MOSFET turned off - strange voltage drop - Page 1
Forum section: electronics / beginners

author: tester43
author_rank: Regular Contributor
timestamp: July 03, 2018, 10:04:08 pm
content: 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?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 03, 2018, 10:12:56 pm
content: Put a pulldown resistor across the FWB output.Tim

author: tester43
author_rank: Regular Contributor
timestamp: July 03, 2018, 10:15:46 pm
content: Hi Tim,Thank you for super quick answer, it's around midnight at my placeBut.... as stupid as it might sound but... what is FWB?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 03, 2018, 10:21:33 pm
content: Full Wave Bridge

author: tester43
author_rank: Regular Contributor
timestamp: July 03, 2018, 10:26:02 pm
content: 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?

author: tester43
author_rank: Regular Contributor
timestamp: July 03, 2018, 10:40:34 pm
content: 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??

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 03, 2018, 11:12:32 pm
content: 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

author: tester43
author_rank: Regular Contributor
timestamp: July 04, 2018, 05:47:16 am
content: 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?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 04, 2018, 10:55:07 am
content: 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

author: tester43
author_rank: Regular Contributor
timestamp: July 04, 2018, 11:00:30 am
content: 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=viewas it's a part of this T12 tip:http://www.ptdreamer.com/wp-content/uploads/2018/02/Wiring-Diagrams.png

author: tester43
author_rank: Regular Contributor
timestamp: July 05, 2018, 09:29:57 am
content: 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?

author: StillTrying
author_rank: Super Contributor
timestamp: July 05, 2018, 09:17:54 pm
content: 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.