilities to go from a junky Radio Shack meter to the Fluke. Still have it, although I need to find a new LCD for it someday.
And I still use my RS meter in the garage, and a $15 one for almost everything else. How time flies.
I still use my cheapo meter because the vast majority of times I measure stuff like continuity or polarity, or test a diode and I don’t see how a $300+ Fluke or Agilent meter is going to perform better at these tasks.
Yet there are people on the internet and Youtube who say that your cheap multimeter will explode in your face and space aliens will analy probe you forever.
Yeah, it will probably explode if I go climb up the pole and stick it to the distribution transformer down the corner of the street. This ain’t gonna happen when I troubleshoot my USB powered LED light though.
For quick-n-dirty loads, I have used cheap electric heaters (with integral fans) and incandescent light bulbs, including automobile headlights. Each has pros and cons. An incandescent light bulb is cheap and readily available, but it is also a highly non-linear load, and has a mean inrush current. Big 'ol resistive elements, such as a water heater element in a bucket of water, are pretty nice. But keep in mind that any resistive load, especially wire-wound ones, may have a lot of inductance.
A couple years ago we purchased a couple Chroma 63804 electronic loads. Not only are they programmable, but there are different modes of operation (constant resistance, constant current, constant voltage, and constant power).
For home use I also use a $10 handheld DMM. But at work we use Fluke 87 III meters. (We also use bench top DMMs too, obviously.) They’re not cheap, but they’re safe & reliable, and safety is something of a concern in an R&D lab. The Fluke 87s, for example, are rated for up to 1000 V according to CAT III (IEC 1010) specs. I don’t think any elcheapo meter – such as the one I use at home – is safety certified.
Also keep in mind that the more expensive handheld DMMs measure true RMS when measuring AC voltage.
Aautomotive H/Ac fan resistor might be helpful. You would need a fan to cool it though.
And you should be able to jump some of the resistor and maybe run a couple three in series??
I use wirewound resistors too, in this case, from old electronic equipment, with a very wide range of values, everything from a fraction of an ohm to 100 k ohms or more and 1-50 watts. Also, you can overload them to quite a significant degree (~5x rated power with a fan aimed at them); they will start to crack and eventually fall apart but that doesn’t matter since I have so many and I rarely use resistors above 5 watts in projects (others suggested putting them in water but that won’t work when you are testing a 300-400 volt PFC circuit). To test the power supply’s transient (load step) response, I just use a MOSFET fed by a square wave to switch the load at around 1-10 Hz.
When I had to test a 600 watt (12 volt, 50 amp) power supply I built, I just used a long length of magnet wire in a bucket of water as a load, adjusting the length to get the desired current. Note that copper wire has a temperature coefficient of about 0.39% per degree C, about an order of magnitude higher than nichrome, so this needs to be considered, even for wirewound resistors if they get so hot they start to glow red hot (I just put another resistor in if the current drops too much, but I usually test with a slight overload to start, say 120% of rated continuous current). For example, starting at 25C, the resistance of copper will increase by about 30% at 100C.
Also, heating elements make a good current limiter for when you work on AC mains-powered projects, so you don’t have to use a fuse (where again I am so cheap I reuse fuses from discarded electronics, and I don’t want to waste any this way) or reset the house breaker or to limit damage every time an “oops” occurs (I set several up so it limits short circuit power to 600-2.4 kW, in increments of 600 watts, with a 4 amp breaker in series with half the elements so it will trip and limit power to 1.2 kW; the voltage drop when loaded (60 volts at 600 watts on full power) provides a good test of a power supply to handle undervoltage, I add a transformer to boost voltage if necessary). Similarly, I use wirewound resistors when designing/testing the primary side of a SMPS (never, ever just connect direct rectified mains voltage to a circuit being designed).
