back to back mosfet circuit not turning on

Thread Starter

super7800

Joined Apr 3, 2019
36
here is my circuit. the FET-Control flag is connected to digital pin 7 of an Arduino, and the Shunt_Input Is Connected To AC-. (24VAC) The Arduino is outputting the 5v signal (4.7v), but for some reason the circuit is not activating (i.e. the heater is not heating up). I know I must be overlooking something stupid (either in my circuit or component selection), but cant seem to figure out what that is. Any ideas about what my problem is are appreciated. thanks.

APV1121 Digikey
MOSFET Digikey

back to back.PNG
 

Alec_t

Joined Sep 17, 2013
12,153
The opto's output has minimum ratings of 6V and 5uA, which may well not be enough to turn the FETs fully on fast enough, given that each has a rated gate charge of ~100nC. At 5uA, 200nC takes 40mS to charge.
R6 will slow down the turn on/off, and seems to be wrongly positioned (as it affects only one gate).
 

Thread Starter

super7800

Joined Apr 3, 2019
36
The opto's output has minimum ratings of 6V and 5uA, which may well not be enough to turn the FETs fully on fast enough, given that each has a rated gate charge of ~100nC. At 5uA, 200nC takes 40mS to charge.
R6 will slow down the turn on/off, and seems to be wrongly positioned (as it affects only one gate).
R6 was to prevent oscillation, but i could remove it. the opto is being fed basically a constant 5v from the digital pin. if the opto is the wrong type, would there be any way to fix it using the current hardware, or would i need to spec a different opto? Thanks.
 

Thread Starter

super7800

Joined Apr 3, 2019
36
ok well this is embarrassing, but it works now. i was using alligator clips, and one of them was faulty, thus all my measurements where wrong. i rechecked the connections again, but this time hard connected everything and it heats up now. but anyways thanks. Btw could you point me to an article explaining more about what you talked about in post #2? id love to learn more about properly selecting things in the future, thanks.
 

crutschow

Joined Mar 14, 2008
27,426
id love to learn more about properly selecting things in the future
One of the most important things is reading a data sheet.
If you don't understand how to read the data sheets for the components you are using then you won't be able to properly select and design with them.
 
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