Testing A Cheap Bench Power Supply

By Ben Everard. Posted

Supplying power to your projects sounds like a pretty simple task, but can be surprisingly complex. The first and most important thing is how much power you need. If you’re dealing with simple electronics, just a few volts and a few hundred milliamps may be enough, and if it is, you may find that the built-in regulator on your microcontroller can supply enough current (which probably comes via USB). However, some projects need a little more oomph.

There’s a myriad of options for getting power into your projects, including batteries and ‘wall wart’ style plugs – these are all great when you’ve got your project working, but for prototyping they can leave a little to be desired. That’s where the bench power supply comes in. These are specifically for experimenting with bits of your project, rather than running the final thing.

Up to 15 V at up to 2 A

We got a YIHUA 1502DD Mini Laboratory Power Supply for £32.16 from JZLTool Store on AliExpress. It can output up to 15 V at up to 2 A. There are three connections on the front: positive, neutral, and ground. The last two aren’t interchangeable. The neutral connects back to the power supply, so should be used for completing the circuit. The ground should be connected through to the mains ground that the power supply is plugged into. However, as we don’t have a mains ground on the supplied plug, this connection doesn’t make sense. Upon opening it up, we found that it was connected to the metal chassis (as you’d expect); however, as the metal chassis wasn’t grounded, it’s left floating.

This isn’t a huge problem for many basic uses, but it’s infuriating to have a connection that doesn’t work, and would cause problems if you were trying to use it. This does hint at a bigger problem – that the power supply isn’t grounded. It’s not essential that any mains-powered device is grounded. However, if it isn’t, it needs to be properly protected to prevent accidental electrocution (this isn’t just good practice – it’s the law that all items sold in the UK do). What we have here is a metal case, with no additional insulation, that’s not grounded. It’s not dangerous now, but if anything were to go wrong inside that pulled a live wire loose, it would be easy for the whole box to become live.

"As a rough rule of thumb, the more you're interacting with the real world, the more power you're going to need"

This is easy to fix – a proper three-core flex could be added, and the box grounded, but it’s not the sort of thing a manufacturer should expect a user to do. Let’s get back to looking at the other features. As with most bench power supplies, there are readouts showing the voltage and current being supplied. These are shown to two decimal places, but they don’t seem to be anywhere near this accurate. This isn’t a huge problem, as we’d recommend checking bench power supply readings using a multimeter anyway.

Perhaps the biggest advantage of a bench power supply over a regular power supply is the ability to run at a constant current as opposed to a constant voltage. In this setup, you set a voltage and a maximum current. The power supply will provide power at this voltage unless the current threshold is reached. At this point, constant current mode kicks in and it adjusts the voltage so that too much current is never supplied.

Our bench power supply does have this, but the lowest current it can limit to is 0.6 A (it’s adjustable up to 2 A). This will help with things like not burning out motors, and not blowing the power supply, but it’s far too high to protect delicate electronics components.

Cross-border conundrums

This power supply shows the problems of buying non-reputable products from overseas. Anything sold in the UK or EU is required by law to follow high-standard safety protocols (other countries vary).

We want to be clear here – the problem isn’t that it’s made in China, since much high-quality hardware is manufactured there. The problem is that it’s bargain-basement hardware that’s not associated with a reputable brand, sold across borders in a way that dodges regulatory compliance. In many cases, this isn’t a problem – a box of LEDs may or may not be good and accurate to their description, but they’re unlikely to cause a safety hazard in prototyped electronics. However, in safety-critical hardware (and anything mains-powered is safety critical), you either need to be confident enough in your own abilities to render something safe, or buy from a supplier that you’re confident is safe.

This power supply has absolutely no bells or whistles. There’s a single channel (it can only output one voltage at a time), there’s no ability to control or monitor it remotely, or other features that more expensive power supplies offer. If it were simply properly earthed then we’d be happy to recommend it as an entry-level power supply. However, as it stands, we simply can’t recommend a product that potentially puts users at risk.

Beware of the fake earth connections

More articles from HackSpace magazine magazine