# AC transformer 500:5A

Discussion in 'General Electronics Chat' started by superway, Dec 10, 2013.

1. ### superway Thread Starter Active Member

Oct 19, 2009
128
0
Hello,

I am looking for an AC current transfomer 500:5A on omega.com website.
There are 2 versions of this product, 500:5A with Burden(VA) 15, and 500:5A with Burden 7.5.
What is the differnce between these Burden value (VA)?

Thanks

Jul 18, 2013
12,890
3,527
Believe it or not, 7.5va
Max.

3. ### superway Thread Starter Active Member

Oct 19, 2009
128
0
I will install 100 Ohm, 25Watt accross to secondary of CT and then monitor AC current on Scope. So is Scope a VA load? What O'scope VA rating? can I use 7.5VA or 15VA CT?
Thanks

Jul 18, 2013
12,890
3,527
VA, Volt x Amp is the power rating of the device, and is directly proportional to any ratio change, as in a transformer secondary etc.
You need to state the voltages you are working at.
If you placed a 100Ω across a supply to draw a maximum of 15va
Or e.g. a voltage of 38.75v with the 100Ω load will be .3875 amps or 15va..
Max.

5. ### crutschow Expert

Mar 14, 2008
16,186
4,324
The burden VA determines the maximum load that can be connected on the current transformer output as determined by VA = I$^{2}$*R. Thus for 7.5VA and a 500:5A transformer the maximum load (burden) resistance is R = VA/I$^{2}$ or R = 7.5/25 = 0.3Ω (thus you can't use the 100Ω resistor you selected). This gives a maximum output voltage of 5A * 0.3Ω = 1.5Vrms. You can do a similar calculation for a 15VA maximum burden.

Note that this resistance includes any lead resistance at the transformer output.

6. ### crutschow Expert

Mar 14, 2008
16,186
4,324
Current transformers do not care about the primary voltage, only the current. The only voltage of concern is the secondary voltage across the burden resistor as determined by the VA burden rating.

7. ### superway Thread Starter Active Member

Oct 19, 2009
128
0
Thanks.
For this CT rating 500:5A, 5A is maximum on secondary rating as your calculation and required 0.3 Ohm.

But for my product application, a maximum 40 Amp is running through CT primary and the secondary will be produced 0.40A ( 500:5A).

So VA = I^2 x R = 0.40^2 x 100 Ohm = 16 VA, is it correct?

8. ### crutschow Expert

Mar 14, 2008
16,186
4,324
No. It's the maximum output voltage that's critical since core saturation is determined by the voltage and frequency, not current. For a 15VA transformer, maximum output voltage would be 15VA/5A = 3V. Thus, for a maximum primary current of 40A, the maximum output burden resistance would be 3V/.4A = 7.5Ω

9. ### Jaguarjoe Active Member

Apr 7, 2010
770
91
FWIW-

Never have current flowing thru the primary side (ie-the "hole") of a CT with an open secondary ckt. This is like having a burden resistor of ∞Ω. Sparks will fly!