Why transistor BC556 burns out frequently in this circuit?

Hi friends,

Attached schematic is the part of a bigger circuit. The voltage applied is +24Volts but it could go upto +35Volts in reality. I'm facing a problem with this part

1) Where the transistor BC556 burns out (get's shorted) every once in a while. And I have to replace it with the new one.

2) Also another issue is that when I touch around the base of the transistor BC556 and the collector of phototransistor of MCT2E, the relay sometimes get's activated or starts chattering.

Could you please help me identify the problem and solve this.

Thanks a tonne in advance.

It depend on what relay the transistor is driving. measure the current flowing through the transistor tto the coil when actuated. See wether the relay coil winding are not shorted. 10K pullup to the base for 24 volts+ supply is a bit less. Try using 100K for pullup and 10K between opto to base. Also measure the Diode and 100Nf across the relay coil for any leakage. Rest all looks fine.
Cheers

Re: Why transistor BC556 burns out frequently in this circui

Hi,

Put one capacitor or varistor in parallel to the transistor. Also one diode in the same direction like D8 in parallel to the transistor and problems will disappear.

Re: Why transistor BC556 burns out frequently in this circui

pranam77 said:

It depend on what relay the transistor is driving. measure the current flowing through the transistor tto the coil when actuated. See wether the relay coil winding are not shorted. 10K pullup to the base for 24 volts+ supply is a bit less. Try using 100K for pullup and 10K between opto to base. Also measure the Diode and 100Nf across the relay coil for any leakage. Rest all looks fine.
Cheers

Click to expand...

Thanks for the reply pranam. The relay current is around 25mA at 35 Volts it may got max 37-40 mA that's all. The transistor is capable of 100mA. I chose this transistor after calculating that. I also checked the relay and also the cap and diode, all seemd fine to me. As the matter of fact after replacing the new transistor it works for some time.

Initially I had designed it with 22K pull-up and 2.2K base resistor still it use to get burnt once in a while in operation.

Added after 7 minutes:

luben111 said:

Hi,

Put one capacitor or varistor in parallel to the transistor. Also one diode in the same direction like D8 in parallel to the transistor and problems will disappear.

Click to expand...

Thanks for the reply luben. I'd try this. But could you also tell me what could be the issue that this would solve so I'd learn something. Is it the rapid dv/dt on transistor or the back emf from the relay as it is an inductive load?

Thanks again.

You may also try using the resistor value suggested by me in my last post. My easiest solution for you will be using a BD140 transistor which is more sturdy and more caable than the one used now.

Re: Why transistor BC556 burns out frequently in this circui

yourdreamz said:

Hi friends,

Attached schematic is the part of a bigger circuit. The voltage applied is +24Volts but it could go upto +35Volts in reality. I'm facing a problem with this part

Click to expand...

The supply is unregulated emitter of BC556 is also fluctuate according the Supply Input or the Load on power supply.
Insert Diode Between +24 and emitter of BC556 and place >1uf capacitor from emitter to Ground(0Volt) the transistor will have stable voltage.

1) Where the transistor BC556 burns out (get's shorted) every once in a while. And I have to replace it with the new one.

Click to expand...

C26 may be Small capacitor (0.1uf) but what about inrush current? every time transistor switching on it encounter short circuit.

2) Also another issue is that when I touch around the base of the transistor BC556 and the collector of phototransistor of MCT2E, the relay sometimes get's activated or starts chattering.

Click to expand...

sometimes when?? the transistor on the way to short circuit or New transistor??

From your circuit i think your driving Actuator(Inductor) have you provided diode
across Actuator.
C28 & C31 should be RC not C only.
http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3236.pdf

Regards
P.Ashok Kumar

Re: Why transistor BC556 burns out frequently in this circui

ashokok said:

yourdreamz said:

Hi friends,

Attached schematic is the part of a bigger circuit. The voltage applied is +24Volts but it could go upto +35Volts in reality. I'm facing a problem with this part

Click to expand...

The supply is unregulated emitter of BC556 is also fluctuate according the Supply Input or the Load on power supply.
Insert Diode Between +24 and emitter of BC556 and place >1uf capacitor from emitter to Ground(0Volt) the transistor will have stable voltage.

Sure. I'll try that, but do you think this could cause the burn out?

