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Topic: IRF3205 circuit (Read 833 times) previous topic - next topic

JCA34F

Did you do those voltage checks? Earlier you said your coil resistance was 5Ω and you had 5A through it. That would take 25V. Did you put 25V on the MOSFET's Gate?

tjones9163

20V Vgs is in the ABSOLUTE MAXIMUM table, if you put more than that voltage on the gate the transistor may already be damaged.
Do this: Put your black voltmeter probe on 12V-, red probe on top of coil. What voltage with button pressed and not pressed?
Put red probe on bottom of coil. What voltage with button pressed and not pressed?
Ok, i just set my benchtop power supply to 10v. I am using the same 10v line through the push button to gate

Top coil (side closer to 12v+)
button pressed - 10.2v
not pressed- 9v

bottom coil ( side attached to drain)
button pressed- 10.2v
not pressed- 0.8v

tjones9163

Did you do those voltage checks? Earlier you said your coil resistance was 5Ω and you had 5A through it. That would take 25V. Did you put 25V on the MOSFET's Gate?
no the max I put was 20v, that is basic OHMS law right 25/5 = 5 amps

So that is my problem I believe, I am sharing my gate voltage with my power supply, but since it took a higher voltage going across the Mosfet to achieve 5 amps, i should probably power my gate with a separate 10v supply with a common ground, so i can not damage my MOSFET and still be able to use a higher voltage to achieve my acquired 5amp draw.

I believe that was my problem, i am right?

JCA34F

You either have a normally closed button or the MOSFET's damaged or connected wrong. Do you  have another MOSFET?
Quote
button pressed - 10.2v
Does that mean MINUS 10.2V?
With the right components the values should have been closer to:

Top coil (side closer to 12v+)
button pressed , > 9V
not pressed, 10.2

bottom coil ( side attached to drain)
button pressed, < 1V
not pressed, 10.2v

tjones9163

You either have a normally closed button or the MOSFET's damaged or connected wrong. Do you  have another MOSFET?Does that mean MINUS 10.2V?
With the right components the values should have been closer to:

Top coil (side closer to 12v+)
button pressed , > 9V
not pressed, 10.2

bottom coil ( side attached to drain)
button pressed, < 1V
not pressed, 10.2v
I will double-check my voltages
nut am i correct in what i said in comment #17?
thanks

tjones9163

I feel like im really close to making sense of this, can someone please tell me if my comment #17 is correct?
Thanks

tjones9163

I feel like im really close to making sense of this, can someone please tell me if my comment #17 is correct?
Thanks

MarkT

no the max I put was 20v, that is basic OHMS law right 25/5 = 5 amps

So that is my problem I believe, I am sharing my gate voltage with my power supply, but since it took a higher voltage going across the Mosfet to achieve 5 amps, i should probably power my gate with a separate 10v supply with a common ground, so i can not damage my MOSFET and still be able to use a higher voltage to achieve my acquired 5amp draw.

I believe that was my problem, i am right?
Quote possibly.  Was the supply decoupled?  Switching large currents can cause voltage spikes on the supply due to stray inductance of the wiring (or even the coil itself if the free-wheel diode isn't doing its job).

12V is the standard gate-drive voltage for standard MOSFETs.   They are designed to work with 10V or more, 12V gives a bit of headroom and is a commonly used voltage.

The gate-source voltage limit mustn't be exceeded even for a fraction of a microsecond, the device will be damaged or completely fail in a very rapid manner.
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

tjones9163

Quote possibly.  Was the supply decoupled?  Switching large currents can cause voltage spikes on the supply due to stray inductance of the wiring (or even the coil itself if the free-wheel diode isn't doing its job).

12V is the standard gate-drive voltage for standard MOSFETs.   They are designed to work with 10V or more, 12V gives a bit of headroom and is a commonly used voltage.

The gate-source voltage limit mustn't be exceeded even for a fraction of a microsecond, the device will be damaged or completely fail in a very rapid manner.
Thank you for the reply,
what do you mean by decoupled?

I have the push-button that is attached to Gate and the other side straight to my benchtops power supplys red alligator clip that is also powering my circuit- as shown in my schematic.

Thanks again

MarkT

#24
Nov 25, 2019, 04:03 am Last Edit: Nov 25, 2019, 04:04 am by MarkT
Ah, loose flying leads across the bench, plenty of stray inductance then, that's probably done it.  It wires are all spread out in space with large loop areas enclosed, they have significant stray inductance (its a one turn large coil).

If you search here for GRumpy Mike's decoupling tutorial I think you'll achieve enlightenment about that.

[ actually its a little hard to find: http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html ]
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

tjones9163

Ah, loose flying leads across the bench, plenty of stray inductance then, that's probably done it.  It wires are all spread out in space with large loop areas enclosed, they have significant stray inductance (its a one turn large coil).

If you search here for GRumpy Mike's decoupling tutorial I think you'll achieve enlightenment about that.

[ actually its a little hard to find: http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html ]
You the man Mark. Ill look into it

tjones9163

Ah, loose flying leads across the bench, plenty of stray inductance then, that's probably done it.  It wires are all spread out in space with large loop areas enclosed, they have significant stray inductance (its a one turn large coil).

If you search here for GRumpy Mike's decoupling tutorial I think you'll achieve enlightenment about that.

[ actually its a little hard to find: http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html ]
From what i read i believe that i should put a capacitor with the (+) connected to the Drain and the (-) connected to the Emitter?

but I could be wrong, the only other place that i could think is right across my negative and positive clips from my power supply?

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