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Registered Member #1107
Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
I need to design a voltage meter for my variac to monitor the input voltage of my big coil. I need it to be led so i can see it clearly at night. I have a led dc voltage panel meter that has a inpedence of 10mohms. I need to find a resistor calculator or formula that will take the rectified voltage and drop it to the unrectified voltage so i can tell what the actual ac voltage is on the panel meter. Any help would be appreciated.
Registered Member #690
Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616
I would think something like this:
Of course the capacitor would have to be pretty small in order for it to bleed fast enough that it doesn't just remain charged when you turn down the variac.
And the resistor, to properly divide the voltage, would have to be 4.14Mohm. It might be a good idea to make it a 5M or 10M trimmer pot to adjust for discrepancies caused by ripple in the capacitor, etc. So, for example, you would set the variac to 100V with no load and then adjust the trimmer so the value on the voltmeter accurately matches that on the variac.
And also because 4.14Mohm resistors are non-existent (just remembered )
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
You run into problems when you use a diode rectifier for precision voltage measurements, because you have some voltage drop from each of the diodes (about 5v in shaun's diagram). If you don't mind this drop (5v out of 100v isn't that bad) then your job is pretty easy. If you want to measure lower voltages then you need to rethink your design... Also, the divide by 1.4 to to get rms value instead of peak-peak value only works for prefect sine waves, so when you start to use 'weird' inductive/etc loads it looses more accuracy.
Registered Member #690
Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616
Oh right, I forgot about the voltage drop in the diodes. But all that is really needed is the meter value to match the value the variac is turned to.
If any inaccuracies are introduced, they can be compensated for by using a variable resistance in series with the meter. That is assuming the magnitude of the inaccuracy remains the same over the full range of input volatges.
Registered Member #1107
Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
Shaun wrote ...
I would think something like this:
Of course the capacitor would have to be pretty small in order for it to bleed fast enough that it doesn't just remain charged when you turn down the variac.
And the resistor, to properly divide the voltage, would have to be 4.14Mohm. It might be a good idea to make it a 5M or 10M trimmer pot to adjust for discrepancies caused by ripple in the capacitor, etc. So, for example, you would set the variac to 100V with no load and then adjust the trimmer so the value on the voltmeter accurately matches that on the variac.
And also because 4.14Mohm resistors are non-existent (just remembered )
Yes that is exactly what i was looking for. I will get a 10mohm 15 turn pot and adjust the voltage so it matched with a dmm. As far as diode losses i will just the pot accordingly. Also What approximate value of capacitor should i use? It has to be rated for at least 250v. (i am thinking of something like a 2.5uf poly cap)
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
It doesn't have to be anything fancy because it's only going to be 60Hz. I don't think it should be all that big because the meter's reading will be delayed if you want to go down in voltage and the cap is too big.
Registered Member #1157
Joined: Thu Dec 06 2007, 12:11PM
Location: Houston, TX
Posts: 307
Why not just use a cheapie VOM with a backlit display? Harbor Freight has em for like 3 bucks.
Take it apart, set it on the AC voltage, and mount the display panel to whatever you wanted it in. Want it with Bright LED's? Rewire the LCD display to use 7 segment LED's instead.
If it blows up, well it only cost ya a couple of bucks. I bet with a little creativity, you could have a toggle switch on there to monitor your current draw as well.
Registered Member #1107
Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
Actually i thought of that before i posted this thread but unfortunatly there are no Harbor Freight stores within 50mi of were i live. I would pay more in gas then i would to get this panel meter working.
Registered Member #1157
Joined: Thu Dec 06 2007, 12:11PM
Location: Houston, TX
Posts: 307
I'll throw a couple of them in the HV Migratory box of junk. I wouldn't imaging up there in the Arctic Ocean (That's where your push pin has you on the member map) they would have much of anything. Polar bears?
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Help with ac voltage meter
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