im trying to light up a strip of LEDs (about 1.3amps worth) using a transistor (nte261) to switch the strip on from a pwm pin on my arduino.
i removed the arduino to try and isolate the problem, so my circuit has a nte960 voltage regulator putting out 5v, powered by the same 12v as my light strip. everything works, with or without the arduino, i can get the light strip to fade on and off, everything looks good...
except the transistor heats up pretty quick. when i attach a smaller strip of LEDs everything works fine, and stays cool.
so my question is why am i getting so much heat when the datasheet seems to indicate i could power 4 times as much with the same transistor?
Switch the LEDs on and measure the voltage drop across the transistor. Multiply by the amps. That's the heat it will produce, it's probably about a watt.
One watt with no heatsink? It's not very surprising it gets hot....
You should expect it to be how as setup. If you had a nice heatsink on that transistor, and maybe a small fan, it will be a lot cooler. Is it so hot that you get a blister on your finger after only 5 seconds of touch? These transistors are expected to get hot. If you have multiple LED strips, you can also use a transistor for each strip to prevent over heating.
Use a suitable MOSFET (logic level, low Rds(on)) and you won't need a heatsink. Darlington's are
inefficient since they cannot have a low saturation voltage.
My advice: Stay away from NTE components. Another guy on the net jokingly says their tagline should be changed to "Worldwide Supplier of Overpriced and Ambiguously Marked Electronic Components". Did you get these at Fry's? They always have a bunch and they are great for a laugh or two at the prices they want.
I agree wholeheartedly about NTE components. Don't like them. I've done electronics repair for many, many years. NTE are my absolute last choice for replacement parts.
You don't have any capacitors on the input or output of the voltage regulator IC. Most regulators need at least an input capacitor. Without one, the regulator is likely to oscillate.
You don't have a series resistor between +5V and the base of the NTE261. So you are providing far too much base current.
dc42:
2. You don't have a series resistor between +5V and the base of the NTE261. So you are providing far too much base current.
Not true.
Why comment when you haven't consulted the datasheet for the NTE261? Its a darlington
pair with in-built resistors so its perfectly sensible to drive the "base" from 5V
dc42:
2. You don't have a series resistor between +5V and the base of the NTE261. So you are providing far too much base current.
Not true.
Why comment when you haven't consulted the datasheet for the NTE261? Its a darlington
pair with in-built resistors so its perfectly sensible to drive the "base" from 5V
Mark,
While I normally respect your replies, you really should not make unjustified accusations. I did check the datasheet before posting my reply, and it (HTTP 301 This page has been moved) shows quite clearly that the NTE261 has base-emitter shunt resistors but no series resistor. Furthermore, the datasheet quotes Vbe(on) as 2.5V max @ Vce=3V, Ic=3A. This darlington should definitely should not have its base connected directly to +5V when the emitter is grounded.
dc42:
This darlington should definitely should not have its base connected directly to +5V when the emitter is grounded.
The whole point of darlingtons is that they only need tiny amounts of base current.
Quite so, which means that you can use a relatively high value of base resistor. For example, using the figures quoted for Vce(sat) on the datasheet, you can use 12mA base current (corresponding to a series resistor of about 200 ohms - so not all that high really) to switch 3A and get a Vce(sat) of no more than 2.0V. But just like a normal transistor, a darlington still needs a base series resistor to prevent the base current being too high - except for a few types that have such a series resistor built-in.
My favorite Darlington is the ULN2803A which DOES have a base resistor built in. I could see myself making this mistake because I don't always use Darlingtons, but when I do, I prefer ULN2803As.
dc42:
2. You don't have a series resistor between +5V and the base of the NTE261. So you are providing far too much base current.
Not true.
Why comment when you haven't consulted the datasheet for the NTE261? Its a darlington
pair with in-built resistors so its perfectly sensible to drive the "base" from 5V
Argh, my bad, my eyes were in a hurry I think - I saw the resistors on the base in my mind.... Sorry.
I'll be double checking in the future.
Mark,
While I normally respect your replies, you really should not make unjustified accusations. I did check the datasheet before posting my reply, and it (HTTP 301 This page has been moved) shows quite clearly that the NTE261 has base-emitter shunt resistors but no series resistor. Furthermore, the datasheet quotes Vbe(on) as 2.5V max @ Vce=3V, Ic=3A. This darlington should definitely should not have its base connected directly to +5V when the emitter is grounded.
