Adding a voltmeter bezel to a Bench Power Supply

Harry

In this post, we explain how we added a digital voltmeter to a case.

Bezels are not only useful for adding displays to plastic boxes – although that is a very common use for our bezels. In this example, we are adding a low-cost 3-digit voltmeter, readily available from a number of online stores, to a bench PSU. We show here a pretty standard 0-15V 1A variable power supply. It is, in fact, a terrific little power supply – for projects up to 1A.

The front of the power supply features a 0-15V knob, but when measuring the actual voltage with a multimeter, the output voltage didn’t quite match up to the dial setting of the knob. And although there are trimmers on the internal PSU PCB (for adjusting things), we still wanted a more accurate way to determine the precise output voltage. So, instead if fiddling with the trimmers, we decided to add a low-cost digital voltmeter.

In this post, we focus on the hardware aspects of adding a voltmeter with its matching bezel. We have covered some of the electronic aspects at the bottom of this article, only because that information might be of useful for you.

We are only discussing the addition of a Bezel to this case as an example of using a bezel. We are not discussing the internal electrical or electronics components, or any other similar aspect. But, because we just happen to be using a bench PSU for this example, we wanted to raise an  IMPORTANT WARNING. Specifically, this particular bench power supply, like many others, uses mains voltage internally. so appropriate safety steps are essential. If you are not sufficiently experienced and qualified to open up a mains operated device safely, then do not do it. Rather find someone who is qualified, or go buy a new ready-built digital power supply. A qualified engineer can also do a proper PAT test on the final project, for your safety.

The challenge

We faced a common challenge: how do we add a display/instrument to a good-looking commercially manufactured box/panel without ruining its appearance by making an untidy hole for the display and then further ruining the look by adding visible mounting screws?

It is was made more challenging in this case because the panel was made with pretty-tough 2mm steel, so making a clean cut-out was going to be more difficult.

The solution

Of course, you can buy a ready-make panel DVM that, with carefully cutting, will slot into the case. But if you don’t have much room behind the panel you may prefer to add one of those small 0.36″ digital voltmeter modules. These come without a mounting bezel, but have 2 screw holes in its PCB. But that the screw holes are very close to the display so any visible screw heads will be pretty close to the 7-segment display. Not the most pleasing result.

The easiest solution was a voltmeter bezel. This bezel:

  1. Mounts the voltmeter, retaining it with 2 screws, from the rear (not the front)
  2. Does aware with the need to use front panel mounting screws
  3. Most importantly, it hides the (inevitable) untidy cut-out in the front of the unit, thus retaining a clean, professional appearance.

The project steps

We used masking tape to mark out the required cut-out, so that we didn’t scratch the panel’s surface too much! Then we drilled the corners and used a junior hacksaw to cut the rectangle. It was then finished with a file to attain the correct cut-out dimensions.

front panel showing the best cut-out we could achieve

Despite our best efforts to make a “perfect rectangle”, we all know from experience a perfect cut-out is rarely the outcome!!

Removing the voltmeter from the bezel, so that we didn’t spill glue on the display, we then glued the bezel into the panel with a gel-type superglue. Finally, we re-fitted the voltmeter and wired it up.

   

The result

This was a simply project, but because of the steel case there was a bit of hard work. However, the result is that we have upgraded a traditional 1A linear bench power supply with a mechanism to more-precisely set the voltage. And the result looks neat, tidy and professional.

     

The electronic aspects

Wiring up a digital voltmeter should be straightforward for most electronic hobbyists. This particular voltmeter has two modes of operation: 2-wire and 3-wire.

2-wire

The voltmeter comes with 3 wires (power+, power-, and measurement). In 2-wire mode you connect the power+ to the measurement wire. But, the voltmeter will not operate below about 5V, and can only be operated up to about 30V. Although the upper limit was OK for this project, the lower limit was not acceptable for a variable-voltage bench power supply.

3-wire

In this case, we used an internal 12V regulator (7812) to supply the voltmeter with its operating power. This separated the measurement wire which was then connected to the output of the unit. In this mode, we can measure a wider voltage input, and our tests show the voltmeter will register from about 0.6V. Much more useful for a bench power supply! And although we don’t need it here, we could (with this particular voltmeter part) measure up to 99v.

In summary

We have shows that this bezel can make an otherwise “tatty-looking” outcome into something that looks more professional. We sell the voltmeter and bezel here. You can choose the bezel only, or the bezel with a matched voltmeter.

Again, to repeat our safety statement, we have shown this project as an example of how a bezel can make a big difference. Most electronic project work on 3.3V/5V/12V which are OK for the hobbyist. This particular project example had mains voltage internally, so we strongly state you should only ever work on such voltage if, and only if, you are qualified and sufficiently experienced to do so.
We have obscured the makers name and model number of the rather excellent power supply that we used for this project.