I have an interesting issue with my heat bed,

I have an interesting issue with my heat bed, running Marlin on a ramps 1.4. Power supply is 350 watts.

Normally the printer bed hits 110 without issue in about 20 minutes. Today I fired up the printer and it spends an hour and never starts to print. It will get to 110, then drop to 107ish and climbs again over and over. The LEDs on the bed are always on. So I tell it to use 108, and it starts printing, I then tell it to hit 116 just to see what happens and it climbs no problem and is bouncing around 116…

This is definitely abnormal behavior. Any ideas what to check / look for?

Make sure you are cooling your heater MOSFET on the ramps, I have this issue as well sometimes if the MOSFET gets to hot it will trigger the resettable fuse and will wait until it cools down a bit to resume heating. Once the bed is at temp it has no problem jumping to a higher temp. A 70mm fan is what I use.

I have a 50mm fan blowing on the ramps board all the time. I can try a larger one and see if that would help.

Have you tried using Auto-pid for the bed thermistor. This helped me a lot. Otherwise you can shorten the time the temperature has to stay stable before the print starts in the firmware.

You say you have a 350W power supply, but is it a single-voltage power supply, or a PC power supply that can only source ~10A on the 12V line.

It’s a converted PC power supply.

How many amps on the +12v rail(thick yellow wires) you can limp a printer with 17amps (I’ve done it) but 30amps is ideal. There should be a sticker on the side that tells you how many amps on which rails and what color wires they are.

Get a switching psu, they are cheap!, you can get a 12v-20amps, you will notice a difference on heating speed and noise.

A few extra volts helps out as long as your ramps is equipped to handle it, (so you don’t over volt your Arduino) the amps is what heats your bed and hot end.

For example I had a 800w dual +12v rail 20amp psu that I was using, still took forever to heat up. Bought a led strip switching 350w 12v 30amp power supply thing heats up in no time. The extra 10amps makes a difference.

My ps claims 18 amps on the 12v rail. I’ve ordered a 30amp switching ps (was going to get around to that anyways) and will put a better fan on the ramps board to see if that helps. Thanks for all the help guys!

John, now I see what you were saying about the extra volts.

I measured the voltage going into the ramps board and it was in the mid 11 volt range.

So I would be better off if I ran a 10amp or less 12v power supply for the system, then relay a 15 or 18 volt ps for the heat bed?

Is there a website that explains how you setup and wire dual ps’s? I like the idea of separating them like that. Just don’t want to damage anything.

Cool thanks for the help. I have ramps 1.4. I just wasn’t sure if the board was able to handle the extra load. I’ll look for a 12 volt 5 amp power supply and a 18 volt 15 amp power supply tonight.

I know one of my mosfets has a small metal heatsink on it, the other two don’t. I’ll have to check when I get home…

Your not kidding that 18volt 15amp power supplies are hard to find. I am trying to find a 20amp one that doesn’t break the bank…

If you use eBay I know of a 12v 360w 30a psu for I think 30$ shipped from the US

It’s actually the one I’m using right now and have had no issues with it at all.

Alternatively, you could get two prusa boards and wire them in parallel. Then you’d draw a few more amps at 12V (and get limited by the thermal fuse). If you’re sufficiently motivated, you can make your own heater out of nichrome wire with lower resistance, too.