Right transistor chip to use?

[mindctrl]

Hi there,

I'm working on my LED Flasher Array, using 20 Infrared LEDs (here's the datasheet for the ones I will be using: https://www.electro-tech-online.com/custompdfs/2004/12/QED233.pdf )

I've just got a few questions. I'm going to use Nigel Goodwin's PIC LED flasher (), and then just connect a transistor to one of the pins on the PIC, then connect LEDs in series to the transistor. Here's a crude drawing of what I want to achieve; P=PIC, T=Transistor, L=LED, R=Resistor and the dashes are wires.

P----T----R---L---L---L---L

So on and so forth. Leds will be blinking at 11hz. What I wanted to know is what transistor should I use for this? I used the MetkuMods LED calculator to calculate my resistance I should be using (http://metku.net/index.html?sect=view&n=1&path=mods/ledcalc/index_eng) but I wasn't sure that I was putting in the right numbers (Wasn't completely sure what Voltage Drop Across LED was), but if somebody could double check my work, that would be great. Here's the numbers I got:

.: LEDs in Parallel:

Supply Voltage: 12 Volts (since I'll be plugging it into a 12V socket in my car)
Voltage Drop Across LED: 1.5 Volts
Desired LED Current: 100 Milliamps
How many LEDs connected: 20

And that should give me a resistor of 5.6 Ohm that rates at 35 Watts... Does any of that look right?

If so, then it looks like I'll be needing a transistor that can take the relatively weak voltage from the PIC(+5V) and use that to switch on and off the 12V supply from the car. Let me know what you guys think, I appreciate it.

Thanks,

--mindctrl

 

[williB]

no..
12V / .1 A =120 ohms

 

[williB]

just about any TO 92 N channel FET will work..TO 92 is the type of package a lot of transistors come in..
or this one is a bit of an overkill but it will work at logic levels of a Pic..
and it is only $0.67 cents..

IRL520N N-ch MOSFET logic-level gate voltage, 10A, 100V TO-220 $0.67

TO 220 packages have tabs with a hole in them..

 

[williB]

hold on , i dont think that 20 LEDs in series will work with 12 V
because with 1.5V per diode drop, that means about seven diodes is about the maximum that u could use with 12V..
12V/1.5V =8
and with the voltage drop on the transistor itself seven is the max you could use..

 

[mindctrl]

OK, I can hook them up in parallel. How does that compute out to? Still use that same transistor?

 

[williB]

OK, I can hook them up in parallel. How does that compute out to? Still use that same transistor?

with three parallel branches you could use 21 diodes , each branch would require its own transistor and current limiting resistor.

 

[mindctrl]

Sounds about right. I can just hook the LEDs to the transistors, then each of the three transistors to a different pinout on the PIC, right? So I'll still only need one PIC, hopefully.

 

[williB]

Sounds about right. I can just hook the LEDs to the transistors, then each of the three transistors to a different pinout on the PIC, right? So I'll still only need one PIC, hopefully.

sure the Pics have (well most ) at least 12 possible outputs..

 

[mindctrl]

what resistor should I use?

 

[williB]

what resistor should I use?

120 ohms will give you around 100mA per branch. 1/4 watt would do fine i'm sure.