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Finding a MOSFET

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2007-2009 » Archive through 28 November, 2008 » Finding a MOSFET « Previous Next »

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amr_bekhit
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Username: amr_bekhit

Post Number: 381
Registered: 06-2005


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Posted on Tuesday, 25 November, 2008 - 10:47 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hello all,

I'm having trouble finding a MOSFET which matches my specs. I've got a DC input that varies from about 3V to 24V and I'd like that voltage the turn on a MOSFET which will in turn give a signal to a PIC.

Here's the circuit diagram of what I plan to do (rotated to fit within 450x600 limit - why can't is be the other way around??):

Switching circuit

The best match I've found so far is the 2N7002 - this has a Vgs(th) value of about 2V, so it will switch at the bottom of my input range. Unforunately, the Vgs max value for the transistor is 20V, so it won't be able to take the 24V max input. I was wondering if anyone knew of an alternative?

Ideally I'd like to use a MOSFET due to the small size and simplicity, but I'm open to other solutions.

Thanks

--Amr
Helm PCB - My personal site.
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gajjer
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Username: gajjer

Post Number: 171
Registered: 05-2007

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Posted on Tuesday, 25 November, 2008 - 05:59 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Amr
I'm not quite clear on what you want to do here.
At what voltage do you want the output to switch?
What is the source of the 3 - 24V signal?
Is there a condition where the input is 0V?
In the circuit you show,is D6 the wrong way round. If not what is it for?

If you want to switch at somewhere around 10V why not just use a divider to make sure the gate never goes above 20V? The switching point wont be accurate because of the variation in Vgs(th) so I guess the switching point is not critical.
You could replace the FET with a bipolar transistor and a divider to the base. You will just drive the transistor harder into saturation.


Another way of doing it is a comparator - again with the signal divided down.



Need more information really.

cheers
gaj
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amr_bekhit
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Username: amr_bekhit

Post Number: 382
Registered: 06-2005


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Posted on Tuesday, 25 November, 2008 - 08:24 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi gaj,

What I would like to do is send a logic low to a PIC whenever the input is greater than 3V (in my case it will be between 3-24V), and send a logic high whenever the input is floating or 0V. The signal is the one which detects a car door is open (need a low voltage input as some of these work at 5V) or directly from the voltage that's driving the interior light (which can be 24V).

The circuit shown in my first post does do that, but unfortunately, the absolute maximum Vgs for the 2N7002 is 20V, whereas I would like to apply up to 24V. My question was whether I'd be able to find an alternative transistor that can accept a Vgs between 3-24V.

D6 is just there to prevent a negative voltage from being applied and D5 is a TVS diode to protect the MOSFET from static.

--Amr
Helm PCB - My personal site.
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zeitghost
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Username: zeitghost

Post Number: 1237
Registered: 01-2006

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Posted on Wednesday, 26 November, 2008 - 08:31 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Zener gate to source.

Series resistor to limit zener current.

Job done.

At the same time, it stops any spikes from rupturing the gate oxide... (i.e. dead fet).

I doubt that the 2n7002 would be worried about a negative gate source voltage unless it has an internal "zener"... which it doesn't.

In addition, in the 1988 Siliconix mosfet catalogue I just consulted, Absolute max Vgs (pulsed) is +/-40V... seems a bit high to me... 20V is more typical.
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amr_bekhit
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Username: amr_bekhit

Post Number: 383
Registered: 06-2005


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Posted on Wednesday, 26 November, 2008 - 05:38 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Just the thing I needed Zeitghost - a zener regulator. I've got it working nicely now - voltages as low as 3V can switch on the MOSFET and anything above 12V (the Vz which I chose - a safe distance away from the MOSFET AMR of 20V) just gets clipped.

Yeah I tried looking for a MOSFET that'd go above 20 but couldn't find any - using a zener regulator is much better - only 2 cheap components needed.

Thanks!

--Amr
Helm PCB - My personal site.

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