Re: newbie question on probing transformerless ac power supplies with an isolation transformer
- From: Tom MacIntyre <tom__macintyre@xxxxxxxxxxx>
- Date: Sat, 09 Jul 2005 19:58:51 GMT
On 9 Jul 2005 12:34:14 -0700, "costab06" <costab06@xxxxxxxxx> 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/en/AppNotes/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
.
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