relay control circuit

[brettt777]

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?

 

[Laplace]

You may not have to do anything at all, as long as R3 & R6 are rated at least 1/4 watt. Measure the relay coil current when driven with 28 volts. DC current gain of Q3 is specified at least 100 in the 10 - 150 mA range so Q3 should remain saturated up to (28 - 0.6)/(4.7K + 10K)*100 = 180 mA of collector current. As long as the relay coil current is less than 180 mA then it seems like it should work without circuit changes on 28 volts.

Correction: On second look it appears that half the current going through R6 will flow through R7 and the other half through R5 so the maximum coil current with these resistor values would be (28 - 0.6)/(4.7K + 10K/2)*(1/2)*100 = 140 mA.

 

[brettt777]

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?

 

[Laplace]

The R2C1 time constant is 1 second so it takes C1 about 5 seconds to become fully charged. Just change R2 to something around 100K so it will charge up in 0.5 seconds. That is the first thing I would try. Changing C1 to a smaller value could make the reset unreliable.

 

[skydivewags]

Not that building the circuit wont be fun, but couldn't you just use a latching switch?

Wags

 

[RAMSELECTRONICS]

I would do this. (attached PIX) change Q3 2n2222 to 2n3055 more strong current and voltage for safe.

Hope it helps.

 

[(*steve*)]

2N3055 may be going a little over the top, especially considering the base current you're providing (the 2N2222 may even switch a higher collector current!)

Changing from a 2222 to a 3055 is a bit like swapping a moped for a Mac truck. Perhaps a motorbike or a small car is required?.

For a larger current I'd perhaps look at either a small power mosfet, or perhaps a darlington transistor, or even just a *slightly* beefier transistor.

 

[RAMSELECTRONICS]

2N3055 may be going a little over the top, especially considering the base current you're providing (the 2N2222 may even switch a higher collector current!)

Changing from a 2222 to a 3055 is a bit like swapping a moped for a Mac truck. Perhaps a motorbike or a small car is required?.

For a larger current I'd perhaps look at either a small power mosfet, or perhaps a darlington transistor, or even just a *slightly* beefier transistor.

Yea.

 

[KMoffett]

Another option. For a 24-28V supply use a 12v relay. R1 and R2 are selected to have the the same resistance as the relay coil. C1 should have at least a 16VDC rating. The down side is that the circuit is always powered, whether the relay is open or closed.

Ken