Like the electric meter that monitors power supply and activity to the home, the PMM was designed to constantly display the status of an electronic circuit's main D.C. voltage and current supply during the design and build stage. Although the typical Volt/ Ohm/ Ammeter is capable of the same, it cannot do both jobs at once and switching over to check the state of each is both time consuming and interruptive. This device allows the circuit builder to employ the V/O/A meter more effectively for measuring a project's circuit activities instead of the supply status.
The advantages of this handy and portable instrumentation are that it has a:
Step 1: Safety First
Striking tools may be used, eye protection is recommended.
Step 2: Gather Some Parts
Before proceeding I like to make sure I have all the necessary items for the build, the only thing I spent any real money on was $7 for the panel meter itself, the rest of the components: Black Cherry wood for the side, ABS plastic for the enclosure and front panel, the speaker push tab connector block, screws, etc. were all surplus shop items I had laying about. At this point I'm pretty confident about how I will arrange the components, and as shown in this image I've already constructed the chassis shell and mounted the display in the front panel, I had set the slope at a 15 degree angle, comfortable viewing for me as it sits on my benchtop.
Step 3: Marking Out The Enclosure
It is a good idea to clearly identify the areas that need tooling done, as during operations it is easy to put a hole anywhere but the proper place, so I make things loud and bold. Laying down masking tape keeps the work face clean and provides an excellent background for the markings.
Step 4: Good Machining Practices
With the rear panel marked out, I chain- drilled a series of small holes to facilitate removing the core of the rectangle, and squared it up with a sharp chisel to the lines, then finishing the work using a file. The ABS plastic- .094" [2.4mm] thick- cuts easily with a few light taps of the hammer on the chisel, and the same method was used for the front panel display cutout as well. The mounting hole for the female power connector was purposely drilled undersize and opened to final diameter with a hand- held taper reamer so a close tolerance could be obtained as there isn't much room for error on the fitment. Attachment holes were then drilled and countersunk as the last step in machining the part.
Step 5: The Enclosure's Final Construction Steps
Once again, I used a heat gun to soften the ABS and also used my shop made brake to put a nice square, crisp bend in the enclosure cover's rear panel. I then positioned it on the chassis, drilled pilot holes for small flathead fastening screws, and ran them in. Having cut the cover overly long ensured that I could trim any overhang back. I let this extra flow towards the front, then marked and cut off the excess stock as the last operation.
Step 6: Wire It Up
I thought it a good idea to snub any transient EMF in the unit, so I applied ferrite cores in the traditional manner to the meter board's wiring plug leads. This should also prove beneficial to any downstream circuitry. Wiring is so straightforward that I didn't even bother with my own schematic, using the information supplied by the meter's manufacturer was sufficient. The auxiliary connector was simply wired in parallel to the push tabs + and - terminals on the input side. One thing to be aware of is that the ground (-) is split between input and output, and the positive side is railed together- odd, but that's how it is.
Step 7: Calibration Of Meter Functions
Voltmeter adjustment: I used my Harbor Freight meters as a standard since they both pretty much agreed on the measured voltage, I then calibrated the voltmeter function to match. Rotating the potentiometer clockwise decreased the reading, and after some fiddling I got the result I wanted.
Ammeter adjustment: Wanting a consistent load, and again using the average of both H.F. meters, I used an old hair dryer nichrome heater element powered with a wall wart, calling it my "load tower". I then adjusted the potentiometer as above until I got the number I wanted, and called it good.
I found that using a flat screwdriver with a 2mm wide tip to be the best tool for adjusting these micro- sized pots. I did not do a live adjustment, safest way was to disconnect power and make very fine tweaks until it rang up the number I wanted.
Step 8: No Skidding Folks, We're Almost Done
A cut section of a cut section of shelf liner, some double face tape and Finis!
Step 9: Parting Thoughts
The auxiliary input receptacle is intended to take it's source from the typical "wall wart", be it a transformer or switcher type.The push tabs immediately below are for conventional input hookup. Care must be used to ensure the PMM only sees direct current energy, and is not exceeding 30 volts and/ or 10 amps. I do not know if any circuit protection is employed by this meter, and hope not to find out the hard way it doesn’t.