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Help with frying transistor

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Hi everyone,

I have a problem with my circuit where a transistor keeps frying, and I don't understand why ...

Here's my circuit : http://tinyurl.com/29vbmd3

I have a 12DC source, and 2 switch mode power supplies. One outputs 3.3V/0.5A (to an LED, and a PIC not shown), the other outputs 24V/1A, for a solenoid valve  . Eventually I'll have the PIC control my valve, but for now I just connected the valve as shown in the diagram to make sure things are working ...

At the bottom of the diagram I implemented a transistor switch using the ZTX1048A NPN transistor (http://tinyurl.com/2azppvc), which according to specs can handle Ic of 4A. I chose the resistor value for R1 to be 4KOhm so the transistor Q1 saturates.

When there is no valve connected, everything works great. I turn on my switch, the 3.3V power supply turns on my LED (drawing about 16mA), and it powers my PIC and LCD just fine too. Great. When I connect my valve as shown in the diagram and turn my switch on, the LED turns on for maybe 1/4th of a second, and during that time I can hear my solenoid valve open and then close. Then, unless I immediately turn my switch OFF, my transistor Q1 starts smoking and then it just dies. While the LED is ON for that short time, it also makes some vibrating sound ... don't know why.

The valve's specs say it wants 24V and consumes 6.9W : http://www.valvestore.com/prodinfo.asp?number=42703
So that's less than 300mA of current.

- Why is Q1 frying ?? It seems to me that it should be able to handle the current going through it.
- Why is my valve causing some vibration on the LED ?

Thanks a lot !


Problem is your transistor selection/design.  With the 4K base resistor, you have a base current of 2.7 mA.  Multiplied by the beta of about 300, you only have a collector current of about 800 mA.  Your load is likely about 1 amp, since the load is 7 watts, and figuring the  efficiency of the boost converter,  you end up pulling more current than can be supported.  Add to that the second converter, you need to improve your switch current capacity.  You need to keep the transistor in hard saturation.  This means take the min beta and min supply voltage and calculate your base resistor based on that.  Don't forget to subtract the current loss from the 90k resistor.  Running in saturation, usually a forced beta should be used, meaning that you overdrive the base to assure saturation.  Using a forced beta of 50 should be safe.

Again looking at the curves, you are also getting close to the SOA limits for DC operation.

I would use a TO220, if space is not an issue, and make it an easy design thermally. 


I was thinking about the same: the transistor is not saturated, so its Vce is high for the 1W max power.
Even if the BJT wouldn't fry, you would like it to dissipate the lowest power possible.

Aha !! That was it !! I thought I had enough base current for saturation, but I swapped my base resistor for a 1K resistor for a quick test, and now it works.

Thank you for your help !!


Beware that the Hfe values given on the data sheet often apply when VCE is 5V which is fine for a linear amplifier but not what you need from a switch. As a general rule of thumb make IB=IC/10 and it should be fine, if in doubt look at the data sheet.


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