Page BottomI bought a variac (a variable output transformer) years ago, and put up with it's problems as i used it periodically. Then i recently pulled it out to so a series of tests on some TM506 mainframes, and darned if the variac had a rattle to it. There was a aluminum plate loose in the plastic wiring box. The box contained the 120vac outlet, the fuse holder, the on-off switch, and a cheap hard to read ammeter. The outlet pulled out of the box when trying to unplug anything from the variac.
So i figured it was well past time to "upgrade" the variac. The minimum box size would be 7 inches by 5 inches by 10 inches. So i went with a
Here's a couple of first takes on front panel layout.
Two analog meters, one for output voltage and one for output current, are necessary. The digital meters are for accuracy and quicker reading. The digital meters also correctly implies a microprocessor inside, for remote reading. The duplex outlet is a safety feature, because the bottom outlet is 10% voltage of the top outlet, which is handy for testing 2kv transformers, or keeping the power supply voltages in most devices low enough while checking out a fuse-blowing problem that nothing will explode. With two outlets, there is no way accidently flipping the wrong switch will put full voltage on the device being tested. This box will have a fan in it to keep things cool. In addition to the variac, there is a dual 12vac@8amp regular transformer. One winding will go to the lower outlet on the duplex. The other will be switchable into the top outlet, for pushing the voltage as high as 150vac, for severe stress testing. The line voltage here once went to 135vac and higher for several weeks, due to a bad auto-regulator on the power company's main line, so the option to test for this situation is desireable.
And here's the front panel as cut and drilled with most stuff set in place to see how it will look when done. The box isn't even started at this point, this is only the front panel.
Below the meters is a row containing a 10amp slow blow breaker, the on-off for the whole box, the switch to boost the output capability to 150vac, and the switch to turn the duplex outlets on-off. That last switch operates a DPST relay to do the actual switching, so the outlets can also be switched off remotely. Each outlet is separately fused, the fuses located above the outlets. So the top outlet is 150vac max, the bottom outlet is 15vac max. The outlet is drilled across the slots, with IR led shooting across, to detect which outlet has a plug in it, to adjust the metering.
I need to drill attachment holes for the rest of the box's sheet steel panels, the second power transformer, and the pcb to mount the led readouts to. And pick a color scheme, i am thinking off-white or light grey background, and possibly red or blue or black to denote the areas of "control switches", "metering", etc..
Here's some of the panels in place.
Ok, i got all the exterior panels on, drilled and threaded, screws are in.
Same pic, but with bits dropped in place.
View inside the portion where the variac will live. You could be thinking that a 7 inch tall rack mount is too large for such a piece of equipment, but you can see the outline drawn onto the bottom of the pit, and that the left side top and bottom feet must be sawn down to fit. The variac itself does indeed take up nearly half the space, and sets the minimum box dimensions (other than the 19 inch rack dimension). The hole in the internal divider on the right is for the fan.
The holed drilled thru the outlet for the IR emitter and detector to determine which outlet is in use. One is 150vac max, the other is 15vac max. I want the electronics in the box to have a clue which they are metering. I'll possibly re-do this later, i don't like the low voltage circuits next to the 150v line.
Done, buttoned up, painted badly. It works nicely, i've been using it to test and repair Tektronix TM506's. The handles are required, this thing is heavy.
Well, that's all for now.
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