Back in Cheap Power! (pic at right) we showed you how to build a variable voltage power supply. It was capable of producing up to 3 amps peak (and 1 amp continuously) and, if you used an old laptop power supply as part of the build, it cost you only peanuts.
Well, here we are back again with another power supply – but this one is capable of lots more current. How much then? Well, depending on how you build it, up to a short-term output of 8 amps at 12 volts, and a continuous 5 amps at around the same voltage. But is it limited to 12V output? No! You should be able to crank the output to 25V without issues.
Those specs make this a power supply that can be used a battery charger, can test headlight bulbs and other 12V items – or can be turned down in voltage to check airflow meters, MAP sensors and other engine management bits and pieces.
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To make this design really cost-effective, you’ll first need an old laptop or industrial power supply module.
Laptop power supplies are readily available in currents of about 3 amps and voltages up to about 20V. Any that has anything like these specs written on it will be fine – and because these power supplies are often thrown away when a laptop is discarded, they’re not hard to find.
Industrial power supply modules (like the one used here) are commonly available with currents up to 3 amps at 27 volts, and – because of their solid-state design – can produce higher currents at lower voltages. For example, the one shown here developed an easy 5 amps at 12V.
Note that the max voltage you’re looking for must be under 32V.
Basically, keep your eyes open for discards or cheap sale prices on these components – and the higher the spec’d max current, the better.
Note: Industrial power supplies like this one need to have an earthed 3-pin plug fitted. If you do not know what you are doing with mains power, stick to laptop-style power supplies that are pre-wired on their mains input.
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Building block
The main building block of this design is a pre-built eBay module. Available from a variety of sellers for around AUD$9 (including postage!), it’s called a “DC-DC Step Down 12A 200W Adjustable Converter Buck Module 4.5-30V to 0.8-32V”. As its name suggests, you can feed it any DC voltage up to 32V and it will turn it into a lower voltage. If the input is 32V, the output can be anything from 0.8 to 30V. (If the input is lower, so too will be the maximum output.)
(Note that the module states that it is good for 12 amps, but more on this in a moment!)
But here’s an important point. There are lots of ‘buck’ converters available on eBay but many of them are very fragile – they work for a few moments and then when overloaded or otherwise mistreated, they die. In fact, in research for this story, three other eBay buck converter designs were trialled but none proved to be up to the task. Ensure the module you buy looks exactly like the one pictured!
To the module I added an external 10-turn pot (a 50 kilo ohm unit) to allow the voltage to be easily and precisely altered, and a toggle switch to allow the output to be turned on an off (often useful when you are testing a circuit and want to quickly disconnect power to make a change). If you don’t have these parts, they’re available from electronic stores like Jaycar.
If you don’t want to make any changes to the module at all, the voltage output can be altered by using a small screwdriver on the pot (arrowed), and the module turned on and off by connecting or disconnecting power appropriately.
I also added a cheap eBay voltmeter (a 2-wire design) and mounted the lot in a kitchen-style sealed container – a very cheap but durable box.
Because builds will vary depending on what parts are available, here’s just an overview of how I did it.
Our approach
I managed to pick up a brand new ABB YSM01-PS power supply module for chickenfeed ($5!) on eBay. As shown, this module can develop up to 3 amps at 27V. Not shown is that at lower voltages it will produce more current. It’s a great starting point for a variable power supply.
The buck converter was taken from its metal box (the box adds nothing to the heatsinking)…
… and then, because I wanted to replace the on-board pot with an external one, the existing pot was crunched with a pair of pliers until it fell apart, revealing its connections. (Taking this approach is easier than unsoldering the pot and then soldering to the PCB pads.) The wires to the new pot were then soldered to these wire connections.
The new pot. Note that because the pot I had wasn’t the right value (you need 50 kilo-ohms) I added some external resistors to correct the value – if you pick a 50 kilo-ohm pot, you won’t need to do this.
A toggle switch was added and two ‘binding post’ style output terminals were used. Note that I chose to switch only the low voltage output – the mains power switching is done by turning the power supply on and off at the power point.
To monitor output voltage, I wired a low cost eBay voltmeter to the output. These 2-wire designs are very easy to connect, although note that they won’t work down to 0 volts – about 5V is as low as they go.
Testing showed that when running continuously at currents of over 5 amps, the heatsinks on the buck converter started getting quite hot. I added a salvaged 24V fan and then drove it via some dropping resistors from the power module’s output (ie the buck converter’s input). The dropping resistors slowed the fan speed, so making it quieter. Even at a slow speed, the heatsinks stay much lower in temp. Unless you want to drive high current loads continuously, you won’t need a fan. When the current load is high, always unclip the box lid to allow air to better circulate.
Here’s the completed power supply running a 50W automotive light bulb at 13.1V. Voltage regulation is good (ie output voltage doesn’t vary much when you add or disconnect the load) and the module is over-load, over-temp and short-circuited protected (although you may also want to add an external fuse).
Keep your eyes open for a few bits, and the result can be a cheap and effective bench power supply.
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