Hi, I am working on an electronics project where I am attempting to computer control some model train lights with my Arduino. The plan is to control the lights with my Arduino using transistors to switch the lights on and off. The lights would be powered by an external power supply and the Arduino would be powered via either USB or a wall wart. I recently bought these transistors TIP120 Power Darlington Transistors - 3 pack : ID 976 : $2.50 : Adafruit Industries, Unique & fun DIY electronics and kits to use in the project. My problem with them is that I cannot get them to work properly when connected to both the Arduino and the external power supply, as they are always dim, but when only connected to the external power, they work fine.
According to the datasheet, Pin 1 (the leftmost pin in all of the pictures) is the base pin http://i.imgur.com/5sgaqHK.png
In my tests with just the power supply, I was able to wire up the light to the power supply and transistor, and use the positive voltage from the power supply to Enable/Disable the transistor. The wiring is as follows:
Positive Voltage(Red)-Collector
Collector through Transistor to Emitter
Emitter- one light lead(Blue)
light lead(blue) through light to other light lead(Blue)
light lead(Blue) to ground(Black)
I used an extra wire as a jumper(Orange) to bridge between the positive voltage and the Base.
Wiring Picture: http://i.imgur.com/rpetvXm.jpg
Wiring Diagram(Fritzing): http://i.imgur.com/n5ELYNI.png
In tests with this setup, the transistor worked as expected. I was able to switch the light on and off effectively. As a side note to this part, I have also tested this using just my Arduino and treated it like a normal transistor and set it up normally, and was able to get it working properly using proper digital pins.
The problem comes when I tried to use my Arduino to Enable/Disable the transistor. I used the 5V pin on my Arduino to simulate a HIGH output on a pin and unplugged it to simulate LOW.
Positive Voltage(Red)-Collector
Collector through Transistor to Emitter
Emitter- one light lead(Blue)
light lead(blue) through light to other light lead(Blue)
light lead(Blue) to ground(Black)
The only change is that not, instead of connecting the base to the positive voltage from the power supply, it is not connected to the Base of the transistor.
I also added a common ground wire(White/Gray) between the ground of the Arduino and the ground of the power supply.
Wiring Picture: http://i.imgur.com/CwdOg0W.jpg
Wiring Diagram(Fritzing): http://i.imgur.com/gQQkAuC.png
Now, when I use the transistor, the external power supply seems to be doing nothing, as the lights are always dim as if they were only plugged in to 5V.
What am I doing wrong? The common ground seems to be right from the googling that I have done and I cant figure out anything else that I could be doing wrong. Thanks for any help you can give me.
Also, I just tried putting the load on the collector, instead of the emitter, like you said. Now the light is always on and adding voltage to the base has no effect on the light. The way I put the load on the collector was by swaooing the collector and the emitter, please correct me if that is wrong.
Since you were not using a resistor on the base of the transistor, you have probably damaged it. With no current limiting resistor on the base of the transistor, the base junction is probably blown.
See attached datasheet.
The dc gain is 1000, so the collector current is 1000 times the base current . If you need 1 A through the collector, then you only need 1 mA through the base
The maximum base current is 120 mA. By connecting the base directly to Vcc you most likely blew the base junction.
If the logic HIGH on the output pin were 5V and the VBEon voltage is 2.5V then to supply 1 mA of base current,
Let Ibase =1 mA = 0.001 A
Let Voutput= 5 V
Let VBEon = 2.5 V
then,
Rbase= (Voutput-VBEon)/Ibase = (5V-2.5V)/0.001 A = 2500 ohms
Use a 2.5 k ohm resistor between the base and the driving pin to get 1A through the transistor.
Use a 500 ohm resistor to get 5 A (5 A /1000 = 0.005 A base current)
(5V-2.5V)/0.005 = 500 ohm.
FYI,
This:
powered by an external power supply
is too vague. If you're posting for a hardware issue we need specifics, like voltage and current rating of the supply.
FYI,
As a side note to this part, I have also tested this using just my Arduino and treated it like a normal transistor and set it up normally, and was able to get it working properly using proper digital pins.
There is nothing normal about using any transistor without a base resistor to limit the current . That is an absolute no no.
You should NEVER do that. In certain circumstances you can get away with that if the maximum current output of a digital output pin is 40 mA and the maximum current rating of the base is 120 mA, but that's just lucky.
What am I doing wrong? The common ground seems to be right from the googling that I have done and I cant figure out anything else that I could be doing wrong. Thanks for any help you can give me.
Ok let's check that. Post a link of any transistor circuit that does NOT have a base resistor.
If you used google, every circuit you found had a base resistor or some element to limit the current.
I defy you to post a schematic you found that shows a transistor with the base connected directly to the positive terminal of a power supply.
jreeve17:
Sorry About that. Here are the Fritzing schematics
I don't have Fritzing installed so can't open those. I think Fritzing can export a pic, like a jpeg or similar: attach those for people to see more easily. (Make sure they're scaled down to 1-200k for easy viewing without too much (any...) scrolling.
But I'd go with what raschammel says, re the base resistor. And there is no doubt that with an NPN transistor you ought to have the load on the collector.
Ok, thanks for that raschemmel. I though that since this was only a quick test I wouldn't need any of that just yet. Ill get one on immediately. I folloewd the quick tutorial that jimbo posted and it is working now, even though I am not really sure what changen in mine. Thanks for the help. Are there any suggestions for base resistor value? The external power supply is 650mA, 15V
Quick Edit
I attached PNG images of my fritzing schematics
Positive Voltage(Red)-Collector
Collector through Transistor to Emitter
Emitter- one light lead(Blue)
light lead(blue) through light to other light lead(Blue)
light lead(Blue) to ground(Black)
The above is an emitter-follower configuration which is used almost exclusively for analog power supply regulator circuits .
Any normal load switched or controlled by a transistor should be with the load connected from the +terminal of the supply to the colector and the transistor sinks the current.
Just a suggestion. Fritzing is like training wheels on a bicycle. Google schematic symbols and learn to read and draw schematics and then when you need to post one you can just draw it on a piece of printer paper and an take a photo and post it. Yeah, it is old school, but everyone respects you more for it, even if the schematic is hand drawn without a ruler. If you want to be really old school, draw it on a cocktail napkin from a strip club... XD
If I were you I'd go with the schematic from the tutorial, perhaps with the bigger base resistor.
Look here and you'll see confirmation that the load goes above an NPN.
I realise this is academic and you probably know you don't need a transistor to switch an LED, but you certainly don't need a Darlington. Look at that page's pic of a Darlington and you'll see the gain is huge. That's because it's two transistors in a sort of chain.
Look at the table, and you'll see that for a lot of cases you can just use a "normal" (as opposed to Darlington) transistor like a BCxxx.
) like this. In the example below I added a space after the [ so that it shows rather than working.
