# Output current of a ballast

#### Jane Panovski

Joined Sep 13, 2011
2
Hello everyone

I have a Tektronix TDS 210 oscilloscope and I'm supposed to measure the output current of several types of ballasts. So far I found out that I need to use a shunt ressistor because I don't have a current probe, and that I have to use both channels considering I dont have a differential input. As a shunt I tried to use a 3,7 m, 0,25 mm^2 copper wire (if I calculated corectly it has about 1.4 ohms resistance). I connected this wire in series with the lamp (ballast). The lamp is connected directly on 231 V. I used the math function Ch1-Ch2 (one probe on each end of the 0,25 mm^2 wire), and what I got was a normal sinusoidal wave. This can't be correct! No matter how high quality the ballast is, it has to change the shape of the current signal. It seems to me I'm getting the input voltage from the socket.I don't really care about the amplitude of the current, only the shape of the signal so the shunt doesn't have to be precise. Could someone suggest what am I doing wrong please?
I' ve posted this to other forums as well and any help is appreciated.

Joined Dec 26, 2010
2,148
I can think of a few things that could be worth trying:

First (if your set-up has a high-potential and a ground potential or neutral line) is to ensure that the shunt is on the neutral or low side. This will be less taxing on the common-mode rejection capability of the scope.

Next, you could try connecting both scope channels to the higher potential end of the shunt. If the subtraction is working effectively, you should not see much. If there is still a significant signal, say more than 5% of what you expect to see normally, check that both channels have equal sensitivity, including any probe attenuation that may be relevant. You also need to be sure that the correct mathematical function is actually displayed, not the raw channel.

Finally, you could try displaying the current wave from something definitely non-sinusoidal, say an old-fashioned fluorescent lamp with no PF correcting capacitor.

#### #12

Joined Nov 30, 2010
18,210
I recently found that the connector on one of my scope probes had developed an intermittent resistance (crusty connection in the BNC end). Just stating the obvious, make sure your test equipment hasn't degraded over the years.

Joined Dec 26, 2010
2,148
That's a very good point. Shared probes in a busy lab. can be particularly treacherous. Hook ends don't always mate properly with the probe body, cables etc. fatigue...

One of my real pet hates is a cheapish variety of 1X/10X switchable probe, where the nasty little slide switch may not stay set where it should. Alternatively, we may simply neglect to set the wretched things.

Actually, the OP should definitely make sure that both her probes are identical, or at least the same attenuation and impedance rating. They should also both be checked on the internal calibrator, assuming there is one provided. If there is one, it also gives a convenient source for checking the subtraction is working: both probes on = no signal?

#### Jane Panovski

Joined Sep 13, 2011
2
The probes are fine (both are 10x) and I've tried them with the internal calibrator. Even if they weren't, it doesn't explain why I'm getting a perfect sine wave. I get the same signal by substracting, as when I put one probe in the neutral line. I expect to get about 500 mV. But this sine wave I'm getting, suggests that one channel doesn't work, or something like that. But I've tried them both. I still don't get why I'm getting this signal.