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Anomaly in Edge trigger

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Anomaly in Edge trigger

Postby tonigau » Tue May 18, 2010 12:53 am

I am using Picoscope 6.3.55.0 & 3224 hardware to capture turnoff waveforms, using both simple edge & advanced edge with hysterisis to trigger off the digital control signal rising edge of ch1, I also get triggers when the other channel 2 rising edge coincides (within nom. 300uS window) with the digital signal falling edge. So its capturing some turnons as well.
Ch2 is a current measurement across 1 ohm of a nominal 280mA load (250mA resistive, 29mA LED PFC Lamp operating off 110V 50hz AC.
The 110V supply is transformer isolated , the Logic board is Earthed & the Laptop is running off battery so there are no ground loops to interfere.

I dont miss any turnoffs, just get additional unwanted waveforms in the buffer (for later analysis & evidence)

Sample PSData file attached...
(PS: add file error "The extension psdata is not allowed.", I'll try zipping it up)

Thanks
Attachments
20100518-0001.zip
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tonigau
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Re: Anomaly in Edge trigger

Postby ziko » Mon Jul 12, 2010 2:57 pm

Hi,

We cannot setup the same experiment as you, as it is not a recommended way to measure such voltages. It looks like you are probably getting some sort of coupling. I would recommend using something like a differential oscilloscope probe.

http://accessories.picotech.com/active- ... robes.html

Kind regards
Ziko

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Re: Anomaly in Edge trigger

Postby tonigau » Mon Jul 12, 2010 9:25 pm

Thanks ziko for the reply,

You might be right, there is probably enough parasitic capacitance in the isolating transformer & my setup to affect the 3224.

The logic circuit actually is not earthed as I stated, it is powered from a topward dc bench supply & Vicor DC-DC converters so has double isolation from the mains supply.(but is sitting on the bench in metal frame) .The laptop is mounted on an insulating base.

We have a job coming up soon where we should be able to purchase an isolated differential probe. I will repeat the test & advise results.

Tonig
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