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Old 16th December 2003, 07:06 PM   #1
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Question Where to buy a low gate voltage p channel MOSFET or make an alternative?

Ok, here's the deal. I have some nice electronic gauges for my car. These gauges have adjustable preset levels at which a warning LED is activated (for example, if my exhaust gas temperature reaches 1600F a LED comes on).

A LED is nice, but I want a loud Piezo buzzer (about 80 dB) to go off when the LED turns on. There is a voltage drop of 1.6V across the LED. I am thinking the gate of a p-channel MOSFET should be hooked to the output side of the LED.

This would put my source voltage at 12V (the car's voltage), and the gate voltage at 10.4V, thus allowing the MOSFET to conduct and the Piezo buzzer to go off.

I can't find a place to buy p-channel MOSFETs online, and Radio Shack doesn't have them. I would like to know if there's any place online to buy these MOSFETs and other small electronic parts.

Also, any simple workarounds using an n-channel MOSFET? I could put a resistor before my MOSFET source to drop my source voltage and use an n-channel MOSFET, but then my Piezo buzzer would only have 8V to 10V of input and would not be as loud as I hoped.

Thanks for any help...
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Old 16th December 2003, 07:20 PM   #2
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My first question I suppose, would be to ask why you insist on using a mosfet for this application?

It seems to me that a bipolar transistor would be much better suited in this case. You don't need the ultra-low Rds(on) of a mosfet in this application, nor do you need its fast switching times. Why not use a bipolar? A small high-Hfe bipolar transistor should do just fine and not require enough base current to bother the rest of the circuit.
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Old 16th December 2003, 07:22 PM   #3
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Also, if you're worried about the voltage drop across the bipolar transistor taking too much voltage from the piezo buzzer and making it not loud enough, just use the transistor to turn on a relay - use a small 12V SPST reed relay, and you'll be good to go with no voltage lost across your piezo buzzer. Don't forget the flyback diode across the relay coil, of course, lest you be forever replacing your transistor.
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Old 16th December 2003, 07:34 PM   #4
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I was concerned that a bipolar transistor would require too much current and bother the rest of the gauge circuit... Can one get bipolar transistors that are normally off? I have a small relay but it takes 9.6V to turn on. Thanks for the advice though, I'm going to look at using a BPT tonight.

Any suggestions on where to buy these kinds of things online?
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Old 16th December 2003, 07:45 PM   #5
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I'd say that the 200mV or so emitter-collector saturation voltage of a saturated PNP switch is rather negligible compared to the 12V supply voltage. A PNP with a resistor to limit the base current should work well without a relay.
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Old 16th December 2003, 09:56 PM   #6
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The type of bipolar transistor you're looking at here is so cheap that you're better off buying them locally. Most places have a minimum order of $8-10 or so - the small TO-92-style PNP transistor you're looking at costs on the order of $0.05 in quantities of 100.

You can go to radio shack and get a package of 2 or 5 or so for $2 or so. It's highway robbery, but you pay for the convenience and no minimum order.

Virtually *any* PNP transistor will work fine here, although if you're worried about the base drawing too much current, get one with as high of an Hfe as you can find. TO-92 devices are easily found with an Hfe well into the hundreds.
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Old 16th December 2003, 10:15 PM   #7
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Quote:
Originally posted by ThingyNess
The type of bipolar transistor you're looking at here is so cheap that you're better off buying them locally. Most places have a minimum order of $8-10 or so - the small TO-92-style PNP transistor you're looking at costs on the order of $0.05 in quantities of 100.

You can go to radio shack and get a package of 2 or 5 or so for $2 or so. It's highway robbery, but you pay for the convenience and no minimum order.

Virtually *any* PNP transistor will work fine here, although if you're worried about the base drawing too much current, get one with as high of an Hfe as you can find. TO-92 devices are easily found with an Hfe well into the hundreds.
Any suggestions for online retailers for the future though?

I'll look for the highest Hfe I can find. Thanks again.
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