[AVR] Using PNP S8550 Transistor with Atmega8

[asking]

Hi,

I want to use PNP Transistor being driver from pc817(NPN) Optocoupler which is driven by Atmega8. Can anyone give me stable circuit to drive the relay 155ohm 12V 30A?

 

[KlausST]

Hi,

Show us your circuit.

Klaus

 

[asking]

mistake: Transistor is upside down. (Ignore)

ACtually with this design it works. but transistor blows. As base voltage is more than -5V as per datasheet.

Thanks.

 

[barry]

You need to size R1 so that VBE is less than the absolute maximum. Probably 100-200 ohms.

 

[asking]

I don't understand why datasheet says maximum voltage is -5V to the base ? How do i calculate that from resistor positions ?

 

[KlausST]

Hi,

relay coil is 155 Ohms. Voltage is 12V,
Current = 12V/155 Ohms = 77mA (at collecotr)

For switching purpose let base current about 1/10 (in datasheet they use 1/20) of colletctor current. Lets say about 7mA

V_BE = about 0.6V (to be excat -0.6V)

VCE sat of optocoupler below 0.5V (guessing)
So R2 = (5V - 0.6V- 0.5V ) / 7mA = 557 Ohms.

So use something between 470 and 750 Ohms.

****************

This is about what you did. So why is transistor overheating?
Maybe you don´t drive the optocoupler strong enough. It´s not clear if it is 10k or 220Ohms.
10k is too much. Use 220 Ohms to 470 Ohms here.

To check on this: Switch your port pin high and measure V-CE of optocoupler. It must be below 0.5V

*******

Datasheet.
My datasheet (ONSemiconductor) says V_EB0 = -5V (Note: in some datasheets they use V_BE0, wich is the negative voltage of -5V = 5V)
This means the voltage form emitter to base must not be more than -5V. With your circuit it is between 0V (while OFF) and 0.6V (while ON). This is all OK.

Klaus

 

[asking]

Finally i've got something easy

Thanks for your suggestion..

 

[KlausST]

Hi,

Use an NPN combined with 470 ohms instead of PNP. Emitter to GND.

With your circuit you get folowing disadvantages
* the uC port is (Current limited with about 0.1mA) pulled up in 12V direction.
* switching OFF of relay is not guaranteed, because relay OFF current is about 35mA
* transistor power is about 200mW in OFF state
* transistor power is about 70mW in ON state
**********
With the NPN solution you
* get 0 relay current while OFF
* no transistor power during OFF state
* about 20mW during ON state
* full relay voltage during ON state

There are so many schematics on how to switch a relay.
Why trying to reinvent the wheel, but not calculating the circuit?

Klaus

 

[asking]

I have no direct connection from Microcontroller. Microcontroller DRIVES optocouple in turn opto PC817 drives Gnd path. Do you have any scheme that drives PC817 (NPN Transistor) another NPN transistor's base ? then please show us Schematic.

 

[KlausST]

Hi,

this is not clear in your schematic, that the optocoupler drives the PNP.
Why do you use a optocoupler? It is not necessary as long as you don´t need isolation.
******
Solution for optocoupler and PNP:
Use schematic of post#3
* correct PNP connection (upside down)
* Use 470R as R1
* use 470R as "220R"
finsihed.

*********
Simple and working solution without optocoupler:
Needed: Relay, free wheeling diode, NPN, 470R, uC
* NPN E to GND
* NPN B to 470R
* 470R to uC port pin
* Relay + to 12V
* free wheeling parallel to relay
* NPN C to Relay- (and diode anode)
finished.

Klaus