Hi all, maybe you could help please?
I have a Variac that i picked up on eBay. It was pulled from an old Farnel PSU apparently. It is 2Amp 0 - 270V 50Hz with 6 taps. I have tried to figure out the tapping with a DMM using DC res setting and moving the wiper. There is still a diagram on the transformer that shows the centre tap but i am not able to see what the others are.
I am going to try putting a 12V step-down in front of it and measuring the voltage whilst mooing the wiper to see if i can figure this thing out but since this it my first encounter with an auto transformer, I wonder if anyone could tell me a better way to find what is what aside from connecting it up and fiddling till i get a varying voltage?
Thanks for reading
I have a Variac that i picked up on eBay. It was pulled from an old Farnel PSU apparently. It is 2Amp 0 - 270V 50Hz with 6 taps. I have tried to figure out the tapping with a DMM using DC res setting and moving the wiper. There is still a diagram on the transformer that shows the centre tap but i am not able to see what the others are.
I am going to try putting a 12V step-down in front of it and measuring the voltage whilst mooing the wiper to see if i can figure this thing out but since this it my first encounter with an auto transformer, I wonder if anyone could tell me a better way to find what is what aside from connecting it up and fiddling till i get a varying voltage?
Thanks for reading
you may be able to measure the winding resistances with a good ohm-meter.
Try to find which is the highest resistance compared to all the other tappings.
This will identify the two extreme or END tappings.
Then you can work from ONE of these END tappings to find the order of all the others.
I am familiar with a 5 tapping Variac. One is the wiper.
two are the ENDs The final two are the middle section but one is near the top END and the other is near the bottom END.
The order for a 5tapping Variac is:
top, upper, lower, bottom and finally wiper.
I wonder if your 6th is actually an earth strap for the core in the event of a short to core.
Try to find which is the highest resistance compared to all the other tappings.
This will identify the two extreme or END tappings.
Then you can work from ONE of these END tappings to find the order of all the others.
I am familiar with a 5 tapping Variac. One is the wiper.
two are the ENDs The final two are the middle section but one is near the top END and the other is near the bottom END.
The order for a 5tapping Variac is:
top, upper, lower, bottom and finally wiper.
I wonder if your 6th is actually an earth strap for the core in the event of a short to core.
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Hi, thanks for fast reply.
I will check the resistance of the taps.
To the 6th tap earth/ground.... no. I would assume that an earth would be extended to the bracket as the bracket is bolted securely to the core/chassis and there is no electrical connection to any tap.
Ok I have now found the two taps with most resistance (wiper disconnected). The 6th tap is open circuit.
Thank you.
I will check the resistance of the taps.
To the 6th tap earth/ground.... no. I would assume that an earth would be extended to the bracket as the bracket is bolted securely to the core/chassis and there is no electrical connection to any tap.
Ok I have now found the two taps with most resistance (wiper disconnected). The 6th tap is open circuit.
Thank you.
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Hi,
it would appear that this is a 5 tap.... though I can't think why there is an unused terminal?
I am going to put a 1/1 transformer in front of the variac and connect the to ends of the autotransformer winding across the secondary of the 1/1, then have the load across one end of the auto transformer and the centre tap of the autotransformer.
it would appear that this is a 5 tap.... though I can't think why there is an unused terminal?
I am going to put a 1/1 transformer in front of the variac and connect the to ends of the autotransformer winding across the secondary of the 1/1, then have the load across one end of the auto transformer and the centre tap of the autotransformer.
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The 5tap Variac allows 6 different ways to connect it up.
Mains to top tap and output to top tap. Max out = mains in
Mains to top tap and output from second to top tap. Max out ~90% of mains in
Mains to second to top tap and output from top tap. Max out ~110% of mains in
Then the reverse (using second to bottom tap) to allow the control knob to rotate in the opposite direction.
I don't have the instructions to hand, so that is all from my bad memory.
The idea of using the 1:1 isolator before the Variac sounds good.
Mains to top tap and output to top tap. Max out = mains in
Mains to top tap and output from second to top tap. Max out ~90% of mains in
Mains to second to top tap and output from top tap. Max out ~110% of mains in
Then the reverse (using second to bottom tap) to allow the control knob to rotate in the opposite direction.
I don't have the instructions to hand, so that is all from my bad memory.
The idea of using the 1:1 isolator before the Variac sounds good.
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I do have a couple of 1/1 with a a 240 and 6V sec but I think I shall save those for amps. I'll be using 2 back to back transformers with a 50W 25V sec to 50W 28V pri so with 240V in (Actually it is more like 285V here from the wall!) I should get as near as dammit to 210V to the auto transformer. After measuring to be sure, I may connect the auto up for 110%.
With the DC resistance as indication of which tap is which aside, is there a numbering standard for the taps?
With the DC resistance as indication of which tap is which aside, is there a numbering standard for the taps?
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