Hi guys! I had spend s couple of days trying to connect an npn transistor to a 12V relay. I am quite a newbie at this but I heard it is possible to use an npn transistor to translate a 5v source from a microcontroller to a 12V relay drive signal. If so is there a correct connection for this? I had tried to connect them with my limited knowledge but the relay contactor will not close. Any help is very much appreciated.

Links for the pic layout: http://i41.tinypic.com/302309j.jpg

 

To drive a 12V relay you need a 12Vdc power somewhere. the transistor will be there to activate the relay when receiving a 5V input from your microcontroller but cannot translate into 12V from a single 5V power supply.

So, you must split your circuit in two. One low power input circuit (driven by a 0-5V signal from microcontroller as shown in your schematic) and one 12V power circuit to power up the relay when the transistor is ON.

To properly power up the relay, your transistor must reach saturation when ON. For that, you need to know the current needed to power the relay, as well as the beta factor of the transistor to correctly compute R1.

In your schematic, what is the circuit on the right side?

 

http://www.pcbheaven.com/wikipages/Transistor_Circuits/
vs=12VDC

 

I always like to buy the 5V relay, becuase I can use it at 5V,6V....24V, if the power source is great than 5V, then it just needs a current limited resistor.

Why you want to using a 12V relay?
What voltage that you usually used? (5V,9V,12V?)

 

For TTL and 5v Micro's I tend to use a Mosfet, depending on the power required, e.g. for up to 200ma you can use a the 'Fetlington' 2N7000.
R1 not needed.
Max.

 

I just did this on the Chat page.

 

I just did this on the Chat page.

That's what mcgyvr mentioned on 3#.

 

That's what mcgyvr mentioned on 3#.

True. I just made a drawing of it.

 

I can never understand the preference for bi-polar in this and TTL applications?
As well as not loading the source, one component is eliminated.
Max.

 

I can never understand the preference for bi-polar in this and TTL applications?
As well as not loading the source, one component is eliminated.
Max.

Until now the load still didn't shows up, must be the load ran away and play where.

 

I can never understand the preference for bi-polar in this and TTL applications?
As well as not loading the source, one component is eliminated.
Max.

Most beginners have some BJT transistors. They are cheap, easy to find, and don't have static electricity issues.

 

Hi guys, this is the updated circuit. Does anyone have any idea why is the ammeter U5 having 320.288MA? And is there anything wrong with my circuit? Thanks.

 

You missing the current limited resistor on Base, why?

 

You missing the current limited resistor on Base, why?

But once a put a resistor (33k Ohm), the led will not light up.

 

But once a put a resistor (33k Ohm), the led will not light up.

You have to using the resistor less than 4.7K for Base, when the Vcc is 5V, the resistor will be less than 9.1K when the Vcc is 10V.