Im hoping you guys can help me out with an issue… I have a printrbot plus LC… its been working fine on a 12V PSU no issue… someone told me I could use a 24V PSU and solid state relay to speed up how fast the heatbed got hot… Its setup exactly how it was with the 12V PSU except I changed the SSR to a 24V one and only hooked the 24V PSU to the bed/SSR. When I start a print the bed heats up perfectly. As soon as it reaches temp and turns off the voltage from the Printrboard changes from +12V to -12V and it never flips the relay again. so by the second or third layer the bed temp starts dropping… If i turn the printer off and back on it will flip the relay and heat up and then again -12V and doesnt flip again. Im not sure whats going on. Ive never hooked the 24V PSU to the printer. Anyone know why the signal from the printrboard is flipping to negative? Thanks!
while looking at your drawing one could guess you may switched the connection polarity from the board to the SSR.
Just my idea on that as you drawed the black wire to the left?
picture?
Oh i didnt get that specific with the drawing. I just drew that quickly to better show how it was wired. I checked the voltage with a multimeter and the wires coming from the connector are correct color to the voltage, but I have tried swapping them on the relay after it swapped to -12V and still didnt make a difference. I am going to swap back to just the 12V relay and PSU for the bed and see if it goes away.
The heatbed has a constant +12v. Is that playing nice with your setup or is it looking for +12v to switch on and off?
my hotbed ony has 2 connections (+/-). Its a helios if that matters. When the bed has power it has an led that lights up. the bed appears to work fine. it seems the issue is with the board not switching the relay.
but it is a DC Relay for control and work load side, both right?
Makerfarm has a 12v heat bed relay.
Instead of the 24v power supply, use a 12v with enough watts to power everything.
Ok I made a mistake. The diagram I posted is how I have an automotive relay wired up not an ssr… This relay:
http://www.skycraftsurplus.com/relayautomotive24vdcspst.aspx
This is the one that triggers the first time (I hear the click) then never triggers again cause the board is sending the -12V signal after that first heatup.
The SSR I have is a Crydom D1D40… But I couldnt figure out how to correctly wire that one up. The diagram I found for it appears different from the automotive style relay. It shows + and - for the Output terminals. I didnt try wiring it up. Im trying to find something to ensure the correct way to wire that one up that i understand.
I have a 12V PSU and relay that ran it I just want to eliminate the 25m heatup times. The first time the relay triggers its heated up in 5m this way. Eventually I am going to eliminate the 12V PSU (Its just a crappy PC PSU). I have a DC/DC converter to step the 24V down to 12V that will provide the board power. But thats for when I convert to my azteeg x3 pro with my Kraken hotend and 4 extruders. I also have a small laser cutter that I would be adding… But I am not trying to add a bunch of variables at once…
Everything is fine voltage wise until I heated up once. Something happened once it turns off after that first heatup thats when it turned negative… But since you gave me the info on the SSR I was able to wire that one up and it works perfectly… THANK YOU… Im guessing the -12V is cause of some issue with that relay i was using? Problem could also be I only know enough to be dangerous when it comes to electrical things… 
Thanks again for the help!