I built this device for use after my isolation transformer and variac so that I can monitor the voltage and current draw in whatever devices I have hooked up, as well as to provide a convenient switch (my variac does not have one, though my isolation transformer does, but is on the floor away from the workbench), and a fuse (mainly to protect whatever equipment I am working on, as both the variac and isolation transformer have their own fuses; if the fuse is not critical, I'll leave a 20 amp fuse in there). I really thought a device like this would already be in production and readily available, but after much searching I could find nothing, so I decided to make my own. Overall I spent under $100, and couldn't be happier with the results.

All parts in the monitor are rated for at least 20 amps (except for the NEMA 5-15 plug, but this rating is somewhat artificial as the only difference between the NEMA 5-15 and 5-20 is the orientation of the neutral connector), and 12 AWG wiring is used. If a 15 amp or lower version is desired, 14 AWG may be used, and the other parts are also more readily available. A partial parts list is detailed below.

Description Price (USD) Part Number Supplier
150 VAC Analog Panel Meter (Techman) $13.50 SK1560 (MFG TP-670A150V) Skycraft Parts & Surplus
20 Amp AC Analog Panel Meter (Techman)
OR
15 Amp AC Analog Panel Meter
$13.50 SK1559 (MFG TP-670A20A)
OR
SK1558 (MFG TP-670A15A)
Skycraft Parts & Surplus
Sloped console aluminum enclosure (Hammond Manufacturing) $21.06 87F2478 (MFG 1456FE3WHBU) Newark
DPST 22A 125VAC / 16A 250VAC high-inrush current rocker switch (Eaton) $2.51 46F1857 (MFG 2600HM11E) Newark
Short panel mount 30A 250V 1/4" * 1 1/4" fuse holder (Bussmann) $2.63 63K9355 (MFG 2HKP-CC-R) Newark

A DPDT rocker switch is used, so that the outlet is totally cut off when the switch is off. This is for both noise and safety reasons. If only the hot were switched, the device under load would still be connected up to one end of the isolation transformer. As a safety issue, if the hot line were to short to ground somewhere, the entire load would now become hot with respect to ground, despite the fact the switch is in the off position. As a noise issue, even if the isolation transformer secondary is not connected to ground anywhere, there is always some small capacitive coupling to ground. If I were to probe the device under load with an oscilloscope while the switch is off, I would see some AC signal approximately half of 120V with respect to ground due to this capacitive coupling. This is nothing critical, and is fixed simply by grounding the device to the oscilloscope (which may be isolated from Earth's ground for safety), but I'd much rather have my device totally isolated from the transformer when the switch is set to off.

Another point is to note that the voltmeter is connected after the ammeter. This layout is more accurate than having the voltmeter before the ammeter, as the current through the voltmeter is completely negligible at any voltage as far as the ammeter is concerned, however the voltmeter will register a small deflection from the voltage drop across the ammeter under large loads. This voltage drop is small, and actually within the error of the voltmeter, but still results in a deflection and so it plays with my conscience, causing me to insist on having the ammeter come first.

The voltmeter is generally rather accurate, while the ammeter is accurate from around 5-20 amps, below 5 amps the needle tends to be compressed close to zero.

Making the enclosure:

The enclosure was made using only a drill, rotary tool with a multipurpose cutting bit (looks similar to a small drill bit, but it cuts on it's side), and a variety of files. The first thing to do once all the parts are acquired is to think of possible layouts, and just see if they would work out by eye, moving the parts approximately into place making sure everything will properly fit inside. If anything looks tight, careful measurements should me made to check if there will be a problem. Once a particular layout looks good, it can be drawn up on paper, or if one is feeling lazy, a rough sketch will work. I only made a rough layout of the front panel to make sure everything would fit in and look nice, as well as work out some numbers (seen here), but the other panels were simple enough that there was no need to sketching anything.

The entire enclosure is covered with masking tape, making sure one piece butts up against the next. Not having the tape overlap keeps a nice flat surface to work with. I like to fold over the tape behind the enclosure on both sides, so it is easy to remove afterwards. It is also important that everything is covered, so the enclosure is protected from scratches and dings while being worked with. Center lines are made, and then the layout is drawn up on the masking tape in pencil (This allows various marks and errors to be cleaned up, and changes to be made easily). When circular cutouts are needed, the center is first located, and then marked with an automatic center punch. This keeps the compass firmly in place when the circles are being drawn. In addition, all holes that must be drilled are center punched to keep the drill from walking. Once the layout is complete, it is covered with clear tape to prevent the pencil from smudging.


With the layout complete, the holes can be drilled, and the larger cutouts roughed out with a rotary tool.

The cuts are then brought to size, cleaned up and deburred with files. All parts should be checked to see that they fit, and the holes should be enlarged carefully until everything fits.

Once all filing is done, the masking tape can be removed, and it is ready for assembly.


The completed project: