Question about relays

[crashdodson]

Please excuse my ignorance on the topic of electricity/electronics. I have a project I am working on using RGB LED light strips. The light strips are powered by a mini controller that receives 12vDC. The output from the controller are RGB grounded connections and one 12V DC connection. As I understand the light strip is common anode so the RGB share a common 12V connection. I don't quite understand PWM but I don't think I need to for my project. If I apply 12V DC to the light strip and ground either the Red, Blue, or Green portion of the strip, it will light just that color at full brightness.

I need a very small normally-open relay (I think) to bypass the controller and just provide 12V DC to the LED strip and ground the Red LED's only when a second 12V source is energized. I have found a 14 pin DPDT relay that would accomplish my goal but it is too large for my application. I could even relay each of the wires RBG and +12 individually if I could find very small inline relays. When the second 12v source is energized and the relay is activated I need to maintain power/input to the controller, just cut its output. Please see my bad drawing.

I have been trying to find relays like the ones in the picture but I have been unable to locate anything like them. I could also do this with some sort of PCB if I could find one small enough. Thanks in advance for any help/advice.

 

[Arouse1973]

Hello
How much current does each LED strip draw, i.e. each colour? Can you keep the power supplied to the strip all the time and just switch out the cathodes of the LEDs you don't want to light and then just switch in the red LED.
Adam

 

[crashdodson]

Thanks for your reply. It will be a maximum of 36 watts per strip if I used the entire 5 meters. Its around .6amps per meter I believe. I think I will only be using 4 meters but I have not finalized that yet.

The trouble is the controller uses PWM to make the lights do different effects. If I cut out just blue and green the red would still be doing what ever the controller was sending at the time. I assume I could leave power to the strip from the controller at all times but I would need to cut blue/green/red from the controller and then ground red with a full ground. The application is automotive, so when the brakes are applied and 12V is sent to the brakes I would override/bypass the controller to just light the strip red.

 

[crashdodson]

I attempted to cut power to the controller completely, and bypass the controller with hard wires to the strip on red and +v. I found that applying power to the +v power output of the controller activated the controller and it took over.

 

[Arouse1973]

I don't think you need to isolate the power but just isolate the cathodes of the LEDs. I can't see how this would hurt if you just switched out the cathodes of the LEDs you don't want to light. I was thinking maybe some transistor switches instead of a relay.
Adam

 

[crashdodson]

Thanks for your reply. How would you use a transistor switch, (which I have only learned about in the last few minutes via google). Do they have transistors with 4 legs? If power was applied to one leg (my 12v source) I would need it to close the circuit, but not send power through the transistor on to the strip. At that same time I would have to open up a ground for red, as I would need a new ground since I cut it with the transistor.

 

[Arouse1973]

I was thinking of maybe using the transistor to switch the PWM to the LED strips. And when you want to turn off the PWM to all you simply turn off the transistors using an external source and you could use this external source to drive the red strip fully on via another transistor. The PWM will still be coming out of the controller but because the transistors are off nothing gets through.
Just a thought
Adam

 

[crashdodson]

i guess I am confused on how I could use a transistor to close the connection from the controller to the strip without then providing power to the strip. The only transistors I can find have 3 poles. If I energized one of the poles causing the transistor to "switch", wouldn't it then start sending that power to the strip.

 

[Gryd3]

i guess I am confused on how I could use a transistor to close the connection from the controller to the strip without then providing power to the strip. The only transistors I can find have 3 poles. If I energized one of the poles causing the transistor to "switch", wouldn't it then start sending that power to the strip.

Transistors have 3 legs...
Think of it this way... you will have a common leg, a signal leg, and an control leg.
So, you put current into the signal leg and it flow to the common leg, when it does this, it also allows current to flow from the control leg to the common leg. As no time will the signal back-feed anywhere.
There are two types of transistor... PNP and NPN, which dictate which way the current needs to flow (in or out) or the signal leg. This is chosen on your application.
Unlike relays, there are no moving parts, and in comparison, they switch almost instantly.

*Terms used are not ... scientific ...
Proper terminals are labelled Collector, Base, and Emitter... behavior will depend on the type PNP or NPN transistor type. More details to follow if this is something you want to try. (They are considerably smaller than relays, but less robust... for example a starter in a car would typically use a relay due to noise, voltage spikes, and current draw)

 

[crashdodson]

Thanks for the explanation. In my scenario, if I apply voltage to the signal leg, I would then need nothing to be able to pass between the Common and Control leg. I would not want the 12V going either direction.

In my uneducated terms I have 4 paths. 3 are grounds (as i understand it) for the PWM and one is hot. When an external 12v power source is turned on I need to shut off 3 of the paths/grounds and open a new one. Basically I need to break the ground to the 3 paths and re-open a new ground to one of the 3 paths. If that makes any sense at all. I can either re-use the existing 12v power source coming from my controller or I can use the newly switched on power source.

If I had a lot of room I could just use a cube relay.

 

[Gryd3]

Thanks for the explanation. In my scenario, if I apply voltage to the signal leg, I would then need nothing to be able to pass between the Common and Control leg. I would not want the 12V going either direction.

In my uneducated terms I have 4 paths. 3 are grounds (as i understand it) for the PWM and one is hot. When an external 12v power source is turned on I need to shut off 3 of the paths/grounds and open a new one. Basically I need to break the ground to the 3 paths and re-open a new ground to one of the 3 paths. If that makes any sense at all. I can either re-use the existing 12v power source coming from my controller or I can use the newly switched on power source.

Do you want to apply 12V to turn off the strip, or apply 12V to turn it on?

The NPN type transistor would not allow the current to return from the strip to one of the 3 grounds until you turn the transistor on with by providing current to the base (the signal pin mentioned above) .. This will allow current flow from the 12V to one of the 3 grounds, and also allow the current from the strip to pass through the transistor to one of the grounds.
This is considered an active high logic, but you can make it function in reverse, to only turn the strip on when you don't put 0v to the transistor's base.

 

[crashdodson]

I want to apply 12V to turn off Green And Blue on the strip, but to power red to a full ground.

 

[Arouse1973]

Funny, this morning while I was looking in my draws... of components that is. I came across a very small relay EA2 series by NEC.

http://uk.farnell.com/nec/ea2-12nu/...NoRrqibeNWDdIM6xqWVPf2sKkzw9eqLVU7hoC0nTw_wcB

These are small, they fit on my thumb nail. But quite expensive though and obviously the coils will draw quite a bit of current.

Here is a solid sate version that might work. T2 and VG1 is what I am thinking might be inside the controller. I show only one LED as the strip for simplicity sake and T4 and components are just for the red LED. You could remove the protection diodes if you wanted but this leaves the MOSFETs with no protection. Oh you will need another two of these for the other colours, green and blue. But you don't need the red on circuitry.

Thanks
Adam