1) Where the transistor BC556 burns out (get's shorted) every once in a while. And I have to replace it with the new one.

Click to expand...

C26 may be Small capacitor (0.1uf) but what about inrush current? every time transistor switching on it encounter short circuit.

This makes quiet a lot of sense. I'll remove this and try.

2) Also another issue is that when I touch around the base of the transistor BC556 and the collector of phototransistor of MCT2E, the relay sometimes get's activated or starts chattering.

Click to expand...

sometimes when?? the transistor on the way to short circuit or New transistor??

The new transistor or the one that is working fine.


From your circuit i think your driving Actuator(Inductor) have you provided diode
across Actuator.
C28 & C31 should be RC not C only.
http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3236.pdf


Sure I get that. I'll take care of that. Thanks.


Regards
P.Ashok Kumar

Click to expand...

Thank you Ashok. I'll try and check how it goes.

Added after 2 minutes:

luben111 said:

Hi,

Put one capacitor or varistor in parallel to the transistor. Also one diode in the same direction like D8 in parallel to the transistor and problems will disappear.

Click to expand...

I tried this luben, but it didn't worked. The transistor still got burnt :-(.

Re: Why transistor BC556 burns out frequently in this circui

Try put a diode across R27 with anode on the base side. You may be getting a positive switching voltage spike through on the base that exceeds the reverse breakdown of 5V between emitter and base. This may blow out your junction causing the transistor to go short collector-emitter as well.

Re: Why transistor BC556 burns out frequently in this circui

pranam77 said:

You may also try using the resistor value suggested by me in my last post. My easiest solution for you will be using a BD140 transistor which is more sturdy and more caable than the one used now.

Click to expand...

Thanks Pranam, I tried BD140 and it does work fine for now with some testing. Let's see for a longer run. About the resistor value, I understand that that would be a better design but would it burn a transistor if it is 22K and 2.2K instead? You think so?

Though I would really want to understand the reason why BC556 is getting burnt out.

hi dear
What is the Relay spec.
24V ... resistance value 100 Ohm or ????
24v 25 ma is not possible ( more than 25 ma)

Re: Why transistor BC556 burns out frequently in this circui

C26 may be Small capacitor (0.1uf) but what about inrush current?

Click to expand...

Yes, C26 has no reasonable purpose. Although the collector current is limited by the base resistor, C26 may be the final cause to damage the transistor.

C28 & C31 should be RC not C only

Click to expand...

Quite right. But it causes only problems for the relay contacts, not the transistor.

What is the Relay spec.
24V ... resistance value 100 Ohm or ????

Click to expand...

According to the above reported measurements, it's about 1K. Then the worst case transistor power dissipation would be (35/2)²/1K = 0.3 W, which should cause no problems. I'm not aware of a considerable 2nd breakdown effect in BC556 SOA.

Re: Why transistor BC556 burns out frequently in this circui

FvM said:

C26 may be Small capacitor (0.1uf) but what about inrush current?

Click to expand...

Yes, C26 has no reasonable purpose. Although the collector current is limited by the base resistor, C26 may be the final cause to damage the transistor.

C28 & C31 should be RC not C only

Click to expand...

Quite right. But it causes only problems for the relay contacts, not the transistor.

What is the Relay spec.
24V ... resistance value 100 Ohm or ????

Click to expand...

According to the above reported measurements, it's about 1K. Then the worst case transistor power dissipation would be (35/2)²/1K = 0.3 W, which should cause no problems. I'm not aware of a considerable 2nd breakdown effect in BC556 SOA.

Click to expand...

Thanks FvM. I tried removing C26, but it still gets burnt.

And pardon me, I didn't get your last statement. "I'm not aware of a considerable 2nd breakdown effect in BC556 SOA." Can you elaborate?

Added after 3 minutes:

E-design said:

Try put a diode across R27 with anode on the base side. You may be getting a positive switching voltage spike through on the base that exceeds the reverse breakdown of 5V between emitter and base. This may blow out your junction causing the transistor to go short collector-emitter as well.

Click to expand...

Thanks E-design, I'd try this.

Re: Why transistor BC556 burns out frequently in this circui

Hi,
In my opinion the main reason for buring the transistor could be the bad PCB design - normally a single diode should prevent the problem but in your case things became too complex. I would suggest to publish the PCB design to analyze it.
I meant - probably the problem for burining the transistor is beyond the schematic.

