I am feeding the LED cathodes from 3 x TLC5940 and planned to use a mosfet to switch each layer on and off, trouble is mosfet selection is clear as mud (to me). I have already wasted money and suppliers time buying the wrong thing, please help.....
I need to use the arduino output to switch the gate of a mosfet to switch the LED anodes (48 x 20mA = 1A) to the + supply.
Please can someone point me towards the correct mosfets on ebay (think should be P type logic mosfets????).
Never choose a MOSFET by current rating - this is not what you think, it is usually in reality a coded way of saying the maximum
power the device can tolerate when on an infinite heatsink. A fairly useless spec really...
You choose by Rds(on) - the on-resistance. Decide how much power you can dissipate (ie decide if you want a heatsink,
how big a heatsink, etc). On-power = I^2xRds(on). So here we have I = 1A, so an Rds(on) of 0.3 ohms or less will mean
a dissipation of 0.3W or less (OK without heatsink for TO220 package).
I'm assuming your circuit is 5V - in which case you need a logic-level p-channel MOSFET, so that it will properly
switch on with 4.5V gate drive (10V is the usual drive requirement). 4.5 rather than 5 to allow for off-spec power supplies
and driver losses.
So you need a p-channel MOSFET with logic-level drive and 0.3 ohms or less Rds(on) [ at Vgs=-4.5V ]
And finally check it is rated for drain-source voltage of 10V or more (unlikely to be an issue, note the factor of two
safety factor). Also check the current rating is > 1A (again this is unlikely to be an issue).
These days its not too hard to find p-channel MOSFETs with Rds(on) down to 0.01 ohm and n-channel MOSFETs down to
0.003 ohms. You might as well find such devices and buy a couple of extra, they may come in useful for other projects
that handle more than 1A.