Hi,
This is a general question about the use of isolation transformers for probing ac powered circuits. The circuit in question is from the following application note:
which describes several designs for transformerless power supplies for PIC microcontrollers.
I built the capacitive power supply in FIGURE 1, and my DMM (battery powered, ungrounded) reads 4.8 vdc at vout as expected. Now I need to see the waveform. I've been trying to use an isolation transformer to be able to see the output wave using my scope, but I can't eliminate an ac potential between the ground for the transformerless power supply circuit and the ground for the oscilloscope probe. I tried the following:
1) The scope and the circuit under test are each on their own isolation transformer. Neither isolation transformer is grounded at the wall, and neither the scope nor the circuit are grounded to the isolation transformers. All three wires are isolated for the scope and for the circuit under test (the line and neutral are isolated via the transformer, and the grounds are disconnected and floating). I checked the potential between the grounds to be sure I wasn't going to burn anything before clipping the probe ground to the circuit. Measuring the voltage between the probe ground clip and the ground for the transformerless power supply circuit shows 20 vac between the two.2) If I tie the grounds for the scope and circuit under test to the neutrals on the secondaries of their respective isolation transformers (recreating a neutral to ground bond at the secondary at each isolation transformer), the voltage jumps up to 48 vac between the probe ground and the ground on the circuit under test.
3) If I put both the scope and the circuit under test on the same isolation transformer, with the grounds floating as in number 1 above, the voltage between the probe ground and the circuit under test ground is then 52 vac4) If I put both the scope and the circuit under test on the same isolation transformer with the ground bonded to the neutral of the secondary of the transformer (as in number 2 above), then the voltage between the probe ground and the circuit ground is 115vac.
In the application note are pictures of startup waveforms for this project. I would like to recreate those, but I cannot seem to eliminate the potential between the probe ground clip and the circuit under test ground, so I haven't actually hooked up the scope....
Any help would be greatly appreciated. Thanks,
Brian