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NPN Transistors

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GatorGnet

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I need to drive some LEDS that could end up using more current then the transistors that I have. Would it be possible to use the transistors in parallel such as this to get a higher current capacity? The schematic is crude but I just wanted to give the idea.

The NPNs I have are rated for 600ma each and the LED(s) I need to use draw 1000-1500ma.

286-led.jpg
 
I need to drive some LEDS that could end up using more current then the transistors that I have. Would it be possible to use the transistors in parallel such as this to get a higher current capacity? The schematic is crude but I just wanted to give the idea.

The NPNs I have are rated for 600ma each and the LED(s) I need to use draw 1000-1500ma.

hi,
I would suggest that you connect the LED in the collector circuits, with a low value balancing resistor in each collector.

DONT use that circuit as drawn.
 
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No. The port pin can only source 25mA or less, so there is not enough current available to drive three bases. Also, you need base resistors, and if you could get the transistors to turn on, you would need a current limiting resistor in series with the LED.

The better way to do this is with a darlington power transistor, or a high-current logic-level NFet.
 
Haha. I like your honesty. As you can see, I need help.


How about if I just break down and spend the $0.45 and buy 2W transistors!?
 
Haha. I like your honesty. As you can see, I need help.


How about if I just break down and spend the $0.45 and buy 2W transistors!?

If you buy the 2W transistors, dont use that circuit.
I would use a N MOS FET, place the LED in the Drain in series with a current limiting resistor.
Choose a N FET with a Vgate turn ON voltage to suit your PIC output voltage.

Whats the Vfwd drop for the LED.?
 
I don't have much info on them but it looks like its 1.8v. Right now I am still hammering out the schematic(its in better shape then the one I posted :)..) and I keep switching some of the components. I will have a better plan once I settle on the LEDs.
 
I don't have much info on them but it looks like its 1.8v. Right now I am still hammering out the schematic(its in better shape then the one I posted :)..) and I keep switching some of the components. I will have a better plan once I settle on the LEDs.

Extract from the web.

LEDcolours.gif
 
How come you like the MOS FET you like over the NPN transistor?

A high current transistor often needs a highish base current which the PIC cannot supply , I know that you could use a Darlington, but they are 'lossy' at 5V

Whereas a MOS FET can give amps in the circuit for about zero Gate current, so that can be attched to a PIC ok.

The attached file is an expresspcb *.sch, change the extension to sch from *.txt to view it.
 

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Do the MOSFETs deal with heat ok? If I used a 400v 20amp one, would it stand up to continuous use left on lets say with 12v/10amp?
 
Do the MOSFETs deal with heat ok? If I used a 400v 20amp one, would it stand up to continuous use left on lets say with 12v/10amp?

hi,
When you choose a FET for high current use, look at the Rds on value resistance and also make sure it can be controlled by a Gate voltage to suit the PIC driving voltage.

For your project of 1.5A, I would look for a 5A rated P MOS FET, that has a Gate threshold of about 2 to 3 Volts.


EDIT:
Something like this spec;

https://uk.farnell.com/jsp/search/b...e=en_UK&catalogId=&prevNValues=500006+1004185
 
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Do the MOSFETs deal with heat ok? If I used a 400v 20amp one, would it stand up to continuous use left on lets say with 12v/10amp?
A MOSFET deals with heat the same as a bipolar transistor. If there's too much it burns up.

The transistor power dissipation is equal to the current-squared times the "ON" resistance of the MOSFET. From that you can determine the size of heat sink you need to keep the transistor below it's rated junction temperature.
 
You shouldn't be connecting the LEDs in parallel.

Connect them in series, recalculate the resistor value for the higher voltage drop and lower current.
 
I haven't had it happen yet but if they are in series and one goes out, will it disrupt the rest? If so, I will have to go parallel due to the number of LEDs in each circuit.
 
I haven't had it happen yet but if they are in series and one goes out, will it disrupt the rest? If so, I will have to go parallel due to the number of LEDs in each circuit.

hi Gator,
Hope you got that overdue sleep session. :rolleyes:

If you mean if one of the series LED's fail s/c the the others will not go off, but they will get brighter and maybe over driven.

If one LED fails o/c then the others will go off.

OK.?
 
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Below is a series LED circuit designed by a member of the fieldlines forum, the LED masterclass is here **broken link removed**

That is the final part however, the previous parts are in the top thread.

I have made several LED lights from the below circuit and not one has failed. The one's I've made vary from 20mA standard led's to 550,000mcd LED's running at 90mA in a string of 4 working off 16 volts ( my nife bank). I reckon this is the best way to string LED's especially when powering them off a batterybank....

Commanda's Led Circuit.JPG


Regards Bryan
 
That should work then. I am just worried about failures. Some of the banks of LEDs will be in the neighborhood of 40-100 leds. Others will only be 3-8.
 
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