My 20 years old variable power supply is beginning to have some imprecision in measuring voltage with his analogical meter. Then I decided to add a digital meter, with which I can know exactly the voltage supplied. Since these digital meters are very cheap this is a good solution for many devices other than power supplies.

Step 1: My Old Friend

This device has been my first electronic instrument, together with this multimeter you usually see in my tutorials. For this reason I don't want to change or dismantle it, and since at low voltages it's not anymore very accurate (it seems that needle sometimes conks out) as you can see from the picture, the additional feature added with a digital meter will be essential.

Step 2: The Meter

This in not an ordinary LED display, although his external appearance is the same, indeed it measures voltage from 3.3 to 30 volt and it shows it on the three digits panel. You can easily find one of these voltage mini panel meters on eBay, they are really cheap, and you can buy one red, yellow, blue or green.
If you prefer reading low voltage values from 0 to 3 V too, you can buy one of these panel meters which have a wire for power supply and a wire to measure voltage.
Since I want to glue the display under the analogical meter, I cut out the frame drilled stands.

Step 3: Open and Drill

I opened the psu, I cleaned it, I also removed some rust from the top front surface.
I drilled by hand a little hole in the right position, just under the voltage authentic meter, then I enlarged it to reach a diameter where to insert the two wires.

Step 4: Secure Wires

I then located the best points where to solder the meter wires. These should be directly connected to the external red and black terminals.
I secured the wires twisting them around the metal pins or connections between components, so to solder them more handy.

Step 5: Soldering

Now you can easily solder the wires ends so that you will not have to worry about the contacts loosen.
Pay attention to not touch the other cables and components with your solder, you have to choose a good location and maybe moving stuff to make space.

Step 6: Glue

I glued the green display with hot glue, and I can remove it if I wish. If you use hot glue it will almost for sure isolate backside contacts of the display from the metal case.
Connect the psu to wall plug, beware to not touch anything inside and check that display works well. If everything is ok you can close the metal case.

Step 7: Check Accuracy

With a multimeter check the accuracy of the new digital voltage meter. As you see from pictures it's very precise from 3.30 volt to 29.9 volt, undoubtedly more accurate than any analogical meter.
My power supply is revived now, what a great pleasure! ;-)

[UPDATE] I also tried to glue the digital display just behind the analogical meter... I was wondering if the plastic of that meter was transparent enough, but actually it's opaque and digits are not been visible in transparency :-|
<p>You could always move the meter. Fill the hole with a black plastic plug. This is a good way to get more life out of an old friend!</p>
<p>you are right, I'll probably do that...</p>
<p>I made it, but it didn't work... plastic layer is opaque</p>
<p>nice one.</p>
Notes about the meter you added would be, there are two wire and 3 wire versions of it. Two wire is supplied by the voltage its monitoring and cant read below 2.5 volts as the display shuts off about there. (I know, they say 3.3v to 30v). The 3 wire version requires a separate power supply, but will read down to 0v. Another note, check accuracy before gluing it down. They have a calibration pot on the back... You glue that with hot melt and calibration is done. A few weeks ago i mounted 3 of these along with 3 buck converters, switches, and 10 turn pots in a frame for breadboard use. Love the displays! Cheap too. Ordered from Hong Kong, they cost under 3 bux a unit. MPJA is 5 bux ea. Nice tutorial for those who needed a nights project!
<p>That's really interesting and useful! Thanks very much!</p>
<p>I like your Instructable and plan to do it myself. Thanks.</p>
<p>good :-)</p>

About This Instructable




Bio: I'm an Italian freelance structural engineer, graphic designer and photographer. I'm also investigating electronics, robotics and science in general. I enjoy hacking and ... More »
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