Circuit digram

[Gregory]

Having desigined the Posted Circuit I powered the circuit up and smoke came out of the Transistor and 555.
The circuit is temperature driven design which trippes the 12V Dc 40A relay which then gives off a audible and visual Alarm (red led)
Is there something wrong with the circuit.

 

[akg]

... circuit up and smoke came out of the Transistor and 555.

accidently reversed the power supply ???

 

[Nigel Goodwin]

Having desigined the Posted Circuit I powered the circuit up and smoke came out of the Transistor and 555.
The circuit is temperature driven design which trippes the 12V Dc 40A relay which then gives off a audible and visual Alarm (red led)
Is there something wrong with the circuit.

Assuming you connected it all correctly, the first thing that 'jumps out' is your transistor, it looks far too small to feed an auto-motive relay?.

Also, what's the speaker for? - it serves no purpose, and it also causes excessive current through the transistor.

 

[akg]

the spkr may be actualy a pizo buzzer?

 

[Nigel Goodwin]

the spkr may be actualy a pizo buzzer?

It does say '8' next to it, which presumably means 8 ohms?, but this could just be the package he's used to draw it?.

 

[mstechca]

I powered the circuit up and smoke came out of the Transistor and 555.

Thats because too much current is used in your circuit.
I have drawn two lines on your circuit. The light red line shows one path the current goes, and literally treats your speaker as an 8-ohm resistor, which then forces the collector to be on.
The dark red line is a problem, because you haven't found out what is INSIDE the 555 timer. and if the output pin of the 555 timer is connected to an internal resistor of say 1K and connected to VCC, then I can perfectly see why the transistor can blow up.

....which then gives off a audible and visual Alarm (red led)

It gives off smoke at the moment. You seem to have the timer wired up in the one-shot mode, and there is nothing in your circuit that shows an oscillator. Look up more on 555 timers, and with some adjustments, you can wire one up as an oscillator (multivibrator).

Try replacing your problem areas with my version (see next attachment).
All my version does is activate the relay when the button is pressed.
the x ohm resistor can be any value. I don't know the maximum current your relay can handle. so find that out, and use ohms law to determine the value for the x ohm resistor.

Notice how I isolated the two stages with a coupling capacitor? This prevents any internal resistors in the 555 from damaging the circuit, while allowing the signal to go through.

Is there something wrong with the circuit.

I have explained that above.

 

[Gregory]

The previous circuit design

Thanking you for your response.
The speaker in the circuit digram is a Pizo buzzer.
The relay drawes 500 MA of current but the contacts that go to the battery charger are normally closed ( this side of the relay is 40 Amp) until the thermistors detect a temperature increase which open circuit is developed for protection of the battery charging circuit.
The red led and the pizo continue to indicate that there is a problem with the charger until the problem is repaired.

 

[audioguru]

you haven't found out what is INSIDE the 555 timer. and if the output pin of the 555 timer is connected to an internal resistor of say 1K and connected to VCC, then I can perfectly see why the transistor can blow up.

No!
What does the output resistance of the 555 have to do with the transistor?
When the output voltage of the 555's output goes high to about 10.3V, then a base current of 20mA will flow through the 470 ohm resistor. 20mA of base current is perfect to saturate a typical transistor. So why would the transistor blow up?

Maybe 470 ohms is too high resulting in not enough base current for a transistor at the low end of its rated gain. Then it won't saturate well enough and will get hot.

Notice how I isolated the two stages with a coupling capacitor? This prevents any internal resistors in the 555 from damaging the circuit, while allowing the signal to go through.

No!
Your coupling capacitor might blowup the transistor, since you removed the series base resistor.
When the output of the 555 goes high with 200mA of output current capability, then the coupling cap charges into the base of the transistor with 200mA.
When the output of the 555 goes low with 200mA of output current capability, then the coupling cap discharges into the avalanching emitter-base junction of the transistor with 200mA.

I don't know why you have a 470k resistor and Rx.

I wish I could see the pins numbers or functions of the 555 in the fuzzy Jpeg pic. :cry:

 

[akg]

Re: The previous circuit design

Thanking you for your response.
The speaker in the circuit digram is a Pizo buzzer.
The relay drawes 500 MA of current but the contacts that go to the battery charger are normally closed ( this side of the relay is 40 Amp) until the thermistors detect a temperature increase which open circuit is developed for protection of the battery charging circuit.
The red led and the pizo continue to indicate that there is a problem with the charger until the problem is repaired.

good that the spkr is piezo..
but 500ma is not good for the selected transistor use BD139/140 better as darlington

 

[mstechca]

What does the output resistance of the 555 have to do with the transistor?
When the output voltage of the 555's output goes high to about 10.3V, then a base current of 20mA will flow through the 470 ohm resistor. 20mA of base current is perfect to saturate a typical transistor. So why would the transistor blow up?

I still think that resistor is too low. There are some transistors that can't handle 20mA.

Your coupling capacitor might blowup the transistor, since you removed the series base resistor.

How can it?
a capacitor can't let DC go through it.

When the output of the 555 goes high with 200mA of output current capability, then the coupling cap charges into the base of the transistor with 200mA.

