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budude
08-25-2009, 09:04 PM
As I was contemplating slapping together my DCSSRs soon (was in group parts buy), I realized that these may have a voltage drop going through the MOSFETs! In other words is there a junction or two to get through that would drop my 5V supply down to 4.3 or less? If so I guess I need to rethink my supply situation for me LED Flood Strips... :(

This struck me as I was toying around with the ULN2803 chips which have Darlington outputs which are two stacked transistors so you lose about 1.5 volts or more.

Is there a way to fool a PC supply to supply more juice from the 5V supply - like playing with the returns or something? I can use the 12V supply but that would mean dropping volts with a resistor (BIG one) or something since the current will be pretty high to run a number of strips (about 1A each!) since a little ol regulator would not work.

Brian

P. Short
08-25-2009, 09:42 PM
Check the specs for whatever MOSFETs that you are using. Unless you are really pushing the current levels I tend to doubt that their voltage drop would be anywhere near 1.5V.

budude
08-25-2009, 09:53 PM
I'm using the MOSFET from the group buy - HUF75309P3 - which indicates a Source-to-Drain diode voltage of 1.25V. This is the drop that I think I would see between my supply and the outputs correct? I don't have the parts on hand to verify how this works - just figured there must be a lot of these out there in DIYC land to get an answer.

Thanks!
Brian

P. Short
08-25-2009, 10:09 PM
How much current would you be drawing? The voltage drop depends on that as well, especially if the current is not too large (i.e. below 4 or 5A with a 5V VGS)

budude
08-25-2009, 10:45 PM
Through each MOSFET I don't expect to pull more than an amp - more likely 1/2A total, so it's not a lot of current. But even a small drop is going to affect things quite a bit since I've set up the resistors on the strips for 5V exactly. Even a half volt drop will translate to 13mA through my LEDs instead of the 18mA I planned for...

WWNF911
08-26-2009, 01:08 AM
I'm using the MOSFET from the group buy - HUF75309P3 - which indicates a Source-to-Drain diode voltage of 1.25V. This is the drop that I think I would see between my supply and the outputs correct? I don't have the parts on hand to verify how this works - just figured there must be a lot of these out there in DIYC land to get an answer.

Thanks!
Brian

MOSFETS in the group buy were initially this one. Good thing we found out it wasn't an ideal substitute when mouser said they didn't have enough. Currently you order is shipping to you but the MOSFETS are the original part specified in the (wiki) BOM which is:
511-STF20NF06.

budude
08-26-2009, 01:19 AM
Thanks Leon - for this particular question I don't think it really matters. I was hoping if someone already has some DCSSRs and if they know the voltage drop acrooss them for a typical load.

edit - the forward voltage on this part is 1.5V so it would appear I'm looking at a 0.8-1.5V drop - dang... Guess the 5V supply is out - I'll either get a 7.5V supply or use my 12V supplies that I already have with appropriate drop resistors...

Brian

n1ist
08-26-2009, 07:52 AM
The body diode is off in normal operation (it's reverse biased). Drop across the FET is just the on resistance times the current. For 0.5A and 0.07 ohms, that's 35mV. It will probably be a bit higher if you are driving it with less than 10V on the gate.

budude
08-26-2009, 10:24 AM
That's what I was looking for! Thanks for that - I was hoping it really wasn't as much as I thought.

Edit - got the parts from Leon and etched out some of LabRat's DCSSRs - works great - voltage drop is neglible - whew!

Thanks!
Brian