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Basic Electronics » Connecting Mosfet

March 19, 2015
by carlhako
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Hey Guys Looks pretty quiet around here and I am having some trouble so opening this thread.

Its been a few years since I have played with some mosfets. I now have a 2 year old that loves buttons and lights as probably most 2 year olds. So to inspire him a little I have been wiring up some leds on a breadboard and a button that will simple switch them on/off via a atmega168.

I have wired up the leds 4 in parrallel and to the mosfet. The problem I am having is the leds glow slightly when the gate is connected to ground. Mosfet connections Source: Ground Gate: Ground Drain: connected to cathode on leds.

I have tried 3 mosfets brand new out of the anti static bag although probably a good 2-3years old now. I get varying amounts of brightness on each one when gate is connected to ground.

pics

March 19, 2015
by carlhako
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the formatting went a little weird. The atmega is not hooked up or anything yet I am just trying to get the mosfet working. Mosfet connections Source: Ground

Gate: Ground

Drain: connected to cathode on leds.

March 20, 2015
by scootergarrett
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I think I could use a circuit diagram to better understand what's going on the orange wire would be your input correct? from what I remember that should have a resistor on it. What are the resistor values going into the LED? Is the objective to run 4 LED's from one MCU pin? I don't think the age of the Mosfet would do any harm.

March 21, 2015
by carlhako
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Yes the orange wire is my input. A resistor is generally used as a pulldown so when the mcu is off or booting for example and not pulling that pin to ground and its left floating its not turned on.

I tried some more mosfets and the next one worked. I cant explain why I had 3 faulty ones. Ive been playing with the circuit a bit more and working perfectly. Any time I switch back to one of the bad mosfets they start leaking current straight away.

Maybe the mosfets were not brand new I dont recall using them in the past but who knows.

Things have gotten alot cheaper since I got my nerdkit. I bought 100 2n7000's on ebay for $5 shipped!

March 27, 2015
by BobaMosfet
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carlhako-

Food for thought. I wouldn't necessarily assume the MOSFETs were bad because they didn't behave as expected. They are easy to specifically test. Normally, no two MOSFETs are going to behave exactly alike in the TO-92 package, and unfortunately (if you look carefully at the spec), you're at the low-end and possibly (depending on circuit) below the 'guaranteed' operating range of the 2N7000.

I doubt the MOSFET is able to allow more a few mA through Vds at your voltage level. I would expect to see an LED intensity variation.

Also-- are those blue resistors actually blue? If so, are you sure they are resistors? it makes me think they are either 1%, milspec, or perhaps are actually axial inductors which would provide no resistance at all. Ohmeter test can confirm.

BM

April 06, 2015
by carlhako
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All the leds are the same brightness I didnt word that correctly, I get a brightness variation when changing the mosfet over.

The 2n7000 mosfet I am using are the same as the ones provided with the nerdkit so should be within spec switching them with 5volt?

The blue resistors are bulk ones i bought off ebay the same as the white leds which I was using for a christmas lighting project I never got around to finishing so I have 100's of them. I just checked the colours they are 570ohm 1% your right.

April 06, 2015
by BobaMosfet
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carlhako-

The 2n7000 MOSFETs (like most MOSFETs) don't get close to fully transconducting until about Vgs = 10+VDC. The key parameters on the datasheet, in this case, are Vgs and Rds. Per the datasheet at about 10 volts, the resistance across Vds is only 1.2 Ohms. As Vgs drops, Id drops, and Rds rises.

You can still use it as Vgs(th) starts at about .8VDC, but again it won't be fully conducting, will only allow Id of a very little current at logic levels on Vgs.

BM

April 08, 2015
by Noter
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I'd say you have tested them in your led circuit and that's good enough, I would pitch the leaky mosfets ... what else would you ever want them for anyway? If they didn't come bad in the package then probably you got them a little too hot early on in your nerdkit days. Love those eBay prices!

April 09, 2015
by BobaMosfet
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carlhako-

Any more parts you want to trash untested, I'm happy to send you SASE so you can send them to me. The parts may not cost much, but if they are actually good, I'm sure I can use them, and I don't mind taking the time to test them.

Just let me know.

BM

April 10, 2015
by Noter
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BM, even if they didn't leak and were somewhat usable that would still be 15 cents worth mosfets you'd be buying for the price of a stamp which I believe is 49 cents.

If you're really serious I'll be glad to send my junk parts to you. Not that I have very many but, you know, stuff happens sometimes. I'll start saving them for you if you wish.

Just let me know.

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