How to make a relay latch circuit.
05-13-2011, 08:47 AM
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Year: 1998
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How to make a relay latch circuit.
First off... this is a circuit... meaning electronics and wiring. Some people are scared off by this... however I will try to explain what this is and how it works so anyone can understand it.
F.A.Q.
"what is a relay latch?"
A relay latch circuit is an version of a stay or latch relay.
"what is a stay or latch relay?"
It is a relay that maintains connectivity to a circuit even after loosing power. One side of the relay turns the circuit on and the other turns the circuit off. Think of it as an electronic on off switch.
"why make the circuit if there is a part available that does the same thing?"
First it eliminates the mechanical linkage on a stay relay and 2nd the flexibility. You can make the circuit with any variation of voltage relays and build it to handle just about any amount of current.
"what is it used for?"
An example would be to keep the radio on after the ignition is turned off until you open a door. Or as part of an automated launch control in drag racing. How about making your own turbo timer? Automated winch? The only limits is you imagination.
"How does it work?"
I will use the radio example. The basic circuit consists of 2 relays and a diode. The basic theory is to tell the radio to turn off when the ignition is on and keep it on until you open the door.
Here is the circuit drawn out...
Yes I know it looks intimidating... bear with me...
NOTICE there are 3 inputs and 1 output.
INPUT #1 from the top left is a red wire. This is the switched ignition source.
INPUT #2 from the bottom left is the yellow "constant" 12V source. This is on all the time.
INPUT #3 from the top right is a red wire that goes to the 12V in on the interior lights. When the lights turn on.
(NOTE: depending on the application you might have to use the ground. If this is the case then run the positive side of the relay to a constant 12V source and use the negative as your input.)
Output #1 from the bottom right is what goes to the radio.
HOW IT ALL WORKS
Looking at the picture...
"Wait!!! What the heck is that diode for?!"
The diode prevents current from going past your circuit. If you don't put it in the circuit there could be unpredictable behavior and/or blown fuses...
I hope this explanation is good enough to spark a relay latch idea thread... Post any questions and/or what you come up with!
F.A.Q.
"what is a relay latch?"
A relay latch circuit is an version of a stay or latch relay.
"what is a stay or latch relay?"
It is a relay that maintains connectivity to a circuit even after loosing power. One side of the relay turns the circuit on and the other turns the circuit off. Think of it as an electronic on off switch.
"why make the circuit if there is a part available that does the same thing?"
First it eliminates the mechanical linkage on a stay relay and 2nd the flexibility. You can make the circuit with any variation of voltage relays and build it to handle just about any amount of current.
"what is it used for?"
An example would be to keep the radio on after the ignition is turned off until you open a door. Or as part of an automated launch control in drag racing. How about making your own turbo timer? Automated winch? The only limits is you imagination.
"How does it work?"
I will use the radio example. The basic circuit consists of 2 relays and a diode. The basic theory is to tell the radio to turn off when the ignition is on and keep it on until you open the door.
Here is the circuit drawn out...
Yes I know it looks intimidating... bear with me...
NOTICE there are 3 inputs and 1 output.
INPUT #1 from the top left is a red wire. This is the switched ignition source.
INPUT #2 from the bottom left is the yellow "constant" 12V source. This is on all the time.
INPUT #3 from the top right is a red wire that goes to the 12V in on the interior lights. When the lights turn on.
(NOTE: depending on the application you might have to use the ground. If this is the case then run the positive side of the relay to a constant 12V source and use the negative as your input.)
Output #1 from the bottom right is what goes to the radio.
HOW IT ALL WORKS
Looking at the picture...
- Imagine the ignition is turned on. This turns the bottom relay on.
- Turning off the ignition power is no longer supplied from the switched 12V source. However notice that the constant 12V on the relay out is connected to the relay's input... This supplies voltage to the relay to keep it on. Pretty nifty huh?
- The second relay is different than the first. It is called a double pole, single throw relay. What this means is if the relay is off it will connect the input to 1 pin. When it is on it connects to the other pin.
- Notice that it is off and is connecting your power relay to ground. activating the relay disconnects the ground from the power relay turning it off.
"Wait!!! What the heck is that diode for?!"
The diode prevents current from going past your circuit. If you don't put it in the circuit there could be unpredictable behavior and/or blown fuses...
I hope this explanation is good enough to spark a relay latch idea thread... Post any questions and/or what you come up with!
Last edited by razor02097; 05-13-2011 at 08:49 AM.
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