Help with frying transistor

[galpi]

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 !

Pierre

[tecman]

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. 

paul

[scrat]

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.

[galpi]

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 !!

Pierre

[Zero999]

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

[Mechatrommer]

good info here. where can i get any link, web or pdf regarding this? step by step designing with transistor? beta, Hfe, saturation, reading graph etc? this simple transistor always is a mystery for me.

[Time]

Designing with a transistor?  There are many books on it.  Whole courses based on it actually... even a degree... EE.

[Mechatrommer]

well, i dont have ee degree. if there is online info? the step by step very basic guide?

[Alex]

What's going on with that LED? You have a 12 to 3.3 V buck converter but then you wrote 12V on the converter's output?

An idea would be to incorporate the LED (if it is a small high efficiency one) in the circuit around Q1 saving you a regulator and a resistor.

EDIT: You can also opt for a boost converter with an enable/disable pin and possibly get rid of Q1.

[tyblu]

First you'll want to get to know transistors as semi-directional switches (current doesn't flow well one way, from source to drain in N-channel or emitter to collector in PNP due to manufacturing techniques). After that, try out an amplifier circuit, and try to get maximum gain out of it. At this point you can read up on the various amplifier circuits, then start touching Art of Electronics. As always, hands-on is best! I searched for tutorials and was simply inundated.

[galpi]

@Alex

>What's going on with that LED? You have a 12 to 3.3 V buck converter but then you wrote 12V on the converter's output?

Oops ... Should be 3.3.V , made the diagram a little too quickly. Thanks.

Pierre

[alm]

well, i dont have ee degree. if there is online info? the step by step very basic guide?

Any good basic electronics textbook/course should cover it. Eg. Art of Electronics, or All About Circuits.

[Simon]

when you select a transistor you need to consider not only it's maximum current capability but also the max wattage, it is common place in situations like this to use well over speced parts just to be sure, a N channel mosfet like the IRF540 would be ideal

[NiHaoMike]

Use a MOSFET instead of a bipolar.

[galpi]

Yeah, I guess I could use a MOSFET here. I have a large enough gate voltage to turn the IRF540 on.

>when you select a transistor you need to consider not only it's maximum current capability but also the max wattage

I'm still having a hard time understanding that stuff from reading the specs. In the ZTX1048A specs, it says it has a maximum power dissipation of 1W, and Icmax=4A. When in saturation mode, the resistance Rce approaches 0, making power consumption low ( <1W), even as Ic approaches 4A. Is that correct ?
But my transistor wasn't fully saturated ... and in this mode of operation Ib controls Ic. I thought it would limit Ic such that maybe the valve wouldn't turn on, but instead it seems the valve drew what it wanted from its power supply and fried the transistor. Wasn't Ib supposed to limit Ic and keep power dissipation under it's absolute max ?

Pierre

[Simon]

with 1W dissipation it only takes a small voltage drop to fry it if the transistor is not fully turned on, if not fully saturated it will quickly have quite a resistance and easily consume 1W, usually a fully saturated transistor will have a Vce of 0.2V so 4A x 0.2 V = 0.8W - almost maximum dissipation and it is unwise to take full advantage of the maximum specs as you should have a safety margin. so if not completely saturated it will burn out, simple. on the other had a nice beefy and cheap mosfet such as the IRF540 (IRL540 if your using a pic of other low voltage output to control it) has a full on resistance of just 0.044 ohms so with 4 amps thats 0.7W but it is capable of much more power dissipation (150W I beleive with heatsink of course) for your application a heat sink is not even needed