MOSFETs...BJTs...Transistors and LEDs...OH MY!

Hey I've posed a few questions elsewhere, so some of you may recognize this question as part of a old thread...please forgive me.

My questions is simple...I've searched BD139s, MOSFETs, BJTs, MOSFET drivers...and anything else i can find yet can't come up with a viable solution for my problem which is that I need to power 3 LED array drawing 1.5A at 5V...anyone have any ideas?

It's a simple problem...arduino sends 3 5V control signals and each array draws 5V at 1.5A on the other side...I've got 3 computer power supplies that give +5VDC and +12VDC....I just need to tie them into the circuit somehow...what do i do?! lol, i'm pullin' my hair out over this issue and can't find viable solution.

It's a PWM control from the arduino btw

Thank you for any leads you might be able to offer

I think up to this point you've asked for theory and parts, and you've gotten good answers...was there something more? Do you need someone to draw you a circuit?

What is wrong with the RTL035N03 FET. It turns on at 1.5V and can take up to 3.5A. There are lots of FETs at:-

Just use one FET per LED array and wire all the gates to the same PIN.

I guess what I am trying to get at is that I'm having trouble deciding on what to buy. I'm not understanding the logic behind these transistors and thus can't interpret their data sheets very well...which kinda sounds stupid if i'm getting into this project but setting that aside what i really am confused about is what specs i should be lookin' for?

So the RTL035N03 FET turns on at 1.5V and can operate at up to 3.5A?...this means that i can supply it a 5v control signal and it will operate? this means that i can pass 5v from the power supply through it? is that all correct?

So the RTL035N03 FET turns on at 1.5V and can operate at up to 3.5A?...this means that i can supply it a 5v control signal and it will operate? this means that i can pass 5v from the power supply through it? is that all correct?

Yes all correct, it is capable of working with a supply much higher, that's what the maximum ratings are all about in the data sheet. You can supply it with 3.5V from your system which is more than enough to turn it on, more is good until it turns into too much. In this case too much is when you take it higher than the drain.

Yes data sheets are hard to follow often they give you far more information than you would normally need and occasionally not quite the data you are looking for, but keep reading them it will come.

Ok so i'll pick up some RTL035N03 FETs...which side is it better to resist on? the power supply side or the array side?

I didn't see the RTL035N03 FET at the site you suggested, any other ideas? what is the RTL035N03 FET in lay man's terms?

Thanks!

Just go to:-

and put in RTL035N03 in the product search. This is just one of many that will do.

which side is it better to resist on?

Sorry I don't understand the word "resist"

Just connect the gate to the output pin, the source to the common ground between your arduino and external PSU, the drain to the -ve power input of your LED array and the +ve power input of your LED array to the +ve of your external PSU.

If i'm supplying 5V and the LEDs require 3.8, 3.2, and 3.5 I'll need to put in some resistors to get from 5V to the respective voltages...Is it better to put the resistors on the side going INTO the FET or on the side coming OUT of the FET...Does that clarify my question?

So...to help further my education...does this product http://uk.farnell.com/1525551/semiconductors-discretes/product.us0?sku=rohm-rtl035n03tr&_requestid=309710 mean that:

I can supply between 1.5V and 30V to turn it off and on?
Up to 3.5A of current?

What does this refer to?

Voltage, Vgs th max:1.5V

Voltage, Vgs th min:0.5V

Thank you for your patience!

I just went to checkout on Farnell and found out there's a minimum 20 pound order...needless to say i'm only ordering 2 pounds in transistors so this leaves me slightly up the creek...will this item perform the task I need? Transistor - NPN, 60V 200mA (2N3904) - COM-00521 - SparkFun Electronics or any suggestions on where to get MOSFETs other than Farnell...again thank you

Vgs (th) is the threshold voltage between the "gate" and the "source" needed to turn the FET on. Why there is a min & max for these, I'm not completely sure, but I think that the voltage or current between the source and the drain can effect Vgs (th). In your case, it doesn't matter since the 1.5V you are giving it meets the max . I don't think it hurts to exceed the Vgs (th) max. either. (Someone will correct me if I'm wrong.)

The MOSFET will fully conduct (saturate) as long as Vgs is over the threshold and the voltage between the source and the drain (your case 30V) is greater than [Vgs (5V) - Vgs (th) (1.5V)] or 3.5V.

As to locating other sources of MOSFETS, if you can get your hands on an old battery backup (UPS) there are generally 4 hefty ones in there.

I don't think those BJT transistors from Sparkfun will cut it. They can only switch 200ma.

(Radio Shack has a IRF510 (60V @ 3A) for USD$2 if that helps.)
(Your in the UK? try http://www.t2retail.co.uk/Semi_Conductors/?sortmethod=name&page=4)

Is it better to put the resistors on the side going INTO the FET or on the side coming OUT of the FET

It doesn't matter a jot.

Why there is a min & max for these

Because devices vary and some may turn on with less voltage than others. When designing a circuit you need to know this to ensure you can always turn every device on and off.

I don't think it hurts to exceed the Vgs (th) max

Quite correct.

Try looking at Maplins now you know what to look for in a FET spec you should be OK.

Quote:

Is it better to put the resistors on the side going INTO the FET or on the side coming OUT of the FET

It doesn't matter a jot.

Well, it does matter, if I understood the intent of the question...a resistor between the MOSFET source terminal and ground being "out" of the FET. The source terminal should be tied directly to ground, otherwise a resistor will lower the possible Vgs and cause the MOSFET to conduct less.

It doesn't matter which side of the string of LEDs the resistor is on, if that was the intent of the question.

Ah! You all are being so helpful, thank you

I'm actually in the US, someone put up a UK site so i took a look into it.
Since radioshack is around the corner, let's see if i get this right

This IRF510 is rated at 60V and 3.0A...This would mean that I can use anything up to 60V to turn it on and off which is satisfied by the 5V i am supplying it, correct? It will take 3.0A and that is satisfied by the 1.5A the circuit will be drawing through it, correct?

So if i'm supplying the circuit with 5V and i have a set of LEDs that need 3.2V then I will need a 1.2Ohm/2.7Watt resistor, correct?

Thanks!

Macegr...the intent of my question was: Should i step the voltage down to 3.2V(from the 5V provided by my power supply) before it enters the FET or let the 5V pass through and then step it down to 3.2V on the other side?

Thanks!