EEVblog Electronics Community Forum
A Free & Open Forum For Electronics Enthusiasts & Professionals
Welcome,
Guest
. Please
login
or
register
.
Did you miss your
activation email
?
1 Hour
1 Day
1 Week
1 Month
Forever
Login with username, password and session length
This topic
This board
Entire forum
Google
Bing
Home
Help
Search
About us
Links
Login
Register
EEVblog Electronics Community Forum
»
Electronics
»
Beginners
»
MOSFET turned off - strange voltage drop
« previous
next »
Print
Search
Pages: [
1
]
Go Down
Author
Topic: MOSFET turned off - strange voltage drop (Read 1918 times)
0 Members and 1 Guest are viewing this topic.
tester43
Regular Contributor
Posts: 90
Country:
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?
Logged
T3sl4co1l
Super Contributor
Posts: 22436
Country:
Expert, Analog Electronics, PCB Layout, EMC
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
Logged
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life? Send me a message!
The following users thanked this post:
tester43
tester43
Regular Contributor
Posts: 90
Country:
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?
Logged
T3sl4co1l
Super Contributor
Posts: 22436
Country:
Expert, Analog Electronics, PCB Layout, EMC
Re: MOSFET turned off - strange voltage drop
«
Reply #3 on:
July 03, 2018, 10:21:33 pm »
Full Wave Bridge [Rectifier]
Logged
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life? Send me a message!
tester43
Regular Contributor
Posts: 90
Country:
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?
Logged
tester43
Regular Contributor
Posts: 90
Country:
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??
Logged
T3sl4co1l
Super Contributor
Posts: 22436
Country:
Expert, Analog Electronics, PCB Layout, EMC
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
Logged
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life? Send me a message!
tester43
Regular Contributor
Posts: 90
Country:
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
»
Logged
T3sl4co1l
Super Contributor
Posts: 22436
Country:
Expert, Analog Electronics, PCB Layout, EMC
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
Logged
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life? Send me a message!
tester43
Regular Contributor
Posts: 90
Country:
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
Logged
tester43
Regular Contributor
Posts: 90
Country:
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?
Logged
StillTrying
Super Contributor
Posts: 2850
Country:
Country: Broken Britain
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
»
Logged
.
That took
much
longer than I thought it would.
Print
Search
Pages: [
1
]
Go Up
« previous
next »
Share me
Smf
EEVblog Electronics Community Forum
»
Electronics
»
Beginners
»
MOSFET turned off - strange voltage drop
There was an error while thanking
Thanking...
EEVblog Main Site
EEVblog on Youtube
EEVblog on Twitter
EEVblog on Facebook
EEVblog on Odysee