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Topic: Arduino PWM massive transient (Read 1 time) previous topic - next topic

gnusmas

HIHI

SO i am designing a buck converter, but before i get to the actual buck converter I first wanted the mosfet to switch correctly. Now I am using a IRF2117 with a IRF540N Mosfet. The pwm signal gets feeded through a opto coupler (TLP250) and from there into the IRF2117 mosfet gate driver. Now the issue I am having is that there is a massive transient at each high rise of the pwm signal, and as the PWM amplitude gets amplified by the opto coupler and IRF2117 so does the transient also gets amplified. The attached pictures are the PWM output directly from the arduino board.


Any ideas?

retrolefty

That is most likely a artefact of your scope probe/scope input. Many probes have a compensation adjustment to handle fast transitions (scope probe frequency response), typically an adjustable C or R that works with the X10 attention resistors in the probe. My Tektronix 2213 has a test point (a 1Khz square wave source) on the front panel that one can hook the scope probe to and then adjust the probe for the cleanest square wave response on the screen with level tops and bottom and no overshoot or undershoot.

So what kind of scope probe and scope are you using?

Lefty

gnusmas


That is most likely a artefact of your scope probe/scope input. Many probes have a compensation adjustment to handle fast transitions (scope probe frequency response), typically an adjustable C or R that works with the X10 attention resistors in the probe. My Tektronix 2213 has a test point (a 1Khz square wave source) on the front panel that one can hook the scope probe to and then adjust the probe for the cleanest square wave response on the screen with level tops and bottom and no overshoot or undershoot.

So what kind of scope probe and scope are you using?

Lefty



Hi thanks for the quick response. Unfortunately I cant access the lab where the scopes are at the moment, i will only be able to tell you tomorrow.... However I am surprised to hear that it might be the scope?? I have tested it on 2 different oscilloscopes yielding the same result? Is there a full proof manner to test/check if it is actually a transient or just a result of the probes i am using.

Grumpy_Mike

Quote
Is there a full proof manner to test/check if it is actually a transient or just a result of the probes i am using.

Yes knowlage of what you are measuring.
Are you driving an inductive circuit? If not then the inductance is in your scope lead. This is a very basic observation so I am guessing that you are very new to using a scope.

gnusmas

#4
Jan 17, 2013, 05:59 pm Last Edit: Jan 17, 2013, 06:06 pm by gnusmas Reason: 1

Quote
Is there a full proof manner to test/check if it is actually a transient or just a result of the probes i am using.

Yes knowlage of what you are measuring.
Are you driving an inductive circuit? If not then the inductance is in your scope lead. This is a very basic observation so I am guessing that you are very new to using a scope.


Yes I am new to using a scope, and like i stated I measured directly from the Arduino output pin where the PWM signal is generated from and I yielded those results. Thus proving the inductance is in my scope lead.
Eventually my circuit will be used for a buck converter.

Note:

Thanks for the comment, went to read now on oscilloscopes and was amazed at how much the probe effected results! :)

here is the link for anyone that wants to read up on it!

Link:

https://www.google.co.za/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CDYQFjAA&url=http%3A%2F%2Fwww.home.agilent.com%2Fupload%2Fcmc_upload%2FAll%2FProbing-6-Hints-Scopes-Dec12-2007-webcast.pdf&ei=VC74UNzxNsORhQekr4DgCA&usg=AFQjCNFSv6_Qn4qW6Wv534yNTXwaTIZnBg&sig2=dNx6wYU77B6nnQJEJCVr_g&bvm=bv.41018144,d.ZG4

gnusmas

Mornings!!

I used a new probe (600MHz) this morning and it yielded the same results. I even adjusted the the probe with a flat screw driver at the BNC connector as stated in the probe manual. yet still getting those spikes. I think I can say with certainty that it is not my circuit because I used a signal generator as well and got the same results as from the arduino uno without connecting it to my circuit.

Is that a fair analogy?

oric_dan

#6
Jan 18, 2013, 08:36 am Last Edit: Jan 18, 2013, 08:45 am by oric_dan(333) Reason: 1
What wasn't mentioned above is that, besides having the probe properly compensated, you
also need to have the probe ground lead connected properly. This means as close to the
point being probed as possible. Otherwise, you have a big inductive loop that can ring.

Also, in section 6 of the pdf file cited, about Damp the Resonance, what they are using
there is called "source series termination". You can google this term. However, that sort
of thing usually isn't a big problem at low freqencies.

http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0517b/BGBGJHJB.html
http://www.sintecs.eu/download/series_termination.pdf

Grumpy_Mike

Yes it sounds
Ike it is not your circuit.

It could be your scope.

gnusmas


Yes it sounds
Ike it is not your circuit.

It could be your scope.


Cool, quick question.

For example, If my cap is rated for let say 63V, and without connecting the scope probe, There is currently sitting a 60V potential on that cap. Will the cap blow up then if i measure the potential across it? Because if the probe induces that spike there, does it not "feed" the spike into my circuit?

Grumpy_Mike

First off current only flows it does not sit.
Second that spike is on the amplifiers inside the scope, it is not real and it does not come out of the scope.

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