N Channel Mosfet switch with load on source?

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
I have a eight LEDs already wired up with a wire on the anode. Since only one LED is planned om being on at a given time I placed the current limit resistor on the cathode that they all share.

I need to be able to switch these with an N Channel logic level MOSFET. Every configuration I have seen for MOSFET switches put the load on the drain.

I guess I wasn't thinking when I wired up the LEDs because I probably should have made them common anode to suit that configuration. Well it is too late they are already wired.

The only think I could think of was to place the load (LED) on the source. With the anode of each LED tied to the source and the cathode via the current limiting resistor tied to ground.

I gave it a try but when I do this, my source appears to oscillate then the gate is tied high.

As there anyway to get this to work? Or am I going to have to have current limiting resistors for each LED?
 

ebeowulf17

Joined Aug 12, 2014
3,282
I have a eight LEDs already wired up with a wire on the anode. Since only one LED is planned om being on at a given time I placed the current limit resistor on the cathode that they all share.

I need to be able to switch these with an N Channel logic level MOSFET. Every configuration I have seen for MOSFET switches put the load on the drain.

I guess I wasn't thinking when I wired up the LEDs because I probably should have made them common anode to suit that configuration. Well it is too late they are already wired.

The only think I could think of was to place the load (LED) on the source. With the anode of each LED tied to the source and the cathode via the current limiting resistor tied to ground.

I gave it a try but when I do this, my source appears to oscillate then the gate is tied high.

As there anyway to get this to work? Or am I going to have to have current limiting resistors for each LED?
Why not just use a P channel MOSFET? I think that'll be cheaper and easier than the circuitry you'd need to do high side switching with an n channel.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
Why not just use a P channel MOSFET? I think that'll be cheaper and easier than the circuitry you'd need to do high side switching with an n channel.
I don't have P Channels.

I will go with individual current limit resistors. That will give me the option to turn on all the LEDs at the same time. .
 

ebeowulf17

Joined Aug 12, 2014
3,282
I don't have P Channels.
Do you have multiple voltage rails available? Specifically, do you have something that's ~5V higher than your LED driving voltage? If not, I think your options are limited. You might need to do some shopping!

Another possibility might be a bootstrap capacitor circuit. I've never used one, so can't advise, but it's worth googling to see if it's feasible in your application.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
Do you have multiple voltage rails available? Specifically, do you have something that's ~5V higher than your LED driving voltage? If not, I think your options are limited. You might need to do some shopping!

Another possibility might be a bootstrap capacitor circuit. I've never used one, so can't advise, but it's worth googling to see if it's feasible in your application.

I have a 12V rail available. But that would be a lot of current to dissipate.


I think I have some redesigning to do but the good news is, I can save most of what I already designed and built.
 

cmartinez

Joined Jan 17, 2007
7,525
I have a 12V rail available. But that would be a lot of current to dissipate.


I think I have some redesigning to do but the good news is, I can save most of what I already designed and built.
N-Fets are devices that are easiest driven on the low side of the load, and P-Fets on the high side. If you wish to invert their normally functioning configuration you're gonna need a driver, such as the IR2101.

It would be real helpful if you posted a sketch of what you're trying to do.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
N-Fets are devices that are easiest driven on the low side of the load, and P-Fets on the high side. If you wish to invert their normally functioning configuration you're gonna need a driver, such as the IR2101.

It would be real helpful if you posted a sketch of what you're trying to do.

Too late. I have moved on but thanks. Actually I realized another error I made in my design. :)
 
Top