Tangent alert: Sanity check on adding DMM display to a variac?

[angelodp]

Gonna do the meter with velcro on a slant like this.

[angelodp]

Hey Steve, seeing as how you have a similar Powerstat as mine, how did you manage to cram the CT in there? I have it lodged in a corner but that does not allow me to use one of the longer mounting bolts. It looks like I can get it buttoned up with just one long screw at the bottom and the middle screw that engages the cover to the shroud. I get 60 ohms on that little CT.... that about where they all sit? It's a trick to get it all crammed in there. You maybe have a picture of the inner guts of the CT/Switch/fuse area?

A

[sluckey]

Our units are laid out slightly different. It was tight inside my unit also. My power switch was originally on the top but I moved it to the front so I could mount the meter on top. I 'may' have replaced the original fuseholder with a smaller one to make room for the switch relocation. Can't remember. Then the CT fits snuggly beside the power switch. It was a challenge.

I think if you replace that DPST switch with a smaller SPST switch you may gain enough room for the CT. Replacement switch should be rated for 10A or more.

Here's a pic of my unit. There are only two long screws to attach to the variac body. There is no small screw at the top like yours.

[angelodp]

Hi Steve, thanks for the comparison. Definitely a tight little area. Here is how I solved it. I am going to use the original switch as I managed to squash the CT in that corner. My thought is to use the switch to energize the circuit through the hot and simply use the other side to pass the neutral signal forward. I took the wires for the PZEM unit from the top of the switch N & L and then the CT wires per the usual. I think this works as well? I am asking.

[sluckey]

Looks like you're ready for the smoke test.

[angelodp]

Akkkk, the dreaded smoke test!!!

[angelodp]

Ok so after a glitch I got this unit installed correctly, or so I thought. I am including two shots, a gut shot and a diagram. The gut shot shows a Silverface Champ plugged in, but the power and current readings seem off. When I last was in that Champ I was under 20watts calculated. Any thoughts on where this is possibly mis-wired. The CT was crammed in there and I pulled it back out but that did not have an affect.

[sluckey]

There is no problem with your meter. It shows the measured input voltage and current required from the wall to run your champ. The power shown by that meter is a calculated value based on the measured voltage and current. None of the readings on that meter have anything to do with your calculated OUTPUT power.

That 38.8W number on the meter represents the power required from the wall to produce your "under 20 watts calculated" output power. The input power will always be more than the measured output power. Your calculated output power does not include power required just to light the filaments. Heck, a 5Y3 requires 5V x 2A = 10 watts! And the 2 6V6s plus 2 12A?7s require another 9.45 watts. That's 19.45 watts of wasted power. Your variac meter is including these 19.45 watts too.

So don't worry about the variac meter readings. They will never directly relate to your calculated tube power.

I use my variac meter to set the line voltage to exactly 120VAC anytime I will be measuring voltages in my amps. That means the voltages I measure next year should match the voltages I measured today and recorded on my schematic. The three other meter readings are nice to have but I'm mainly interested in the voltage.

I'll end this long winded reply by repeating my first sentence. "There is no problem with your meter." Enjoy...

PS... Put that variac back in the can before you get hurt!

[angelodp]

Steve, as always concise and informative. So that a 15w light bulb draws considerably higher current through this meter is not a concern as well? I saw some earlier tests that showed a check using the various bulbs? Hope I am not beating a dead horse here.

Best A

[sluckey]

A 15w light bulb operating with 120V should pull .125A. If your current reading is not close to .125A then maybe you have damaged the CT again.

[angelodp]

Ok, all good....that was not a 15w bulb...Now have a 60w bulb in there and it checks out perfectly.

Thanks Steve