Introduction: Add a Digital Meter to Your Old Analogical Variable PSU
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 :-|