Can the Mosfet module be setup to pass negative current?

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Araich
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Joined: Mon Apr 21, 2014 7:28 am

Can the Mosfet module be setup to pass negative current?

Post by Araich » Mon Apr 21, 2014 7:44 am

Hi, I am making a simple dimming/flickering LED set and have used the freetronics mosfets to power 12v LEDs. All works fine. However I now would like to replace the LEDs with pre-assembled LED sets (car brake lights) that contain a common ground and 4 positive leads. The polarity is such that I can only connect the ground common lead to the 'D' or load terminal - when what I need is to be able to connect one of the positive leads (having a mosfet for each lead, and a common ground).

Is there a way to reverse the current being switched by the mosfet? That is instead of having it switch a positive current through it's shared ground, it could switch a negative current?

I hope that made sense.

cef
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Freetronics Staff
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Joined: Wed Nov 09, 2011 12:53 am

Re: Can the Mosfet module be setup to pass negative current?

Post by cef » Mon Apr 21, 2014 8:15 am

You really need to use a different type of mosfet to switch a positive voltage, and you really need a proper driver circuit to do it too.


The NDrive uses an N-type Mosfet, but you really want a P-type. The only issue is that you need to switch the gate from the full voltage of your supply (12V) down to 0V.

Easiest way to do this is to use a standard NPN transistor and a few basic components like this image shows:
Image

(Image linked from http://cq.cx/interface.pl#13 which is a useful little reference for various things, not the least being exactly what you want to do).

Note: The image references 10V. The same values should work well enough for 12V. Note that the microcontroller in his examples is 5V based, which is what most of the Arduinos use. If you're using 3.3V logic, you might want to reduce R1 to about 680 ohms.

Hope this helps.

Brissieboy
Posts: 206
Joined: Fri Sep 20, 2013 7:25 am

Re: Can the Mosfet module be setup to pass negative current?

Post by Brissieboy » Tue Apr 22, 2014 1:10 am

Araich,

Another approach is to use an high-side switch, such as a BTS432E2 or VN920-E.

These are aimed at the automotive industry and are extremely rugged with capabilities way in excess of what you require (>30Amp with very low on resistance at around 30-40mOhm).

They are virtually indestructible.

Another good thing about them is that they operate directly off logic levels with very few external components.

A similar low-side switch is the IPS1021PbF.

Do a google search for data.

Brissieboy

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