I fail to understand how that is possible. If a capacitor could do it like that, then why can't I connect a battery, a light and a capacitor in series and make the light continuously on?

I thought the capacitor pin touching the transistor will only charge to the base bias current.

I don't know why you have a 470k resistor and Rx.

470K is for the transistor bias.
Rx is extra resistance to prevent the relay from malfunctioning dueto excessive current.

 

[audioguru]

I still think that resistor is too low. There are some transistors that can't handle 20mA.

He selected a transistor rated at 800mA max to drive his 500mA load. The BC338 is shown in its datasheet to saturate very well with a base current of 50mA and a 500mA load. Do the math for the value of the resistor for it to provide 50mA of base current. (Hint: 192 ohms)
A transistor that can't handle 20mA of base current wouldn't have anywhere near a 500mA collector rating.

a capacitor can't let DC go through it.

No!
A capacitor charges and discharges with DC current. Try discharging a big capacitor with a screwdriver. LOTS of DC current. Try charging a big capacitor through a diode. If nothing limits the DC current then the diode will disappear!

I thought the capacitor pin touching the transistor will only charge to the base bias current.

No!
The base-emitter junction of a transistor is a diode with nothing to limit the current! The 555's output current is 200mA to charge the capacitor. The base-emitter diode is in series with the capacitor so it also must pass the 200mA.

470K is for the transistor bias.

With the 12V supply, a 470k resistor will provide only 24uA of base current if it connected to 12V. If the gain of the transistor is 100, a load of only 2.4mA could be switched. The relay is a load of 500mA!
You have the 470k resistor connected to the transistor's collector, so the transistor will conduct about 1.2mA all the time, until it is turned off momentarily when the coupling cap is driven low.

Rx is extra resistance to prevent the relay from malfunctioning due to excessive current.

How can a 12V relay have excessive current from a 12V supply?
Your Rx will prevent the relay from working.

 

[Gregory]

Circuit Digram with photoes

There a photoes attached for you to view and to place some light on my problem.

 

[Roff]

Guys, here's a redrawn and simplified version of the schematic. I left out the piezo sounder and the LEDs. I can't figure out what it does, but maybe it will give you something to chew on that is drawn in a somewhat conventional form.

 

[audioguru]

Thanks, Ron. It is clear now.
The 555 circuit isn't a mono-stable, astable nor comparator.
The schematics don't show the thermistor nor those two little blue things.
It looks like the two little blue things are connected between pin 7 and the positive supply. Maybe they are the very low resistance thermistor.
Therefore whenever the 555 is reset, which could easily occur at power-up, the discharge pin 7 must conduct a heavy current to ground which makes smoke. :lol: :lol: :lol:

 

[audioguru]

The BC338 transistor smokes because it is mounted backwards on the pcb. :lol: :lol: :lol:

 

[Roff]

Thanks, Ron. It is clear now.
The 555 circuit isn't a mono-stable, astable nor comparator.
The schematics don't show the thermistor nor those two little blue things.
It looks like the two little blue things are connected between pin 7 and the positive supply. Maybe they are the very low resistance thermistor.
Therefore whenever the 555 is reset, which could easily occur at power-up, the discharge pin 7 must conduct a heavy current to ground which makes smoke. :lol: :lol: :lol:

AG, I didn't see any little "blue things". :?: :?:
Nothing is connected to pin 7 in his schematic. The thermistor I simplified to a 50k resistor. I see now that he probably just used 100k resistors as symbols, as the thermistor is actually low resistance. I just wanted to make it readable so all could see that it apparently will do nothing as temperature changes, except possibly destroy the transistor. :cry:
EDIT:
Oh, you mean on the photo. I was just redrawing the schematic. I think they go between pin 2 and VCC - the thermistors ("100k") in the schematic.

 

[mstechca]

correct me if I am wrong, but his photo shows the 555 timer backwards (the groove in the IC is on the right). Also, the VCC pin should be connected to +ve.

It is unclear from the photo what half the components are. I couldn't even determine what is connected to +ve, if anything.

Have you thought of maybe inserting the components all the way in (like your resistors?)

I managed to fit my transistors all the way in.

and another thing. get acquainted with the IC BEFORE using it, or you will have the same problem again. When you deal with an unfamiliar IC, do a search for the number you see on the IC. Chances are that you will get a PDF file showing everything about the IC.

The manufacturer doesn't matter. It is the part number that does, and determines how the IC works in different circuits.

 

[Roff]

I don't think the IC is backwards. See the annotated photo below. He has labeled + and - for the power supply connections, and pin 8 definitely connects to +.

 

[audioguru]

Did you notice the little hole blown in the top of the backwards transistor?
It would be very difficult to re-insert the smoke so I think the transistor should be replaced and installed THE CORRECT WAY AROUND!

 

[Gregory]

Thanking you for your continues help.

The two Thermistors are for heat detiction to turn the 555 off at the set temperature.
The green led is to indicate the unit is on and working.
The red led is there so when the green goes out due to the 555 being tripped then the red is lit up and piezo gives off audible sound.
Pin 7 on thr 555 is not conected to any thing.