# Measuring AC wave from transformer

Discussion in 'General Electronics Chat' started by Gibson486, Mar 11, 2014.

1. ### Gibson486 Thread Starter Active Member

Jul 20, 2012
217
12
I am measuring a 300Khz AC wave (300V pk pk) from a transformer output. The problem is that, over time, it drifts to a lower value (depends on the cable length). I am trying to see what is causing the drift. If I remove the cable, the drift does not occur. I am guessing that the cable has a capacitance and the scope probes acts as a leak for the cable's capacitance. Does that sound feasible or am I just silly?

2. ### #12 Expert

Nov 30, 2010
17,847
9,181
There is an interaction between all 3 elements. The supply of the signal might be changing with the power (heat) required to drive the capacitive load of the cable. I have actually calculated the maximum resistance allowed when required to drive a cable capacitance over x amount of distance at a particular frequency. This is usually a low number at 300KHz.

3. ### Gibson486 Thread Starter Active Member

Jul 20, 2012
217
12
Thanks.....I am going through the process doing calculations (arggg...opening up the Emag book sucks). On the cable spec sheet, it has 2 capacitance. 1 is C from wire to wire, the other is C to ground. Do I simply add those two? Also, they give the capacitance at 1kHz. I am guessing I have to resolve for C with my new frequency?

4. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
429
capacitance does not change with frequency. Xc does.

5. ### Gibson486 Thread Starter Active Member

Jul 20, 2012
217
12
ofcourse....hahahaha...need coffee....

6. ### #12 Expert

Nov 30, 2010
17,847
9,181
Either add the capacitance or use a separate amplifier to drive the shield with the same changes that are on the data wire. Zero voltage difference causes zero current to flow and, mathematically, the shield disappears.

7. ### crutschow Expert

Mar 14, 2008
16,236
4,343
If it drifts with time then I would suspect that whatever circuit is driving the transformer is heating up due to driving the cable capacitance and that causes its output voltage to drop.