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New guy here with a MOSFET question (as in recommend me one)...

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kpatz

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I've dabbled in electronics on and off ever since I was 12. Which means for 30 years now. That doesn't mean I'm an expert by any means...

Most recently I've started to delve into microcontrollers. For now, BASIC Stamps, since I have a couple handy, but down the road, will probably go the PIC route.

So where does my MOSFET question come in? I'd like to build a circuit that can be turned on and off using a momentary push-button. The idea being, the circuit being switched by a MOSFET, when I initially push the button, the button would apply voltage to the gate, turning the circuit on. Once the circuit switches on, the microcontroller would latch the MOSFET on using one of its output pins. Then to turn off, pressing the button again would tell the microcontroller to turn off the output pin controlling the MOSFET and wait for the button to be released, and the circuit will turn off.

I picked up a IRF510 at RadioShack (I wish they were like in the late 70s/early 80s when they carried lots more parts 'n stuff), and have been studying web posts and the datasheets, and I'm seeing that this MOSFET allegedly doesn't turn "fully" on unless the gate voltage is over 10 volts. :eek: The problems being, the microcontroller runs at 5 volts, and the entire circuit is being run from a 9 volt battery.

So, long story really short, what MOSFET should I be using. I anticipate the circuit won't draw much more than 200 mA peak, so I don't need a super high power one. Just one that reaches saturation at <5Vgs, I think is the term I'm looking for. (And from playing with CMOS ICs and op-amps, I always figured all MOSFETs required very low voltages to turn on...)

Also, where do you guys generally prefer to go for your parts 'n stuff? I'm guessing it will have to be online, since RatShack's part selection is dwindling and I have yet to find any semiconductors at the local pharmacy...
 

ericgibbs

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kpatz

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Thanks... another thought I had was using a 2n2222 or similar switching transistor instead of a MOSFET. It certainly wouldn't need as much voltage/current to turn on to saturation, but I'm not sure how much voltage drop in the circuit I would see. Would that be collector-emitter saturation voltage [Vce(sat)]?
 

crutschow

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Thanks... another thought I had was using a 2n2222 or similar switching transistor instead of a MOSFET. It certainly wouldn't need as much voltage/current to turn on to saturation, but I'm not sure how much voltage drop in the circuit I would see. Would that be collector-emitter saturation voltage [Vce(sat)]?
The "on" voltage drop of a bipolar transistor is indeed the collector-emitter saturation voltage. Typically it's a few tenths of a volt. To achieve the lowest saturation voltage you should drive the base with no less than 1/10 of the maximum collector load current.

To better drive the MOSFET you could also use the bipolar to control the gate of the MOSFET. Just connect the collector of the bipolar to the 9V supply through a resistor (say 10kΩ or so) and also connect the collector to the MOSFET gate. (Of course this provides a signal inversion: when the bipolar is turned on, the MOSFET will be off). That will apply 9V to the MOSFET gate, which should turn it on quite well.
 
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kpatz

New Member
Thanks for the tips guys. Since this circuit is relatively low powered, I'll try just using a bipolar transistor for switching. If that causes too much voltage drop, I'll try a bipolar driving the MOSFET (still, fewer components is always better IMHO). If all else fails, I'll just use the tried 'n true toggle switch instead. ;)

What I'm building is a EMF meter, using a Basic Stamp microcontroller to control the audio and LED bargraph display. I thought it would be cool to include a momentary power on/off button, but I can forego that feature for the initial prototype.

In the next week or so I'll post a schematic, so I can get input on how to make it work better. I'm waiting for some parts to come in the mail so I can finish tweaking it on the breadboard, and then I'll start moving it over to a more "permanent" setup.
 

mneary

New Member
Although it's tricky to rely on 'typical' specs, you'll notice that a typical IRF510 has an ON resistance of just over one ohm with 5V on the gate. In your application, this might be good enough. Just don't go into commercial production and try to get away with this.

At only 200 mA, you could also consider 2N7000 or 2N7002. The same cautions apply.
 

kpatz

New Member
I did some experimenting last night.

I inserted the MOSFET into the circuit's power. When I put 9V on the gate, the circuit turns on fine. 5V into the gate, the LEDs were dim, so I get a voltage loss there. I haven't taken voltage drop measurements yet. I'll also measure current draw. I said 200 mA but it's probably a lot less.

Getting things set up so I could control the circuit's power with the MOSFET, *and* be able to get the µC (microcontroller) to maintain or kill power to the MOSFET's gate was trickier. I wound up using a PNP transistor to drive the MOSFET gate. The base of the PNP is tied to the power button through a resistor, and also to an output pin of the µC through a diode (which is there so the power button doesn't feed 9V into the µC pin). When the circuit powers up, the µC sets the pin to 0 so it sinks current through the diode and PNP transistor to keep it, and in turn, the MOSFET, on. When the µC wants to kill power, it sets the pin to 1, and a B-E pullup resistor cuts off the PNP, which in turn cuts off the MOSFET (which has a G-S pulldown resistor).

The next thing I need to do is get the same power button (the one that turns on the MOSFET to power-up the circuit) to also signal the µC on an input pin, so I can use the same button for power off. I'll probably do this through a voltage divider (to reduce the 9v coming off the button down to <5V) and pullup resistor. I might switch from the PNP to a NPN in emitter follower configuration to see if it makes things simpler by sourcing instead of sinking base current. I may also try just a bipolar transistor, foregoing the MOSFET entirely, since this is a low power circuit.

I'll have to start sketching a schematic. Is there any cheap/free software for drawing schematics quickly? I typically use Paint or Paint Shop Pro but it's a bit of a pain.
 

crutschow

Well-Known Member
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Try TinyCAD.
 

kpatz

New Member
Got my DMM hooked up to the circuit with the MOSFET turned on. Idle, the voltage drop is a measly 9.7 mV, that's right, millivolts. With all the LEDs lit, the drop is around 25 mV. This is with the MOSFET gate at 9V, well, 7.8V since my 9V battery is getting tired... let me try a fresh one... ok... 10 mV idle and 29.5 mV w/all LEDs lit.

As for current draw, with a fresh 9V battery, 26.16 mA idle, 71 mA w/all LEDs lit. I might bump up the current-limiting resistors on the LEDs to drop the current draw some.

Not to shabby me thinks!
 
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