Author Topic: MOSFET turned off - strange voltage drop  (Read 1918 times)

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Offline tester43Topic starter

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MOSFET turned off - strange voltage drop
« on: July 03, 2018, 10:04:08 pm »
Hi,

My Mosfet is handling AC unfiltered half_waves.
When it's on - everything is perfect on my scope.
On the place marked in schematics (see right side of schematics, drain) I see perfect half waves from 0 to 12V as expected when mosfet is closed (conducts current through load resistor).
The issue I have is: when gate of Mosfet is not charged (mosfet not conducting) I see waves with 0 shifted to 0.7-0.8V (approx.).
Probing between zener and it's resistor is visible as attached.
I do not see any reason to have this voltage at the place... what can it be?

 

Offline T3sl4co1l

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Re: MOSFET turned off - strange voltage drop
« Reply #1 on: July 03, 2018, 10:12:56 pm »
Put a pulldown resistor across the FWB output.

Tim
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Offline tester43Topic starter

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Re: MOSFET turned off - strange voltage drop
« Reply #2 on: July 03, 2018, 10:15:46 pm »
Hi Tim,

Thank you for super quick answer, it's around midnight at my place :)

But.... as stupid as it might sound but... what is FWB?
 

Offline T3sl4co1l

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Re: MOSFET turned off - strange voltage drop
« Reply #3 on: July 03, 2018, 10:21:33 pm »
Full Wave Bridge [Rectifier]
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Offline tester43Topic starter

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Re: MOSFET turned off - strange voltage drop
« Reply #4 on: July 03, 2018, 10:26:02 pm »
Hmmmm.... How would it work.... Are you saying to put a resistor between + and - of FWB? Or do I get it wrong.
Putting a resistor between + and - would put a resistor parallel to mosfet and LOAD resistor....

Do you have any idea what is this 0,8 that I see on scope?
 

Offline tester43Topic starter

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Re: MOSFET turned off - strange voltage drop
« Reply #5 on: July 03, 2018, 10:40:34 pm »
Ok, I could not sleep, I did add this resistor.
You are correct.
My load is 8ohm.
I added around 2k ohm resistor between + / -.
I have my zero now ... but why?
Which characteristics of mosfet is this 0,8??
 

Offline T3sl4co1l

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Re: MOSFET turned off - strange voltage drop
« Reply #6 on: July 03, 2018, 11:12:32 pm »
Consider what the circuit does with a capacitance between FWB +/-, or across the zener diode.

Consider also that all of your components have capacitance (read the datasheet). :)

Tim
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Offline tester43Topic starter

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Re: MOSFET turned off - strange voltage drop
« Reply #7 on: July 04, 2018, 05:47:16 am »
So you are saying that it's just not unloaded fast enough.... due to input capacity (15000pF)?
Issue is I have a thermocouple there in series with load (load is heater).
If I add this resistor in parallel will i not ground my TC voltage in no time?
« Last Edit: July 04, 2018, 07:22:32 am by tester43 »
 

Offline T3sl4co1l

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Re: MOSFET turned off - strange voltage drop
« Reply #8 on: July 04, 2018, 10:55:07 am »
How high of a resistance do you think your TC voltage has? ;)

You are describing a voltage divider.  That means the voltage drop can be known exactly, and compensated for. :)

Tim
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Offline tester43Topic starter

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Re: MOSFET turned off - strange voltage drop
« Reply #9 on: July 04, 2018, 11:00:30 am »
But the circuit is open, there should be no drop - mosfet is not conducting. Scope screen stays the same after I physically removed the zener diode with it's limiting resistor.
TC and load are integrated in one enclosure:
http://dangerousprototypes.com/forum/download/file.php?id=11012&mode=view

as it's a part of this T12 tip:
http://www.ptdreamer.com/wp-content/uploads/2018/02/Wiring-Diagrams.png
 

Offline tester43Topic starter

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Re: MOSFET turned off - strange voltage drop
« Reply #10 on: July 05, 2018, 09:29:57 am »
Hi,

Is this because if bridge not being loaded with MOSFET closed then the whole PCB + wiring works as a capacitor for the circuit? Or is it different effect?
 

Offline StillTrying

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Re: MOSFET turned off - strange voltage drop
« Reply #11 on: July 05, 2018, 09:17:54 pm »
Yes, when the bridge rectifier and everything else has stopped conducting at 0.8V, it's just a floating voltage on the stray capacitances.
The first opto could be replaced by 3 resistors, one from each AC connection, and one to 0V.
« Last Edit: July 05, 2018, 09:20:18 pm by StillTrying »
.  That took much longer than I thought it would.
 


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