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Mosfet noob

kapytanhook

Sep 23, 2012
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Hello people, I have been stuck with this problem for weeks now and I am desperate.
I want to power a 12v motor (that pulls 300ma) with a mosfet.

I have purchased this mosfet:

http://www.produktinfo.conrad.com/datenblaetter/150000-174999/162873-da-01-en-IRLZ_34_N.pdf

I have a little board that outputs a 5v signal.
I have a 12v transformer.

This is what I tried among other stuff, but it is not working.

board.png


Help would REALLY be appreciated,
I am a programmer and working on a hobby projects, hardware is not my strong point yet I
built an entire robotic frame from mecanos and 50$ worth of brand new parts.
And it is just sitting there because I can not figure out how to use this mosfet.

Thanks for reading!
 
Last edited:

Harald Kapp

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Make the following changes:
Connect source of the transistor to -12V
Remove the connection of the motor to -12V, connect it to +12V instead..

This is an N-channel MOSFET, therefore source goes to the negative potential of the 12V source.
Do not swap the 12 V voltage source since the pull down resistor from the gate correctly goes to "-".
 
Last edited:

kapytanhook

Sep 23, 2012
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Thank you for your quick reply, yet it still does not seam to work at all. I have this setup now:

board2.png


But the mosfets do not seem to be responding to the 5v signal.

I have multiple ones and i am pretty sure i did not bust them.
 

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BobK

Jan 5, 2010
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That circuit should work. Very strange, though: I scrolled through the datasheet 3 times and cannot find anywhere where it specifies which pin is which. How did you decide which pins are the s d and g?

Bob
 

kapytanhook

Sep 23, 2012
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That circuit should work. Very strange, though: I scrolled through the datasheet 3 times and cannot find anywhere where it specifies which pin is which. How did you decide which pins are the s d and g?

Bob

Took me a while to find this too, but.

All the way at the bottom there is this:

L E A D A S S IG N M E N T S
1 - G A T E
2 - D R A I N
3 - S O U R C E
4 - D R A I N
 

MrEE

Apr 13, 2012
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Make sure you have applied gate voltage (I see you have 5V, so that should work)
then measure the following voltages:
1- Drain to gnd
2- Gate to gnd
Is the FET getting any warm or hot?
 

kapytanhook

Sep 23, 2012
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I did these measurements:

Drain to ground: 12.4v
Gate to ground: 1.94v

board2measure.png

And there is 2.9 milli amps flowing into the gate yet it does not open at all
They specify gate threshold voltage min 1v and max 2v. Also does not heat up at all.

I also double checked the type printed on the mosfet and it is indeed the one on the spec sheet so no mistake there (IRLZ34N)

Should i just go buy other ones or does one of you skilled people see a major flaw in my setup?

Thanks!
 

MrEE

Apr 13, 2012
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There should be no gate current. This is the main feature of mosfets. You may have a damaged FET. Also the gate voltage should be 5V. The 5k pulldown resistor only draws 1mA from the 5V source. I don't know why you only have 1.94V. Check the wiring of your circuit. Again, you may have a bad mosfet. Do you have another one to try? Also the drain voltage being 12.4 indicates the the mosfet is not turning on at all. I assume your voltage supply is 12.4 as well.
 

kapytanhook

Sep 23, 2012
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I have a few other fets, but they all react the same, i must have fried them all in the same way I am afraid. I know for a fact I burned 1 of 4 since it hooked it up as a normal n-channel mosfet (Not a logic one).

The voltage is 1.98 there since i have a 5k pull-down resistor and the gate for some reason has low resistance and an amp flow as I mentioned before.

It is all very strange I burned 1 fet using high gate voltage and I think i burned the other fets having the source and the drain wrong way around. (as mentioned start of the post).

Anyway thanks for the feedback, guess it is back to the shop to blow another 10$ on fets.
 

kapytanhook

Sep 23, 2012
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HOLY **** IT WORKS

I started dismanteling my setup, i took out the pulldown resistor and it works,

Obviously i tried without before, but back then my wiring must have been wrong all together! YESSSS

You guys rule!

If you ever need anything programmed just pm me XD
 

BobK

Jan 5, 2010
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Are you sure the pulldown was 5K? It sounds like it was a bit smaller. And 5K is not a standard resistor value.

Bob
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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A 5k6 resistor. Did you measure it in circuit? That would probably explain the difference (looking at the circuit again, probably not).
 

BobK

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What is the 5V source then? It must be high impedance to have a 5.6K resistor pull it down to 1.9V.

Bob
 

BobK

Jan 5, 2010
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Aha, the outputs are open collector. That means you need a resistor from the output to a 5V source, it does not produce any voltage itself. And you do not need the pulldown resitor.

Bob
 

kapytanhook

Sep 23, 2012
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There are 2 sets of analog exits. PWM open collectors and 5 volt.

thiscard.jpg.

But the problem is resolved :)

My final circuit for my logic level N channel mosfet:

board3.png

Thanks you all once again.
 

BobK

Jan 5, 2010
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So you were connecting to a PWM output? If so, then it would read 5V * duty_cycle when read with a multimeter, which explains the 1.9V.

Bob
 

kapytanhook

Sep 23, 2012
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No i wasn't, i only used the DAC, GRN unconnected that would read 5v spot on. Yet with my misplaced 5.6k resistor it read 1.9V and was unable to switch the working mosfet. I did some previous tests with a broken one i was not aware of (like i explained in my previous posts). All in all it was a big mess.
 

BobK

Jan 5, 2010
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Okay, now the question is why are you using a DAC output to drive the gate? Are you planning on varying the voltage to control the speed of the motor? If so, this is not a good idea.

Bob
 
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