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Help required for converting milliamps to amps

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koduvayur2001

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I have to convert (0-50)mA to (0-1)A. The lower range of current required to be converted is from a current transformer of ratio 400/1. The CT actually to be installed was supposed to be 20/1. Instead, by some regrettable mistakes, it was wrongly wound to 400/1. Now the maximum primary current instead of 400A would be just around 20A. Hence for the protection relay to kick into action, this primary current of 20A has to transform into 1A. Instead I would be getting only around 50mA. Now if I have a circuit which could linearly transform (0-50)mA to (0-1)A then this should solve my problem.
 
I don't really understand what you're talking about but for a current transformer to work you use standard turns ratio formula. A 10:1 transformer will convert 10A to 1A on the secondary so you'll need a 20:1 transformer to convert 1A to 50mA.

In practice it's a little more complex than this as you need to account for the losses and use an appropriet core for the powerline's frequency. Generally though you should calibrate the device measuring the current rather (for example tweaking the current sense resistor) than worrying about designing a perfect transformer.

Either way I'd consider buying a ready made unit rather than trying you cobble something together myself.
 
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koduvayur2001 said:
The CT actually to be installed was supposed to be 20/1. Instead, by some regrettable mistakes, it was wrongly wound to 400/1.

The guy who made the CT, give him a whack around the ear and tell him to do it correctly.

The guy who ordered the wrong CT, give him a whack around the ear and tell him to but a new one at his own expense.

Either way, I think you need a new CT, not some half baked bodge which probably wont work correctly/reliably.

JimB
 
Buy a new CT with correct ratio as suggested by JimB and consider the mistake a valuable lesson learned.

There is no other ways, especially when protection relaying is involved.
 
Come on guys!

First of all thanks for your suggestions. Unfortunately the CTs are too costly to be bought and replaced. I will accept the whacks and I would also accept to whack the designer, but it would not solve the problem! It is just a digital current amplifier using any op amp circuit which will sence the (0-50)mA current from the wrongly installed CT and amplify this to the required times with negative feedback. I do not have a lab to test or make a circuit from scratch. Hence this request. Incidentally the CTs are for a 33KV Vacuum Circuit Breaker and hence they cost quite a packet.
 
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