[SOLVED] Driving 5V relay by a transistor

Status
Not open for further replies.
D

demonspells

Newbie level 2
Joined
Dec 8, 2010
Messages
2
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,291
hey
am having a problem driving a 5V relay by 2N2222 NPN transistor by the following circuit:
**broken link removed**

when I increase the supply voltage, it works fine
and when I try to energize the relay directly by 5 volts it works too
 

Hi,

Can you explain it more with referring to diagram ?
 

If you mean that shorting the collector and emitter together operates the relay but putting a logic '1' on the MCU input doesn't operate it, the reason can only be that insufficient base current is flowing and the transistor VCE is not saturating. The 2N2222A is a low gain transistor, if your MCU has a 3.3V output, the most base current that can flow is (3.3 - 0.65) / 10,000 or 265uA. A typical Hfe of the 2N2222A is 40 so the most collector current (relay coil current) you will see is about 10.6mA (Ib * Hfe) which is probably too low.
Try dropping the base resistor to say 1K, it will still only draw a small current from the MCU but it will let you sink 10 times more current through the transistor.

Brian.
 
Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…