Humble Electronics #005 - Volt/Ammeter

I was building an ATX bench top power supply recently, until I blew up the power supply. I'll post that project in the future when it can be completed. One facet of the build was incorporating a voltage and current display. When I purchased it from Amazon, it came in a three pack with a max of 100 V/ 10A. I was sure I'd find a use for the others. (Screencap from Amazon)

 

I have this Pad - 234 Digital Analog Trainer my husband found at an estate sale. It included the manual and schematics. It has a variable +15 DC voltage supply along with other voltage supplies and a built in clock and function generator. It's not the most practical because you should only use one voltage source at a time but the variable source can sink up to 1.5 A. The other sources supply less. The function generator needs an oscilloscope to see the waveform values accurately but I have one. Besides those limitations, it's really cool but has no lab manual. It will work for at least a single project at a time. I'll make another post, or several, about it. It doesn't seem very common online.

 

 

I'm going to use one of my spare meters to measure the variable voltage supply and it will show the current as a bonus. I got the wiring diagram from Bill at DroneBot Workshop. It will use the 5V power supply on the same system to run the unit so I can measure under 5V. 

I decided on screw terminal barrel jacks for the input and output. I determined my sizes, plugged in my values and downloaded a simple box from boxes.py. I added cutouts for the screen and jacks in Xtool Creative Space (XCS) and cut it out on the laser cutter. I glued the base and the sides together and hot glued the barrels in place to the top but only pressed in the display because it had clips. I miscalculated the depth of the box and had to push the female barrel in as far as it could go. I soldered a length of wire to the display voltage and fed it out a hole in the bottom of the box. Then I pressed the lid in place. The female jack side of the box still doesn't close right but it is holding everywhere else. I'm not gluing the top down anyway and I may be able to fix it. I took a female barrel jack and attached 2 single strand wires to it and the same with a male jack. I have a little bread board I use as a power indicator and connected it to the power through the meter with the voltage knob all the way to its lowest point then raised it to 5V. The white extension assembly was something I made for the failed ATX supply. I don't really need it here because I can extend the wires on the male jack.

 

 

 

Now I can adjust the voltage without having to get out my multimeter every time. It takes a bit of time to warm up and settle the voltage but then it's nice and steady. Compared to the multimeter, the voltage is accurate to 0.2 V. 

 

I took the wires and zip tied them together to be less like spaghetti everywhere. 

 

Then clipped the tails neat. I can move the ends of the meter to any supply I need as long as I remember to power my circuit from the meter output. 

 

The meter didn't register any current so I placed a 1W 220 ohm resistor as a load and raised the voltage to 12V. It finally registered about 0.4 A but my calculations said it should be closer to 0.6 A. There are two calibration potentiometers on the back I may be able to adjust to get more accurate readings but I don't think it'll ever be sensitive enough to pick up led current. (Found out I was totally wrong on that. I just needed more LEDs)

 

So that's a look at my quick and easy enclosure for a V/A meter. I hope someone gets inspired by this and makes their own. I'm posting more in the Laser Cutter #008 post about the build and problems I encountered.

Happy Making! 

Michelle