[SOLVED] how to prevent TIP122 from shorting collector and emitter

[assadmahmood]

Greetings EveryBody!
I am using TIP122 to drive a Solenoid of 4 ohms working on a 24V-DC, 3.4Amp, I am giving a pulse of 500ms on the base of TIP to operation Solenoid.
In some cases TIP gets damaged and its collector and emitter get shorted. Now I want add some safety in the circuit to protect from short or if it gets shorted then something should be there to cut off the current.
Circuit diagram attached.

Many thanks for your kind consideration.View attachment tip_122.bmp

 

[godfreyl]

The transistor is damaged when it gets too hot. To prevent that, it should be mounted on a proper heatsink with thermal grease.

 

[assadmahmood]

Yes you are right, but I want to add something I mean any alternative way to break down the flow of Ic if it gets shorted. Is there any?

 

[betwixt]

In 500mS and at that current it shouldn't overheat. It's more likely the back EMF from the solenoid is killing it, do you have a diode across the solenoid coil? You should have. If not, use a 1N400x series diode with the cathode toward the positive supply so it isn't normally conducting. The voltage spike as the transistor switches off will be in the oposite polarity so the diode should safely short it out.

Brian.

...and please don't post 790Kb BMP images, I can't see them on my mobile phone!

 

[didoe]

Dear betwixt

He have used a diode to prevent EMF from the solenoid. But TIP still die , another way to solve problem ?

 

[assadmahmood]

Respective Mr.Betwixt I am surely using 6A10 diode for back EMF protection.

Now I have one solution and need guidance, what can we do is, If we add an electrolytic capacitor is series between solenoid and transistor then I would block DC voltage, Since it would let the pulse to pass. Now capacitor selection would be such that its charging time should be slightly more than 500ms.

Hows the idea?

 

[FvM]

I can't follow the calculation. 4 ohm solenoid would result in 5.5 A rather than 3.4 A current. Or ist it that the voltage drop across the transistor is so high due to insufficient base current?

Although slightly above continuous current rating, it could work with
- sufficient bases current (more than the about 3 mA given in your circuit)
- sufficient transistor heat sink

It's probably better to use a MOSFET with low Rdson, that can stay effectively cool at these curent levels.

 

[crutschow]

You might consider changing to a power N-MOSFET of about 10A or greater rating. They tend to be more robust than BJT's.

To protect against a short, just add a fuse in series with the solenoid.