Ramps powered with only 1 PSU?

Hi,

I just switched from Sanguinololu to a RAMPS and everything appears to work great.
The only issue is my heated bed output. I have a custom heatpad which draws more amps than the normal MK2 heated beds and therefore I had the power to the bed going through a relay and the relay itself was switched on by the power terminal for the heated bed.

So I just tried to replicate the setting on the RAMPS. I have a separate PSU on the 5amp input and the 11amp is empty at the moment. When I now give the command to power up the heated bed I dont hear the relay switching on.
So my question is if the heatbed output only comes online when its powered by the 11amp input and if there is a way to get the board working on only 1 PSU (I saw a post somewhere with a similar "1 PSU for the whole board" but cant find it anymore)

Yes - the heatbed output is the one and only thing on the 11 amp rail. You will not be able to control your relay unless you power that rail. It doesn't have to be a physically separate power supply if it is capable of providing the total draw on both rails. Since you are powering a relay, you won't be drawing the full 11 amps.

Edited 1 time(s). Last edit at 08/24/2014 12:16AM by jbernardis.

Aha thanks for the fast answer.

You think I can just bridge from one input to the other like this?

As long as the PSU is rated for the amperage you will be drawing, then yes you can wire it that way. Like I said, the fuse on the left rail in your picture is rated for 11A, but it's expecting to be powering a heated bed. If you only have an SSR, then you will draw significantly less amperage. The power supply needs to be sized for your actual draw.

Seems to work like that. Thanks

Maybe you can help me with another thing.

Im having trouble with my hot end it show correct temp but with my laser thermometer I can only measure 120C thats a bit low even if you calculate it unprecise measuring. So I checked the output voltage at the D10 terminal I it jumps around from 1.3V-2.3V (input on the main power terminal is 12V stable) shouldnt that also be 12V?

I tried with a laser thermometer but the kapton tape and shiny metal wont let me get an accurate reading.
I then tried the thermocouple that came with my multimeter and it showed what i was expecting.
Difficult to hold steady and hard against the hot end but it does work.

Laser thermometers need a nice matt surface to work well.

Gordon

I agree with Gordon. IR thermometers are inaccurate if used on a reflective surface.

The output from D10 should read ~12V when the MOSFET is on. Don't forget that you're not using bang-bang methodology here. The MOSFET is usually switched on and off rapidly as indicated by the accompanying LED. I'm not sure, but I believe that a DC voltage here would show the average of all of those on/off values. You should see 12V though during the initial warm up period though; just don't expect it when you're maintaining operating temperature.

Ok seems i can rule that out. the voltage on startup is indeed 12V and then goes down after it reaches target temp.

Quote
jbernardis
I'm not sure, but I believe that a DC voltage here would show the average of all of those on/off values.

Depends on how good your meter is and what resolution it's samplying frequency is. Cheaper meters can be all over the places and you may not be able to determine accurately what voltage things are running at if it's switching the power on and off faster than what the meter can handle. That is where an analog meter can be handy as the physical delay of the needle sweeping back down is a lot slower than what the power is being cycled so it as a bit of built in "averaging".