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Electronics => Electronics => Topic started by: Jak24 on June 19, 2010, 07:50:47 PM

Title: transistor activation
Post by: Jak24 on June 19, 2010, 07:50:47 PM

HI

my  circuit is attached!
how much ohm should resistor 2 be?.....i tried 500 - 1000 ohm but i didn't work :P

here are the datasheets of the:
transistor http://www.fairchildsemi.com/ds/2N%2F2N3904.pdf (http://www.fairchildsemi.com/ds/2N%2F2N3904.pdf)
and relay: http://www.quazerel.com/datascheet_pdf/12522.pdf (http://www.quazerel.com/datascheet_pdf/12522.pdf)   mine is(10 A 12dc ,
coil resistance  = 400 ohm)
please reply ASAP.
thanks

regards

Title: Re: transistor activation
Post by: Razor Concepts on June 19, 2010, 08:52:19 PM

Try 100 ohm

Title: Re: transistor activation
Post by: Soeren on June 20, 2010, 10:35:35 AM

Hi,

Quote from: Jak24 on June 19, 2010, 07:50:47 PM

how much ohm should resistor 2 be?.....i tried 500 - 1000 ohm but i didn't work :P

If the transistor is OK, then you probably have wired something wrong or shunted something.

Assuming 12V on the relay the collector current amounts to 12/400 = 30mA
The transistor will have a gain of at least 100, so the base current should be 30m/100 = 300µA

Assuming the digital pin is 5V logic, the base resistor should then be (5-0.7)/300µ = 14.3 kOhm
Selecting 10k you have a bit of overhead (which you need due to the other resistor).

Anything lower should work as well, as long as it doesn't get low enough to load the pin down - you can get the output capability in the datasheet for the mysterious "digital device" having the pin.

1kOhm should work, else go upwards towards 10kOhm (don't go lower).

Title: Re: transistor activation
Post by: gamefreak on June 23, 2010, 06:18:42 AM

I am curious as to why you have a 10k Ohm resistor between the base and ground. Since transistors are current devices shouldn't it shut off naturally without the need to be pulled down?

Title: Re: transistor activation
Post by: waltr on June 23, 2010, 07:12:59 AM

Quote from: gamefreak on June 23, 2010, 06:18:42 AM

I am curious as to why you have a 10k Ohm resistor between the base and ground. Since transistors are current devices shouldn't it shut off naturally without the need to be pulled down?

Leakage from the collector to the base can pull the base high enough for the transistor is start conducting. Sometimes this doesn't happen but many times it does. Therefore its good practice to ensure that the base stays off especially when driven from a processor output pin as most processors pins are inputs, hi-Z, after a power on reset until the code sets the pin to output.