4051 + MOSFET troubles.

Other Hardware General Electronics
1

Hi guys, I was hoping to get some help driving some MOSFETs from a 4051 IC.

The circuit I have so far acts strangely, in that the MOSFET won't switch on unless I remove the connection/resistor between the gate and source (at which point its stays on regardless). The threshold voltage is 2-4V so that shouldn't be the problem, and when testing the mosfet directly it works as it should do.

Here are some images of what I am working with at the moment. I'm still pretty amateur, so if I am missing anything obvious I would really appreciate the help :.

I have simplified the layout to try and isolate the problem, so the 4051s address ports aren't wired, so that it outputs to y0.

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2

Yes its a floating gate - don't do that, drive the gate with some definite voltage.

Your analog multiplexer doesn't have anything connected to its select pins, so
they're floating too.

3

How come the gate is floating? The output of the 4051 at that pin is 5v, if I replace the MOSFET with an LED it lights, which is what's confusing me.

Also, the select pins are usually connected resulting in the same problem, I was just trying to simplify things, though I should have tied them to ground in the diagram sorry. The unused IO pins are left floating though, would this cause a problem?

4

An "MOSFET"?
Which?
Something in a IRF-5useless?

Something less than 1M? on that "pull down" - much less than - like 5kto10k?.
(That won't help with an IRF-5useless though.)

5

Why not use an 74HC238 instead of 4051?
4051 are made for analog signals, 74HC238 are digital.

And you MUST wire ALL inputs of an MOS device to high or low.
Input impedance are about 10^12 ohms and just breathing on them change status.

Pelle

6

The MOSFETs I have tried are a SSF6808 and a 3N80C.

Replacing the resistor with any value, including a 10k or even a piece of wire, will still not switch with the gate either high or low.

I will try using a digital multiplexer instead, the 4051 is just something I had to hand.

In the mean time, is there anything else I could try to make this work? I thought that the inputs of the MOSFETs were all high or low. I've tried grounding all the unused pins and reconnecting the selector pins to the arduino too, no luck.

EDIT: Actually, it seems to be working as a switch now, but the led is very dim, what is happening?

7

Neither of those MOSFETs are logic level and require 10V on the gate to switch
fully.

8

SSF6808 can switch almost 100A with only 3.5 volts.
From transfer characteristic in data sheet http://www.silikron.com/upload01//SSF6808%20V2.4.pdf

The other can switch a LED with only 5 volts, I think

Pelle

9

I agree, I don't think 10v is necessary for the SSF6808 because as I said it switches with 5v DC just fine outside of this circuit...

10

Keep in mind that the 4051 has a pretty high (Z to Y) resistance with a 5V arangement.

Do you have the Select lines, 9, 10, and 11 at Gnd.?

I may be wrong, but I think that the Gate should go to 3, and 13 should get connected to the input (in this case, +5)

11

Pelleplutt:
SSF6808 can switch almost 100A with only 3.5 volts.
From transfer characteristic in data sheet http://www.silikron.com/upload01//SSF6808%20V2.4.pdf

The other can switch a LED with only 5 volts, I think

Pelle

No, that's the curve for Vds = 10V, not for fully on. Calculate the resistances
and you'll see. The specified Rds(on) value is only quoted for 10V. Furthermore
the threshold voltage is quoted as 2--4V, so its clearly not logic level.

12

Thanks for the help guys, trial and error with the gate resistor and input voltage for the 4051 made the circuit work well enough. Grounding wasn't as big of an issue as I'd thought :smiley: