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5V to switch a 12V coil relay ?

12v supply on one side of the coil.

NPN transistor on the other side with "C" (Collector) connected to the coil and "E" (Emitter) connected to ground. 5V output signal connected to resistor connected to "B" (Base). If using an N-channel FET instead of the NPN, resplace labels "C", "B", and "E" with "D" (Drain), "G" (Gate), "S" (Source) and you don't need the resistor (just connect output signal directly to "G").

When output signal is at 0V, transistor is off, no current flow through coil, relay is off.

When output signal is at 5V, transistor is on, "shorting" one side of the coil to ground (current flow is limited by resistance of the coil), relay is on.

-- ...The Bit Eimer [remove keinewurst to email me]

"My goal in life is to be the kind of person my cat thinks he is"

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tips

issue.

Reply to
bit eimer
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Also, connect a diode to the two sides of the coil, with the "thick line" on the diode pointed at the 12V side. This is good standard practice on any relay, regardless of the method of driving it. It prevents high voltage spikes when turning off the relay (current keeps trying go go through the coil) - such spikes can damage the rest of your circuit.

-- ...The Bit Eimer [remove keinewurst to email me]

"My goal in life is to be the kind of person my cat thinks he is"

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coil

the

is

82c55
Reply to
bit eimer

Hi Group,

Firstly, I am not an electronics engineer, primarily a software engineer with minimal understanding of electronics.

I have been dropped into a project where I have digital output from 82c55 chips needed to turn a 12V coil relay on or off. How do I do that with minimal components ? I can just about solder without singeing my finger tips off ;-)

BTW, it's a prototype for now and cost, size and heat are all not an issue. Well.. maybe not cost but it is not critical for now.

Any suggestions and diagrams etc gratefully accepted.

Thanks in advance.

S. Harnek

Reply to
S. Harnek

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