Alarm buzzer through MOSFET will not work .

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rogerm123

New Member
I know this is going to be a generalized discussion but I am trying to get a read on whether my thoughts on the problem could be valid.
First of all voltage is 4.75V.
20M2 B.6 pin is outputting to MOSFET gate.
MOSFET is switching on. With no load through Drain/ Source the voltage is 4.75V when on and 0V when off.
MOSFET is a ??44Z?? power mosfet so I know it will handle the amps
MOSFET source is tied to ground. Drain goes to buzzer neg. wire. buzzer pos. wire goes to +4.75V rail.
Note: 1-If I replace the buzzer with a LED it lights up as it should-MOSFET is switching
2-If I hook the buzzer neg. lead to ground and the positive lead to the +4.75V rail it buzzes as it is supposed to-Buzzer not broken.
3- Buzzer is rated for 3-12V DC
4- Buzzer resistance is like 32Mohm. There is no more info on this cheap buzzer in our universe.
So my thoughts are I am starving the buzzer for amps via the Gate/Source voltage relationship for this MOSFET(used what I had). Could a Logic MOSFET solve this issue?
 
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papaof2

Senior Member
Possibly not starved for amps but for volts? Do you have a voltmeter? What is the voltage across the buzzer when the MOSFET is on?
Possibly the buzzer's effective resistance is so high the MOSFET can't be turned on?
Put a white LED across the buzzer and see what happens when the MOSFET is turned on.
 
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tmfkam

Senior Member
Are you certain it is a buzzer and not just a transducer in a housing? I've found that a 12V piezo buzzer will buzz very loudly being driven directly from a processor pin at 4.5V (ish). The buzzers are also polarity aware and don't buzz if fitted with incorrect polarity.
 
Technoman

Technoman

Senior Member
2-If I hook the buzzer neg. lead to ground and the positive lead to the +4.75V rail it buzzes as it is supposed to
Click to expand...
Seems it is a buzzer, a piezo would not make any noise on DC. The high resistance is may due to the electronics components inside.
Your connections are fine. The MOSFET is may be not turned on ; the answer lie in ??44Z??. IRF? IRFL?
 
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rogerm123

New Member
I checked the voltage across the the buzzer: -.237V when the MOSFET gate has 0V and same -.237V when the Gate has 4.75V from the 20M2 output pin.
swapped buzzer leads and same thing.
I am thinking to trash buzzer and buy a piezo unless there is an alternate transistor or reed switch that will work.
I looked at the data sheet and looks like It is at the very beginning of the gate/source curve.
 
Technoman

Technoman

Senior Member
What is the exact label on your MOSFET? Is your transistor ok?
... very beginning of the gate/source curve
Click to expand...
May be the voltage is too low to turn it on ; some MOSFET don't properly work at logic level.
 
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tmfkam

Senior Member
Mosfet is fitted as a LOW side device is it?

Why not drive the buzzer directly from the PicAxe pin?
 
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rogerm123

New Member
tmfkam said:
Mosfet is fitted as a LOW side device is it?

Why not drive the buzzer directly from the PicAxe pin?
Click to expand...
I am just not sure of the amp draw. I assumed it would be more than the output pin could handle. You make a good point though-I may try to test it.
 
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