2nd breakdown is an effect that causes damage of transistors in a particular region of the current & voltage characteristic, but I assumed, it doesn't occur with transistors like BC556. It's not mentioned in the datasheet.

Re: Why transistor BC556 burns out frequently in this circui

as pranam77 said have measured the current flowing through the transistor to the coil when actuated

Also another issue is that when I touch around the base of the transistor BC556 and the collector of photo transistor of MCT2E, the relay sometimes get's activated or starts chattering.

Click to expand...

sometimes when?? the transistor on the way to short circuit or New transistor??

Click to expand...

The new transistor or the one that is working fine.

Click to expand...

the new transistor Chattering is due to pull-up resistor R27(10K Check Value directly between base and emitter pin ) not connected or the MCT2E may be partially faulty. i think if you solve this fault the transistor burning also solve automatically if power supply is stable.
What types of loads are connected
How much is load current sufficient filter cap is provided
This fault only to particular PCB or all the PCB s??

as luben111 said publish the PCB design.

P.Ashok Kumar.

Re: Why transistor BC556 burns out frequently in this circui

ashokok said:

as pranam77 said have measured the current flowing through the transistor to the coil when actuated

Also another issue is that when I touch around the base of the transistor BC556 and the collector of photo transistor of MCT2E, the relay sometimes get's activated or starts chattering.

Click to expand...

sometimes when?? the transistor on the way to short circuit or New transistor??

Click to expand...

The new transistor or the one that is working fine.

Click to expand...

the new transistor Chattering is due to pull-up resistor R27(10K Check Value directly between base and emitter pin ) not connected or the MCT2E may be partially faulty. i think if you solve this fault the transistor burning also solve automatically if power supply is stable.
What types of loads are connected
How much is load current sufficient filter cap is provided
This fault only to particular PCB or all the PCB s??

as luben111 said publish the PCB design.

P.Ashok Kumar.

Click to expand...

Thanks Ashok and Luben. I noticed, when I put a finger on the top of MCT2E without touching any of its pin, the relay starts chattering. Looks like as you said MCT2E could be bad.

You mean shall I upload the gerber?

Re: Why transistor BC556 burns out frequently in this circui

please answer the following
1)This fault only to particular PCB or all the PCB s??
2)from where your feeding the power supply (source of supply)
3)Sufficient filter capacitor connected on the PCB
3)what all the loads connected to PCB
4)How much is the load current
5)Inductive loads are connected with reverse diode like relay coil


P.Ashok Kumar

For better understanding your design and work and the probelm, you may upload the High resolution photos of your PCB track side and the component side.
Cheers

Re: Why transistor BC556 burns out frequently in this circui

you should not connect coil terminal to C28.i think that,s the problem.

Re: Why transistor BC556 burns out frequently in this circui

Thanks everybody and thanks Ashok. Finally the transistor burning issue is resolved. All I did is change the transistor from BC556 to MPS2907 (600mA) and it worked alright. I also changed the Optocoupler from MCT2E to CNY17-4 and even the 'relay chattering when optocoupler was touched' problem disappeared.

But I am facing a new issue. The issue looks like an EMI issue. I am driving a Linear Actuator that has a DC Motor, so inductive load. I'm driving it Forward and Reverse so supply is given both ways. Have a look at the schematic.

What happens is when I drive the actuator forward, it works fine. When I drive it reverse it goes fine until the relay goes back to normal position to stop the drive voltage to the actuator. The microcontroller PIC16F887 hangs. Exactly when the relay's Common terminal separates from Normally Open (NO) terminal.

I have done a proper protection at the Power Supply side using ferrite beads. Also I tried probing the power supply pins and other I/O pins of microcontroller on Oscilloscope, it doesn't show any transient or impulse behavior. It looks very clean. I don't know how and where the EMI is disturbing from and hangs the microcontroller.

I'm driving it Forward and Reverse so supply is reversed. Thus I can not directly connect a reverse Diode across it. Still I tried putting a Diode nearest to the Relay terminals between +24V and GND just before it goes to NO and NC of the relay. But still the issue remains the same.

Any suggestions are welcome.