Transistor to drive 12v relay

Thread Starter

nickmms

Joined Jan 2, 2014
40
Hi

I have made a voltage detector circuit using NE555. This gives the right cut in and cut-out voltage (12/10V)
Pin 3 gives -5V when 12Volts is reached.
I need the output to drive a 12VSPDT relay (coil 50mA).
I tried using a 5V relay to drive the 12V relay but it drew too much current 50mA for the 555 as the voltage dropped to 3.3V
I was hoping someone could help me design a circuit that can drive the 12V relay with a Transistor or Fet
I have in stock 2n2222 (old one so not sure if i may have broken it in the past) , 2 off BC557, Tip42C, IRF1405 and IRF740
Thanks
 

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Reloadron

Joined Jan 15, 2015
7,510
The NE555 family should be able to sink or source 200 mA without a problem. NA555,NE555,SA555,SE555 any of those should handle 200 mA and drive a 50 mA relay coil just fine. This is the data sheet.

It looks like you are trying to use a 555 as a voltage comparator which I have no clue why? A 2N2222 has a max collector current of 800 mA so would be a good choice. You may want to check your wiring as the 555 should have no problem sourcing 200 mA, a 50 mA or even 100 mA relay coil should not be a problem. Yes, you can use the 555 timer chip as a comparator but it's an odd way to do it. What you have is something which looks like this.

555 Comparator.png

Ron
 

Thread Starter

nickmms

Joined Jan 2, 2014
40
The NE555 family should be able to sink or source 200 mA without a problem. NA555,NE555,SA555,SE555 any of those should handle 200 mA and drive a 50 mA relay coil just fine. This is the data sheet.

It looks like you are trying to use a 555 as a voltage comparator which I have no clue why? A 2N2222 has a max collector current of 800 mA so would be a good choice. You may want to check your wiring as the 555 should have no problem sourcing 200 mA, a 50 mA or even 100 mA relay coil should not be a problem. Yes, you can use the 555 timer chip as a comparator but it's an odd way to do it. What you have is something which looks like this.

View attachment 312732

Ron
Thanks Ron
I used this circuit as it was easy to build and works well for my application ( I am pretty well a novice and not sure why it works) ie gives an output on pin 3 when the voltage is above 12V. I just go the circuit off the internet.
Yes I thought driving the relay should be Ok but it drops from 5V to 3.3V so that's why I thought I could just use a transistor but don't know where to put in the circuit
 

Reloadron

Joined Jan 15, 2015
7,510
Just as a side note what is the exact part number of your 555 chip?

OK using a transistor should not be a problem. A simple 2N2222 used as a low side switch should work for you. I can make you a simple drawing if you would like.

I did notice something when you mentioned:
I need the output to drive a 12VSPDT relay (coil 50mA).
I tried using a 5V relay to drive the 12V relay but it drew too much current 50mA for the 555 as the voltage dropped to 3.3V
OK so what you want to do is drive a 12 volt 50 mA relay coil using the 555 pin 3 output, correct?

Ron
 

Thread Starter

nickmms

Joined Jan 2, 2014
40
Just as a side note what is the exact part number of your 555 chip?

OK using a transistor should not be a problem. A simple 2N2222 used as a low side switch should work for you. I can make you a simple drawing if you would like.

I did notice something when you mentioned:

OK so what you want to do is drive a 12 volt 50 mA relay coil using the 555 pin 3 output, correct?

Ron
Yes correct 50mA relay coil from the 12V
thanks Ron
 

Thread Starter

nickmms

Joined Jan 2, 2014
40
Just as a side note what is the exact part number of your 555 chip?

OK using a transistor should not be a problem. A simple 2N2222 used as a low side switch should work for you. I can make you a simple drawing if you would like.

I did notice something when you mentioned:

OK so what you want to do is drive a 12 volt 50 mA relay coil using the 555 pin 3 output, correct?

Ron
and the spec sheet for the 555
 

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Thread Starter

nickmms

Joined Jan 2, 2014
40
In schematic form it would look like this.

View attachment 312767I just noticed a my bad. The junction of R1 and R3 should tie to 12 V and not 5V.

This assumed your 12 volt 50 mA relay coil. D1 is a flyback diode across the relay coil. Now if you want to use a 5 volt relay coil just use:
Thanks very much Ron I will give it a try .
I just want to get the output from pin 3 to drive a SPDT 12V relay so there is no benefit using the 5V relay then ?
 

sghioto

Joined Dec 31, 2017
5,388
Thanks very much Ron I will give it a try .
I just want to get the output from pin 3 to drive a SPDT 12V relay so there is no benefit using the 5V relay then ?
I would choose the relay that requires the lesser amount of current assuming the contacts are rated the same.
 

Reloadron

Joined Jan 15, 2015
7,510
I just want to get the output from pin 3 to drive a SPDT 12V relay so there is no benefit using the 5V relay then ?
No, there is no reason to use a 5 volt relay as a slave to drive a 12 volt relay. You have 12 volts available so I would do it as I drew it out. Not much involved to it. :)

Ron
 

Thread Starter

nickmms

Joined Jan 2, 2014
40
Thanks very much Ron I will give it a try .
I just want to get the output from pin 3 to drive a SPDT 12V relay so there is no benefit using the 5V relay then ?
Hi Ron I couldn't get it to work the only way I got it to work was use the +5V for the base and Pin 3 (-5V) to the emitter
pls see attached
No, there is no reason to use a 5 volt relay as a slave to drive a 12 volt relay. You have 12 volts available so I would do it as I drew it out. Not much involved to it. :)

Ron
thanks Ron that's what I thought too no point with the 6 v relay.
BTW when I used another Lm555 the 6V relay worked so it must have been damaged.
I am a bit confused now which is the Emitter and the Collector I looked at the Jaycar data sheet and has Pin1 as collector and Pin3 as the emitter.. but a lot of other data sheets show the opposite. As I had the 2n222 in stock I guess it could be either config.
 

Thread Starter

nickmms

Joined Jan 2, 2014
40
Basic TO92 case 2N2222 transistor. They also come in a can but the plastic is most common.

View attachment 312830

Ron
Thanks again Ron
I think I fried that one. The ones I bought yesterday are the P2N2222 so CBE.. bit of a trap really as both 2N2222 and can hardly read the numbers with my fading eyes
The way I got it to work was have pin 3 from the NE555 (-5V) to the Emitter and +5V to the base pls see attached
Could you pls check the circuit and make sure I have hooked things right. I know sometimes BJTs can work for a while if hooked up wrong
 

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Reloadron

Joined Jan 15, 2015
7,510
No, pin 3 should go to the base, the emitter should go to ground.
What he said. :) Emitter to Ground and 555 out pin 3 through 1K resistor to the Base.

I think I fried that one. The ones I bought yesterday are the P2N2222 so CBE.. bit of a trap really as both 2N2222 and can hardly read the numbers with my fading eyes
Tell me about it, me who has a nice magnifying glass laying on the bench. :)

Ron
 

sghioto

Joined Dec 31, 2017
5,388
The way I got it to work was have pin 3 from the NE555 (-5V) to the Emitter and +5V to the base pls see attached
Going back to post #1 you said "pin 3 of the 555 gives -5 volts when 12 volts is reached".
When you say -5 volt from the 555 how are you measuring it, with respect to ground?
I think you mean the output of the 555 is 0 volts when the battery is 12 volts.
 
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