# darlington pair question

#### c.marsh

Joined May 16, 2009
100
how do you work out the transistor values for a darlington pair?

We're only talking 5v MAX with up to 2 amps but i have no idea how to work out the transistor values for such a pair.

#### patricktoday

Joined Feb 12, 2013
157
The second transistor (the one connected to the load) needs to have enough of a current capacity rating to supply the full load current and the first transistor needs to have enough capacity to supply the full load current divided by the second transistor beta. If second transistor beta is 50, and the load is 2A, the first must supply 40mA, the second 2A. This is not the exact but approximate math.

#### c.marsh

Joined May 16, 2009
100
okay, so what 2 transistors would you recommend for 5v at 100ma to 2amp?

#### patricktoday

Joined Feb 12, 2013
157

#### studiot

Joined Nov 9, 2007
4,998
A 5 volt supply is very low for an efficient darlington. You will loose 2x BE drops.

Can you explain your objectives further?

#### c.marsh

Joined May 16, 2009
100
driving somewhere in the region of 150 leds on a 5v source being triggered from a atmel tiny chip which can only handle a max of 5v with very little (200ma) output current.

thats the MAXIMUM number though.

#### GopherT

Joined Nov 23, 2012
8,009
Are they individually controlled or are some of the 150 lights turned on / off in groups.

Can you supply a schematic. We may be able to offer an alternative wiring design, lower your current demand and simplify what you are asking for.

#### c.marsh

Joined May 16, 2009
100
they vary in groups, there will be at a guess

1 group of 30 //saucer
2 group of 10 //engineering - neck
1 group of 6 //strobes
1 group of 2 //bussard
6 groups of 4 //nacell blue - violet - torpedos - deflector orange - deflector blue - shuttle
2 group of 8 //rcs - navs
1 group of 20 //spot lights
1 extra circuit triggered by 2313 chip

there could be more added. i mean thats only a rough guess as the final number hasn't beed fully counted yet.

but if someone can work some magic and work out a more efficient way to trigger those from an attiny2313 then amazing!!

ignore the // notes, i was trying to work out numbers lol.

#### Bernard

Joined Aug 7, 2008
5,784
A second power supply would be helpful, 12V to 24V, so that the LEDs can be used in series strings. Need to know color or Vf of different LEDs. A 5 V control signal is then sufficient to drive just about any combination.
Example: One group of 30. With 24V supply, might have 6 blue LEDs [ about 3V VF ] in series, 5 strings in parallel & if 20 mA / LED; .02 X 5 = 100 mA for the 30 LEDs.

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#### c.marsh

Joined May 16, 2009
100
they vary and are mixed, i dont have confirmed colour counts per group yet

#### c.marsh

Joined May 16, 2009
100
although i'd definatly be wiring each led with its own resistor.

so would a darlington pair be alright for a 12v led power source?

also, is there a decent circuit for reducing 12v to 3-5v and garantee it wont exceed 5v?

#### Ron H

Joined Apr 14, 2005
7,063
What is the supply voltage of your Atmel device (need to know input voltage levels for Darlington, or whatever you wind up with).
Can you use surface mount devices? If so, logic-level MOSFETs would be a better choice from an efficiency and heat standpoint.

#### c.marsh

Joined May 16, 2009
100
I'm going with through hole devices at the moment. for ease really.

the voltages of the 2313 are 1.8v to 5.5v max but i'd rather keep it 5v or less

#### Ron H

Joined Apr 14, 2005
7,063
If you run the Atmel on 5V, you could use RFD3055LE Rds(on)=0.107Ω, price=\$0.31 in small quantities.

If you go to this Mouser page, you might find a reasonably priced through-hole logic-level MOSFET with lower Rds(on), if you need it.
As an example, The RFD3055 will dissipate about 0.4 Watts when conducting 2 Amps. It will get pretty warm without a heat sink. The TO-220 package will run coolest. A part with lower Rds(on) will run cooler.
A Darlington drawing 2A will dissipate ≈1.5 - 2 Watts. it will definitely need a heat sink.

#### Rbeckett

Joined Sep 3, 2010
208
Looks like your building a starship from your notes. Have you considered using Fibre optics and using less actual led's? Might be a tad easier and you can get more colors and way more lites on way less current.

Wheelchair Bob

#### c.marsh

Joined May 16, 2009
100
i'm not afraid to use fiber optics but this is meant to be an open to all project, and not everyones comfortable with fiber optics so was sticking with standard led's really.

i have no idea how to use a mosfet either.

#### Ron H

Joined Apr 14, 2005
7,063
i'm not afraid to use fiber optics but this is meant to be an open to all project, and not everyones comfortable with fiber optics so was sticking with standard led's really.

i have no idea how to use a mosfet either.
But you do know how to use a Darlington???
We know how to use MOSFETs here.

#### c.marsh

Joined May 16, 2009
100
i can work out a darlington, but never used mosfets

#### Ron H

Joined Apr 14, 2005
7,063
i can work out a darlington, but never used mosfets
MOSFETs are easier. The hardest part is choosing the right one. We can help with choosing and using, if you want.