# Relay control circuit

• 21st October 2010, 16:34
brettt777
Relay control circuit
I need to control a simple relay with a push button; push on/push off. I found this circuit online which seems simple enough except it's for +6-19 VDC. I need to run it with 24-28VDC. It's been a very long time since my basic electronics and semi-conductors training in the Navy. What, if anything, do I need to do to this circuit to use %30 higher operating voltage? Assuming the coil in K1 is a 24 volt coil, can I just raise the value of R3 and R6 to say, 5.6 K or would I need to do more?
• 21st October 2010, 16:53
nandhu015
Re: Relay control circuit
Use a positive voltage regulator, For example 7812.
• 21st October 2010, 17:38
Zonetronik
Re: Relay control circuit
Quote:

Use a positive voltage regulator, For example 7812.
why?

Quote:

I need to control a simple relay with a push button; push on/push off. I found this circuit online which seems simple enough except it's for +6-19 VDC. I need to run it with 24-28VDC. It's been a very long time since my basic electronics and semi-conductors training in the Navy. What, if anything, do I need to do to this circuit to use %30 higher operating voltage? Assuming the coil in K1 is a 24 volt coil, can I just raise the value of R3 and R6 to say, 5.6 K or would I need to do more?
I think you can feed your circuit without changing the values of R3 and R6
• 21st October 2010, 18:07
Re: Relay control circuit
Just run it. The transistors can handle 40 volts and 1 amp. In this circuit, to switch the transistors, it's more about the relative voltages than absolute voltages. I think the numbers were originally for a 6 or 12 volt relay coil.
• 21st October 2010, 18:10
Zonetronik
Re: Relay control circuit
vce only 30V
• 21st October 2010, 18:18
Re: Relay control circuit
I must have been looking at the datasheet for the new and improved model.
• 21st October 2010, 19:01
brettt777
Re: Relay control circuit
Quote:

Just run it. The transistors can handle 40 volts and 1 amp. In this circuit, to switch the transistors, it's more about the relative voltages than absolute voltages. I think the numbers were originally for a 6 or 12 volt relay coil.

I see what you're saying about the relative versus absolute voltage. So if I do build it and run it as is with a 24-28V source, the circuit will work with a 24V relay coil that draws maybe 50ma?
• 22nd October 2010, 01:59
Re: Relay control circuit
Yes. The transistor that's driving the relay coil can handle it easily.
• 22nd October 2010, 15:54
brettt777
Re: Relay control circuit
Okay so I built this thing up last night and tried it at 12 and 24 volts and nothing worked. So I went back over everything this morning and all is correct. I started checking voltages in various places and found that all three transistors appear to be partially conducting all the time. I even disconnected the base on Q3 to get it to stop conducting and nothing changed. Once I adjust the source voltage up to about 14-15 volts, the relay energizes regardless of what the rest of the circuit is doing. So I'm thinking I have a shorted transistor or something. I pulled another one out of the bag and looked at it more closely. It said 2N2907 on it. I looked that up on Google and found it to be a PNP transistor, the complement transistor for the 2N2222 which of course, is NPN. The package says they are suppose to be NPN switching transistors but guess what? They aren't. That will teach me not to buy parts from Radio Shack. So I am thinking that once I get the correct transistors in there it should work.
• 23rd October 2010, 03:40
siongboon
Re: Relay control circuit
Some reference to interface the digital logic signal to control a relay.
Switch
• 23rd October 2010, 06:18
Re: Relay control circuit
This circuit should work....

Attachment 50161

It uses a digital counter and the last bit of the output is used to drive the relay through 2n2222 transistor. But be careful, when using 40 v. Because I think the 7805 voltage regulator is not able to withstand 40 volts i/p. So either u have to lower the operating voltage (around 30v). Or u have to find some other means to generate 5v from 40v.
• 23rd October 2010, 08:25
sam185
Re: Relay control circuit
first you need to select appropriate relay ,transistor rating for that & I think the pulse-width is based on r1,r2 and c what ever value you calc for r3,r6:- r3=r6 for proper output. That's all.
• 23rd October 2010, 18:33
dimas_lilik
Re: Relay control circuit
I suggest you use ULN2001-ULN2003, with common GND. It's effective and easy.
• 25th October 2010, 22:10
brettt777
Re: Relay control circuit
Okay so once I got the correct transistors installed (the package was labeled wrong) the circuit works okay except that it seems to require a second or two between button pushes to change state. If I push the button to quickly, it does nothing untill I give it a second or two and then push the button again. I don't expect the circuit to be operated all that quickly when it's in operation but you never know. Is there something in the circuit that can be changed to eliminate this required two second pause? Decrease the size of R2 or C1 perhaps?
• 25th October 2010, 22:28
keith1200rs
Re: Relay control circuit
Yes, reduce C1.

Keith
• 26th October 2010, 09:30
sam185
Re: Relay control circuit
You can reduce either c1 or r2, but if you reduce r2 that will be more better.
• 26th October 2010, 09:33
keith1200rs
Re: Relay control circuit
I suggested C1 because if you reduce R2 too much then there is a risk of upsetting the collector voltage on Q1 which is in turn the base voltage for Q2. There isn't that risk in reducing C1.

Keith.
• 26th October 2010, 09:42
sam185
Re: Relay control circuit
Yes you are right keith. i have not seen that part.