Buz11 MOSFET question

Thread Starter

fredric58

Joined Nov 28, 2014
252
Good day everyone. I am using a buz11 enhanced mode N channel MOSFET in a project as a high side switch. power supply is 6v, This "switch" powers up a micro servo. it works very well, the servo takes a signal to power up and sweep from an attiny85. there are several sweeps with a delay between each sweep. depending on the amount os sweep, mAs range from 3 to 15 during the sweeps on my DMM. During the delays the DMM goes to OL. However, there is a 5 minute pause/delay before it cycles the next time through the sweeps. The problem is that during that delay (5 minutes) I think the DMM should be OL, but it seems to be leaking .28mAs during the 5 minute pause between sweep cycles. Not to good as it is a battery powered project. Could it be some residual capacitance in the MOSFET allowing the the .28mAs to trickle through. Would a pull down resister be of any help? How many Ohms?

Thanks, Fred
 

Papabravo

Joined Feb 24, 2006
20,985
IMHO using an N-channel device as a high side switch is problematical. We do not have a schematic diagram which is a glaring omission from your original post. Do you really think it is possible for us to help based on that inadequate verbal description of your circuit.
 

crutschow

Joined Mar 14, 2008
34,002
Are you accounting for the current the meter takes when connected to the voltage?
Typical DVMs have an input resistance of 10 megohms.
 

Thread Starter

fredric58

Joined Nov 28, 2014
252
I'll see if I can clarify it

"the servo takes a signal to power up and sweep from an attiny85"

a signal to power up the servo comes from pin 1 (PWM 255) 100% duty cycle and goes to the gate of the switch (the MOSFET), the servo is now ON. Once the servo is on.........

a signal begins from pin 0 and tells the servo where to go/what to do. The servo sweeps around with delays between the sweeps. During "those" delays the DMM is OL. (an open circuit)

When it stops its loop (C++) for it's 5 minute pause/break, instead of the DMM reading OL, it reads .28mAs for those 5 minutes. Keep in mind, that right before the 5 minute delay/break, pin 1 (that powers the MOSFET that turns on the servo) is written to LOW (0volts). Theoretically, the MOSFET should be open. (OFF) It appears however that it is not! It's like a faucet that didn't quite get turned off all the way. So every 8 hours i will lose 2.24mAs.

So once again......Could it be some residual capacitance in the MOSFET (there is 0 volts at the gate)allowing the the .28mAs to trickle through. Would a pull down resister be of any help? How many Ohms?

I"m kinda really old and have no idea what IMHO means.....
As for problematical, did you mean to say, "in your opinion"? There are many arguments for high side vs. low side. N vs. P, application, expected results and one of my favorites, what's going happen if.......isn't taken into consideration.

Servo schematic.jpg
 
Last edited:

Papabravo

Joined Feb 24, 2006
20,985
I'll see if I can clarify it

"the servo takes a signal to power up and sweep from an attiny85"

a signal to power up the servo comes from pin 1 (PWM 255) 100% duty cycle and goes to the gate of the switch (the MOSFET), the servo is now ON. Once the servo is on.........

a signal begins from pin 0 and tells the servo where to go/what to do. The servo sweeps around with delays between the sweeps. During "those" delays the DMM is OL. (an open circuit) The switch is turning ON and OFF.

When it stops its loop (C++) for it's 5 minute pause/break, instead of the DMM reading OL, it reads .28mAs for those 5 minutes. Keep in mind, that right before the 5 minute delay/break, pin 1 (that powers the MOSFET that turns on the servo) is written to LOW (0volts). Theoretically, the MOSFET should be open. (OFF) It appears however that it is not! It's like a faucet that didn't quite get turned off all the way. So every 8 hours i will lose 2.24mAs.

So once again......Could it be some residual capacitance in the MOSFET (there is 0 volts at the gate)allowing the the .28mAs to trickle through. Would a pull down resister be of any help? How many Ohms?

I"m kinda really old and have no idea what IMHO means.....
As for problematical, did you mean to say, "in your opinion"? There are many arguments for high side vs. low side. N vs. P, application, expected results and one of my favorites, what's going happen if.......isn't taken into consideration.

View attachment 296369
I bet I'm older than you and IMHO means "In My Humble Opinion". So, already you are misleading us by saying the configuration is a high-side switch when it is not, and you may not be aware of the difference between an N-channel MOSFET and an NPN Bipolar Junction Transistor, but you chose the symbol for the latter. There is not enough detail on your drawing to draw much in the way of conclusions.
 

Poyntat

Joined May 24, 2022
60
I’m a little confused with your explanation.
You say that your meter reads OL during the pauses between sweeps.
On most DMMs this indicates ‘Overload’.
The current is out of range of the DMMs set range.
Also it would be helpful if you could indicate where in the circuit you have connected your DMM.
 

BobTPH

Joined Jun 5, 2013
8,655
From your description we have no idea what you are measuring with your DMM. Show us how it is connected. When there is 0 current, the DMM will read 0.

Why is there a diode between the output output and the gate? That means there is no current path to turn the MOSFET off.
 
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