Darlington transistor extremely hot

Why would a Darlington Transistor get hot?

I'm surprised it works at all with the 1N916 diode in series with the bulb, it's only rated at about 200mA continuous.

The datasheet for the light bulb specs a current of 1A when it is fed 28V. In your corrected circuit its voltage is only about 13V or 14V so we don't know the current.

Your circuit does not have a supply voltage for the digital logic so we cannot calculate if the base current is 1/500th the collector current for the darlington to saturate properly.

hFE is used when a transistor is a linear amplifier that has plenty of collector to emitter voltage (5V for this darlington), not as a saturated switch.

Could you put scope between D1 and R2 to observe the wave form from original ckt and your replacement? Do they look alike?

What is the supply voltage on your NOR gate?

Why the heck is this a Poll ?

The problem is that you are using a tiny transistor in TO-92 to control large power in linear region. This cannot end well.
Also increasing C1 means the transistor stays longer in linear region, hence it´s demise.
Either use PWM or get some transistor in TO-220 or larger case.

Why in god's name are u driving a 3Amp load from a 1 Amp transistor.

It should blow up in ur face if u did that u know.

If U cannot get any other transistor then u shud increase R3 Value.

The 60k resistor is causing the transistor's base and collector current to be lower, which, paradoxically, can increase the power dissipation of the transistor. Try shorting out the 60k and see if the heating problem disappears.

The MPSW45 has a very low base current of only 0.35mA so its saturated collector current can be only 500 times which is 175mA. Since the current in your light bulb is higher then the darlington is not saturated and is burning.

The CM1820 light bulb draws only 0.1A at 28V, not 3A.
This circuit has a 24V supply and the saturation voltage loss of the darlington and 4.7 ohm resistor produces only 22.5V across the light bub then its current is only about 85mA and the darlington dissipates (1V x 85mA)= 85mW and will not be warm.

If the supply for the Cmos gate is 12V then the base current is (12V - 1.4V)/62.9K= 0.17mA. Then an MPSW45 darlington should saturate pretty well when its output is (0.17mA x 500)= 85mA.

How about the driving pulse. What's the duty cycle ?

Audioguru said:

The CM1820 light bulb draws only 0.1A at 28V, not 3A.
This circuit has a 24V supply and the saturation voltage loss of the darlington and 4.7 ohm resistor produces only 22.5V across the light bub then its current is only about 85mA and the darlington dissipates (1V x 85mA)= 85mW and will not be warm.

If the supply for the Cmos gate is 12V then the base current is (12V - 1.4V)/62.9K= 0.17mA. Then an MPSW45 darlington should saturate pretty well when its output is (0.17mA x 500)= 85mA.

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I agree..

OP..show us a photo of ur setup

First, build the circuit in its original form, especially Q3, and make sure it works. Replace n1 and n2 each with the NOR from N1C (that's two NOR gates). Then, replace Q1 with your substitute Darlington.

Ron H said:

The 60k resistor is causing the transistor's base and collector current to be lower, which, paradoxically, can increase the power dissipation of the transistor. Try shorting out the 60k and see if the heating problem disappears.

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Did you try this?

Is it compulsory for Q3 to be a darlington ?

It's always better to use a MOSFET.

samridh said:

Hi audioguru,
I didnt understand the point you are trying to make here. Are u suggesting to supply the transistor 12 V instead of 24V?

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No.
You were asked in post #9 "What is the supply voltage for the NOR gate?" but you did not answer.
So I am just guessing that it is +12V. The maximum allowed supply voltage for the HCF4025B Cmos NOR gate is +18V, not +24V.
The circuit should work and the MPSW45 should be barely warm when the gate has a supply between +12V and +18V.

"CMOS GATE" where is that???

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Look at the datasheet for the Cmos HCF4025B NOR gate.

Audioguru said:

No.
You were asked in post #9 "What is the supply voltage for the NOR gate?" but you did not answer.
So I am just guessing that it is +12V. The maximum allowed supply voltage for the HCF4025B Cmos NOR gate is +18V, not +24V.
The circuit should work and the MPSW45 should be barely warm when the gate has a supply between +12V and +18V.


Look at the datasheet for the Cmos HCF4025B NOR gate.

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See Post #13:

The Supply on the NOR gate is 18V and on the original NOT gate it is 10V.

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Have you checked the polarity of D2?
BTW, what is the purpose of D2?
Also- R4 should be 18k to ensure that Q3 saturates.

When the light is continuously lit, measure the voltage across the 4.7 ohm resistor. If it is more than 0.4V then the light bulb draws more current than only 85mA.