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Old 17th October 2005, 08:28 AM   #1
sss is offline sss  Israel
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Default had a weird problem with uc2825 / sg3525

i'm making a new smps so i've tested the PWM chips with my new scope

and something weird was happening :
output B was ok and output A had spikes only , no square wave
i tried both the sg3525 and the uc2825 and the same thing was happening , only one output is working and it was output B on both chips ...
i've changed the chips million times , and was going like

finally i found the problem , i used 2 regular BNC cables with the scope , i've changed one to a 60mhz probe and it started to work .
now i have couple questions :

both outputs of the chip are identical ,how can it be that out B is working with normal probe and out A isnt?
the same is happening with sg3525 ...

update :
i have added a 1K resistor in series with a normal BNC and it worked fine , what is going on here ?
also , do u think i've damaged the chips?
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Old 17th October 2005, 06:34 PM   #2
subwo1 is offline subwo1  United States
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I am wondering if I damaged an SG3525 chip as well since, under a light load, the duty cycle is not staying at maximum even though the feedback is disabled right now.

It seems strange how your probes are acting on your circuit. It is great you have scope now, though.
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Old 17th October 2005, 07:17 PM   #3
sss is offline sss  Israel
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i figured out what was the problem .....
i used an old pc power supply and didnt use any caps on the Vcc pin of the chip , so when i was connecting the probe the signal stopped because in a split of a second the voltage dropped because of the load (the probe) and the chip entered under voltage lockout condition
it started to happen on output b also , then i figured it

now all working fine , even with a regular probe
so i didnt damage anything
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Old 17th October 2005, 07:47 PM   #4
subwo1 is offline subwo1  United States
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It makes sense now.
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Old 18th October 2005, 12:32 PM   #5
Eva is offline Eva  Spain
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There is a little lesson to learn there: when you connect a probe to any node of a circuit, you are always inserting some parasitistic capacitance that may change circuit behavior sometimes.
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Old 28th November 2005, 04:53 PM   #6
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Default What size cap did you use

I'm having the same problem, however I do not have a 60 Mhz probe. Should the circuit have a cap across the battery line, and if so what size. Also the SG3525 get loaded down with the scobe probe but ok. That just seems kinda strange but ok. Any help with the circuit I will be more than happy.

Thank you
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