Need help saturating a transistor for 2 amps

[oldmanbiker]

Hi,

I need to blink an array of 78 white leds.  I figure that I need to switch about 4.0 volts at 1.6 amps.

I want to take a $1.00 blinky led as a signal source and drive a TIP120 with it.  When using a TIP120 as a switch, can I just wire it in series with the LED array ground connection?  Also, I figure that the darlington in the TIP120 is high gain, so I should be able to saturate it with the blinky led.  Will connecting the blinky led from V+ to the base do it?

Thanks

 

[audioguru2]

Hi Oldy,
Welcome to our forum. ;D
It sounds like you are going to connect 78 LEDs in parallel like light bulbs. LEDs are not light bulbs, they are diodes and their individual voltages are slightly different which greatly affects how much current they draw.
4V might be the typical voltage for most or might be the max voltage that a few LEDs will have.

One LED will conduct its 20mA with only 3V across it and the other LEDs might need need 4V so won't get any current. The 3V LED will take the entire 1.6A or more and burn out, followed quickly by the next 2nd lowest voltage LED, etc.

LEDs need a current limiting resistor in series with each one then they won't burn out and will be the same brightnesses.

The TIP120 is a darlington transistor and "saturates" with typically 0.8V across it when its load is 1.6A. Its "saturation" voltage max is about 1.6V. That is a little over 2.5W of heat so it will need a heatsink.

The TIP120 will turn on with an input current of less than 1mA. Therefore in order to turn off you should connect a resistor from its input to ground, to bypass any leakage current of the blinky LED.

The supply voltage needs to be a total of:
4V for the LEDs.
1V for the current limiting resistors.
Up to 1.6V for the darlington transistor's "saturation"
Total supply= 6.6V.

Does the blinky LED operate from about 5V without a current limiting resistor in series with it?

The cathodes of the LEDs connect to the collector of the darlington transistor. The anode of the LEDs connect to its current limiting resistor. The other end of the current limiting resistor connects to +6.6V together with the positive wire of the blinky LED.

 

[oldmanbiker]

Does the blinky LED operate from about 5V without a current limiting resistor in series with it?

The cathodes of the LEDs connect to the collector of the darlington transistor. The anode of the LEDs connect to its current limiting resistor. The other end of the current limiting resistor connects to +6.6V together with the positive wire of the blinky LED.

Thanks the info, audioguru.

I plan on using a readily available led, which means Radio Shack 
  http://www.radioshack.com/product/index.jsp?productId=2062553

This led does need a current limiting resistor.  It is Vf 2.25, 90mA.  If the darlington turns on at 1mA, will 50mA thru the base cause any problems?

 

[audioguru2]

I plan on using a readily available led, which means Radio Shack 
  http://www.radioshack.com/product/index.jsp?productId=2062553
This led does need a current limiting resistor.  It is Vf 2.25, 90mA.

If the darlington turns on at 1mA, will 50mA thru the base cause any problems?

The darlington barely turns on with a base current of only 1mA. It will "saturate" well with a base current of 1.6A/250= 6.4mA or more. Its max base current is 100mA.
Select a resistor for the blinky LED to conduct 30mA: (6.6V-1.3V-2.25V)/30mA= 102 ohms. Use 100 ohms.
Select a resistor to turn off the darlington transistor. RadioShack don't say what is the minimum current of the blinky LED so just try resistors from the base to ground until it works.

 

[indulis]

You could try using a low on resistance (.006 ohm) logic level MOSFET.

 

[oldmanbiker]

indulis:  I'm a newbie.  Newbie + static sensitive parts = poof

audioguru:  I could not get a tip120, I ended up with an NTE2343. Also a darlington npn, supposed to similar and handles more power.

  http://www.nteinc.com/specs/2300to2399/NTE2343.html

Any difference in the calcs?  Is there a link to a tutorial on how to do the calcs?  Thanks

 

[audioguru2]

The datasheet for the NTE transistor doesn't have any curves and has only a few spec's.
Its "saturation" voltage is pretty high at a max of 2.0V with a 5A load and 1/250th of its collector current as its base current. I guess its input voltage is 2.5V max at 5A. Who knows what they are at 1.6A??

Your RadioShack blinky doesn't have any spec's and now this NTE transistor doesn't say what you need to know. Just try them and see what happens.

 

[oldmanbiker]

You're right, the datasheets for NTE semiconductors aren't like others I've seen.  Anyway, I found out that NTE2343 cross references to TIP102.  Luckily, fairchild has plots for the tip102.  I hope that has the necessary info  ;D

  http://www.fairchildsemi.com/ds/TI/TIP102.pdf

 

[oldmanbiker]

By the way, most people seem to refer to a transistor in switching mode as "saturated", is there a more correct term for that?

 

[audioguru2]

A single transistor saturates well with a low voltage drop of 0.05V to 1V. A darlington transistor doesn't saturate, it just turns on and its voltage drop is 0.6V to 2V.

 

[indulis]

A transistors Vce sat voltage is current (Ib & Ic) dependent. For example, with Ic=10A & Ib=3.3A the 2N3055's Vce sat max voltage is 3V, while with a Ic=4A & Ib=400mA, the Vce sat max voltage is 1.1V

A darlington transistor will saturate just like any other transistor. The only thing "special" about a darlington is it's high Hfe. When transistors are connected in a "darlington configuration" the Hfe's of the transistors multiply. In fact, darlingtons are very easy to saturate because of it's high Hfe.

By definition saturation is when Hfe * Ib > Ic the transistor is then considered to be saturated.

 

[audioguru2]

A darlington transistor will saturate just like any other transistor.

Hi Indulis,
I disagree.
A transistor is fully saturated when its collector-base junction becomes forward-biased. This can't occur in the bottom transistor of a darlington, so its "saturation" voltage is always higher than a base-emitter voltage drop.

 

[indulis]

Greetings John

By definition saturation is when Hfe * Ib > Ic ...

at this point voltage has nothing to do with it if the above condition is true.

Beside, that "general rule" to which you refer, only applies to a "single" BJT.