i have a transistor switching a 12v relay on and off. the problem is it burns out after sometime of operation.
transistor used is bc547. the relay coil is 285ohms
calculation of base resistance:
hfe=110 (min. value; from datasheet)
Ic=(12-0.7)/285=39.6mA (ignoring 1kohm resistance in parallel)
Ib=Ic/hfe=0.36mA
Rb=(5-0.7)/0.72m=5.97kohm (Ib is doubled)
are these calculations correct?why is that the transistor gets heated and burning out? is the base current too high for it to handle? not sure what is happening. can anyone please help?
this is the transistor relay circuit.the image is taken from google.
i connected a series combination of 1kohm resistor and led in antiparallel to the diode across relay coil.
the input to base of transistor is from arduino, which is 5v.
Do you have a multimeter? The transistor may not be in saturation.
Your calculations look OK, but it's standard practice to overdrive the transistor "hard" into saturation. I'd try cutting the base resistor in half, or possibly go down to 1K.
Ib is doubled to make sure the transistor is saturated. Isn't that enough or should it be more increased for driving the transistor hard into saturation (as you said)?
but, if resistor is much reduced, i.e. 1k , transistor draws more current. So does this not cause overheating of the transistor?
If you intend to do more projects, what you need is a logic level gate mosfet. This will take the relay closer to Gnd without the CE voltage drop with the transistor. They will also handle more current than the basic transistor.
You need logic level MOSFET because the gate needs to be fully on or the resistance of the drain/source is not at its least so causes heat.
My favourite small mosfet is NTD5867NL. It works with 5v on the gate and handles a few amps with ease. They are cheap. I bought mine from RS Components.
There are others but make sure they are fully on at 5v on the gate.
Adding to weedpharmas comments, I'd like to know why your using both a transistor and a relay.
A mosfet can deal with almost any (DC) voltage/current and it can deal with any that it's safe to play with. There are also solid state devices which deal with AC.
I also find it difficult to understand why you are using a relay that needs 12V to activate it? that is 12V across the coil, you did mean that didn't you?
Burns as in getting hot, or it just pops it's top?
Something fishy going on.
A BC547 driving a 12v/285ohm load will dissipate 0.125watt worst case if not saturated (6volt!).
It might get a little hot, depending on cooling by the wiring/circuit board, but it should survive that.
AFAIK, tested ones have a letter behind the number. e.g. BC547B.
But even an untested BC547 must have a DC gain over 100.
As sugested, measure the collector voltage with the relay 'on', but test also the load current draw.
Relay 'off'. DMM on 200mA current, test leads on collector and ground.
Calculated = ~42mA....
Some relays have inbuild kickback diodes.....
My favorite mosfet to replace a BC547 with is the 2N7000. TO-92, 60volt, 100-200mA.
Cheap, and no extra parts needed apart from the flyback diode.
Leo..
I'd have to agree with the #1 reply and use 1K base resistor.
4mA base current won't kill it.
Don't know where you found the term "anti-parallel" though...maybe Dr. Who...?
holmes4:
I'd like to know why your using both a transistor and a relay.
I also find it difficult to understand why you are using a relay that needs 12V to activate it? that is 12V across the coil, you did mean that didn't you?
Hi, that is because I have a 12V relay and the output from the arduino is only 5V.
Yes, it is 12V across the relay coil.
raschemmel:
You should not put a led in parallel with the flyback diode because it compromises the back EMF protection.
the led has a 1kohm resistor in series. this led blinks whenever the coil is energised.
Wawa:
Burns as in getting hot, or it just pops it's top?
Yes, it gets hot and produces fumes. Thereafter, it doesn't work.
I have connected another 6kohm resistor to the base of the transistor, with a total of 12kohm as transistor base resistance. This works fine. But I dont think this is the right way to do.
bluejets:
I'd have to agree with the #1 reply and use 1K base resistor.
4mA base current won't kill it.
Don't know where you found the term "anti-parallel" though...maybe Dr. Who...?
With a 6k resistor, the transistor failed. I am not sure of 1kohm :-\
Well, this word "anti-parallel'', I got it from my teachers.
Did you already measure the true load current, as explained in post #8?
And yes, a 1k base resistor should be better than 12k... if the load current is below 100mA.
You want this transistor to fully saturate.
I expect no more than 60mA with the LED on.
Leo..
Wawa:
Did you already measure the true load current, as explained in post #8?
And yes, a 1k base resistor should be better than 12k... if the load current is below 100mA.
You want this transistor to fully saturate.
I expect no more than 60mA with the LED on.
Leo..
A 40W bulb is connected as load to the relay. It draws nearly 170mA. The relay coil draws 44mA.
When the relay is on, Vce was 0.2V and when it is off, Vce was read as 12V.
Personally i would drop value of base resistor to make sure its saturated in conduction.. When ever i need to drive transistor as switch i usually go resistor values at 1 KOhm or lower. Transistors beta value is bad at this since it can change from transistor to transistor... Read datasheet, sometimes beta is indicated with massive minimum/maximum spread...
Or use something like ULN2803 ic. it still needs those protection diodes..
None of this is making sense. There is no reason why the transistor should burn out if wired correctly. Are you sure you don't have the collector and emitter reversed ?
raschemmel:
None of this is making sense. There is no reason why the transistor should burn out if wired correctly. Are you sure you don't have the collector and emitter reversed ?
