I want to control the light intensity at once of a 8*8 LEDs matrix. Calculation gives at least 1.28 A as current intensity passing by White LEDs. I want to drive this matrix by a N-Mosfet and Transistor as indicated in this link : http://www.instructables.com/id/Circuits-for-using-High-Power-LED-s/. The Light brightness is controled by arduino (PWM) which in turn is commanded by an LDR.
I know that i should use 10K resistor at the gate of the Mosfet (RC = 6us is a little quantity, C: capacitance of the gate = 600pF) so the Moseft is rapid. I'v chosen IRF530 because it handles 14A>> 1.28A and the power dissipation is not high =(1.5)2*0.16=360mW.
For the resistance linked to the drain, i've chosen 0.5 Ohm = Vbe/1.28A.
My questions are:
1-Is the circuit correct?
2-Are my values correct and how to verify them if not?
3-Which type of NPN transistor should i use?
MrElec:
I know that i should use 10K resistor at the gate of the Mosfet (RC = 6us is a little quantity, C: capacitance of the gate = 600pF) so the Moseft is rapid.
I'v chosen IRF530 because it handles 14A>> 1.28A and the power dissipation is not high =(1.5)2*0.16=360mW.
For the resistance linked to the drain, i've chosen 0.5 Ohm = Vbe/1.28A.
My questions are:
1-Is the circuit correct?
2-Are my values correct and how to verify them if not?
3-Which type of NPN transistor should i use?
Why 10k when you can switch faster with 1k (or less).
The limit is Arduino pin current and 'small transistor' collector current.
The IRF530 is old, and not a logic level fet. It barely starts conducting with 4volt at the gate.
You only have 4.3volt available.
I ran an LTspice test with your circuit and an IRF530. It topped at ~800mA.
Instructables must have used a 12volt supply in their example (three LEDs in series).
10volt at the gate, and they used a modern logic level mosfet.
You have 5volt (supply) -3.3volt (LED) -0.7volt (resistor) = ~1volt across the mosfet.
Dissipation in the mosfet is 1volt * 1.28Amp (your calcs) = 1.28watt
The mosfet could be ~82C above ambient without heatsink.
Not a very good idea to connect LEDs in parallel.
Check your LDR circuit. A0 should be connected to the LDR/resistor.
Leo..
Once you get a logic-level NFET that you can drive from Arduino, use a lower value series resistor to the FET gate. (10K Gate to Ground(Source) is good only to keep FET off when arduino is turned off. I suggest 100 ohms. 6 uS is a long time in FET land.
@Wawa and terryking228, thank you for your responses. I've corrected the link LDR/resistor and changed for low resistor R<1K . The only N-Mosfet which i have now is an IRF830 its Gate-Source Threshold Voltage is 4V i've no choice!!
"The mosfet could be ~82C above ambient without heatsink".
How did you get the 82C value? should i use a heatsink for the N-Mosfet. I'm using a voltage regulator 7805 to pass 1.5A. Does it need a heatsink also?
"Not a very good idea to connect LEDs in parallel."
MrElec:
How did you get the 82C value? should i use a heatsink for the N-Mosfet. I'm using a voltage regulator 7805 to pass 1.5A. Does it need a heatsink also?
"Not a very good idea to connect LEDs in parallel."
A typical TO-220 package without heatsink gets 65C hotter for every watt.
1volt across, * 1.28Amp = 1.28watt, *65C = 83.2C above ambient.
The 7805 has trouble with >1Amp, but say you go from 9volt in to 5volt out. 4volt across the 7805.
4volt * 1Amp = 4watt. That definately needs a small heasink.
12volt in and 5volt out, with 1.5A current, needs a medium size heatsink.
A $1 buck converter from ebay could do this without heatsink.
These LEDs are all in the same package. Shouldn't be that bad.
Leo..
@Wawa, Thanks for your response, I'm going to use LM317T instead of LM7805 i think this is the ideal solution for me now but are the capacitors values as indicated 0.1uF at the input and 1uF at the output?
MrElec: @Wawa, Thanks for your response, I'm going to use LM317T instead of LM7805 i think this is the ideal solution for me now but are the capacitors values as indicated 0.1uF at the input and 1uF at the output?
Yes those capacitors are for stability of the LM317, they should be placed as close to the LM317 as possible.
Tom....
@TomGeorge, thanks for your response, this is my final Circuit in Proteus. Is there any corrections? I think i don't need a heatsink for the LM317T. What are your opinions?
Hi,
Sorry, don't zip any files, and can you export the diagram as a graphics file, jpg or png.
The format you have supplied is not supported on my computer.
I don't know, but I would think Proteusas an export button.
The adjustable LM317 and the fixed LM7805 are both lineair regulators.
They both convert voltage difference between input and output into heat.
Same heat dissipation. Same heatsink.
Note that the tab of the LM317 is not ground, so you might have to electrically isolate the LM317 from the heatsink.
Did you read this in post#4 "A $1 buck converter from ebay could do this without heatsink."
Leo..
I suppose you do have smartphone/tablet chargers there.
My cheap tablet came with a 5volt/1.5Amp plugpack supply.
That could power your Arduino and the LEDs.
Leo..
Wawa:
I suppose you do have smartphone/tablet chargers there.
My cheap tablet came with a 5volt/1.5Amp plugpack supply.
That could power your Arduino and the LEDs.
Leo..
I agree with you, but i wanted to design the AC/DC power supply also.
