How to use transistor(2N3906) to turn on relay?

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babboy12345

New Member

• Mar 22, 2009

• #1

my output from microcontroller's is around 2v,so it cannot to turn on my 5v relay.By using PNP transistor,can i get it on by transistor? please provide me the circit diagram and the resistor value,thx

 

blueroomelectronics

Well-Known Member

• Mar 22, 2009

• #2

Which microcontroller?

 

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babboy12345

New Member

• Mar 22, 2009

• #3

pic16f877a

 

audioguru

Well-Known Member

Most Helpful Member

• Mar 22, 2009

• #4

If your PIC has a 5V supply then its output voltage is 5V when the load current is low.
It can drive the base of a PNP or an NPN transistor through a series current-limiting resistor.

 

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babboy12345

New Member

• Mar 22, 2009

• #5

As what u say,so what value of resistor should i put at my PORTB ??

 

audioguru

Well-Known Member

Most Helpful Member

• Mar 22, 2009

• #6

babboy12345 said:

As what u say,so what value of resistor should i put at my PORTB ??

Click to expand...

Use 330 ohms to limit the current to 10mA. Then the relay's coil can be as low as 50 ohms.
Use a reverse-connected diode in parallel with the relay's coil to clamp the high voltage spike it will try to produce.

 

Sceadwian

Banned

• Mar 22, 2009

• #7

I would start with a 1k resistor to save power in case you need it for other I/O lines. If the transistor gets hot try the 330ohm one, but PIC's have a limited maximum amount of current for the whole chip, and all those ma's add up fast. Ideally you want a resistor that will be JUST low enough to put the transistor into saturation for the coil load, if you tell us what transistor you're going to use what the relays primary coil resistance is we could give you a more specific resistor value to use.

 

Mikebits

Well-Known Member

• Mar 22, 2009

• #8

From the title I would assume the transistor is a 2N3906...

 

Sceadwian

Banned

• Mar 22, 2009

• #9

<cough> You might be right mike.

 

Boncuk

New Member

• Mar 23, 2009

• #10

To minimize the load current of the MCU you could use a low power MosFet BS170 (N-channel) or BS 250 (P-channel).

They require extremeley low gate current (compared to a bipolar transistor) to conduct fully.

Boncuk

 

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mneary

New Member

• Mar 23, 2009

• #11

Boncuk said:

To minimize the load current of the MCU you could use a low power MosFet BS170 (N-channel) or BS 250 (P-channel).

They require extremeley low gate current (compared to a bipolar transistor) to conduct fully.

Boncuk

Click to expand...

With the caveat that these MOSFETs are just barely ON with a 5.0V gate voltage. They should usually be OK with a light load and the full 5V Vcc, but it Vcc is less than 5V (e.g. 4.5V) these MOSFETs aren't guaranteed to turn on.

 

Last edited: Mar 23, 2009