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:

http://ww1.microchip.com/downloads/e...tes/00954A.pdf

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 vac

4) 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

On 9 Jul 2005 12:34:14 -0700, "costab06" wrote:

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:

http://ww1.microchip.com/downloads/e...tes/00954A.pdf

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 vac

4) 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

In practice, in a repair shop, the device to be tested is isolated,
and the testing device is simply plugged into the AC receptacle.

Tom

"costab06" wrote in message
oups.com...

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:

Ground the scope normally.

Isolate the capacitive power supply.

Connect a ground wire from the scope to the capacitive power supply - I
assume you can find the correct point to ground.

N