Author Topic: need help with variac  (Read 1532 times)

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Offline Raj

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need help with variac
« on: January 26, 2017, 10:16:39 am »
I was testing nixie using variac and a mastech meter i just recently bought.
so i had a 22k resistor to limit current from going above 10ma (the spec of my nixie says current 10ma).
The problem is, after i reached the desired voltage on variac, where all digits glow just the right amount ,I went on to measure voltage on variac , I got the result as follows-

117v on ac output

50v on AC setting of meter (dc output)
104v on DC setting (dc output)

the problem is ,even after rectification, why is there still 50v AC
is one of my diode shot?(which would have definitely lead to magic smoke) (ill check soon after this post)

here's a drawing of it
https://drive.google.com/file/d/0B2ffwyRcquZTYWVkUWJtemZGMmM/view?usp=sharing
« Last Edit: January 26, 2017, 10:21:01 am by Raj »
 

Offline jeroen79

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Re: need help with variac
« Reply #1 on: January 26, 2017, 10:30:43 am »
After the rectifier the voltage will still be going up and down, it just won't go below 0.
So it will have an AC as well as a DC component.
You need a capacitor to smooth out this ripple.

« Last Edit: January 26, 2017, 10:33:16 am by jeroen79 »
 

Offline Raj

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Re: need help with variac
« Reply #2 on: January 26, 2017, 12:01:30 pm »
Thanks, with a cap, my meters shown as
Almost same voltage at both side of rectifier.

So it was a meter misunderstanding after all
(Thought that those 50 volts are under 0)
« Last Edit: January 26, 2017, 12:04:11 pm by Raj »
 

Offline Ian.M

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Re: need help with variac
« Reply #3 on: January 26, 2017, 12:06:57 pm »
Its generally a *BAD* idea to use a Variac + bridge rectifier for a DC power supply as nearly all Variacs are autotransformers and provide no isolation from the mains whatsoever.   For the sake of your life and sanity, either put a conventional transformer in-between the Variac and the bridge rectifier or run the Variac off an isolation transformer with a fully floating output.
 
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Offline Raj

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Re: need help with variac
« Reply #4 on: January 26, 2017, 12:11:50 pm »
Yes, I do ( forgot it in drawing. Sorry)
Thanks
 

Offline Seekonk

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Re: need help with variac
« Reply #5 on: January 26, 2017, 01:17:52 pm »
Many meters expect to be reading AC on AC setting.  If you do add a capacitor, it still may measure considerable AC which isn't there.  Place a .47uf in series with probe to measure actual AC instead of the imposed DC.
 


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