struggling choosing a transistor base resistor

Using Arduino General Electronics
1

Hi all
looking for some help on choosing a base resistor for a transistor!
transistor is a ZTX300 (was the only one i could find lying around.) Im trying to run a relay from my arduino,its rated voltage is 24V and coil current is 30mA.
Ive done numerous calcs and came up with different resistor values!! Heres the one im semi confident on! Please let me know id i done it wrong
Data sheet links
transistor HTTP 301 This page has been moved
Relay http://docs-europe.electrocomponents.com/webdocs/0e8b/0900766b80e8b0be.pdf

formula Rb = Vchfe / 5lc my calcs (2450)= 1200/(5.03)=8000
So going by this am i correct in assuming i need an 8K resistor?
If not,does anyone know what resistor i need because my head hurts and i give up

2

The exact value is not important. It will tell you only the maximum value to use. Normally you will want to supply between 2 times and 10 times the current you want to give yourself some margin.
So while 8K sounds reasonable then anything between 1K and 4K7 would be fine.

3

Cheers Mike
when you say 2 times to 10 times the current i presume you mean the base current?
I have also choosen a diode to go across the relay coil (1N4001) i presume this would be fine as it is just acting as a surge protector

4

Yes that is base current.
The diode is fine, it is not surge current however it is reverse voltage caused when the relay turns off.

5

In a perfect world you would use a schottky diode as it has a faster switching speed so can bleed off the back-emf sooner, but a 1N4001 is fine.

6

Cheers guys.
1 more for yer. Do i need to connect the gnd on arduino to the 0v of the 24V or will the arduino be fine gnd through the usb cable

7

Do i need to connect the gnd on arduino to the 0v of the 24V

Yes, yes, yes

This is why:-
http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html

8

Cheers Mike. Remember reading that a month or so back aswell :blush: