PSU for heated bed

Hi,

I just wired up my aluminium plate with nichrome wires and a thermister... I have 2 major problems:

1) Thermister is reading 375C at room temperature..
2) Nichrom wires not getting hot

The bed temp has always been displaying 375C even though I placed my hand over the thermister trying to heat it up, with a multimeter it measures about 1.8 Ohms, could this be caused by the aluminium plate shorting the thermister legs?

I am using RAMPS 1.3, when I set the extruder to hot, the LED on the RAMPS board switches on, so I expected this would be the same when I power up the heated bed, but there was no light even though it measures 12V (11.86V) from the power terminal (D8) ... I cannot be sure whether there is not enough AMP from the PSU as I don't think there is a (affordable) multimeter out there can measure currents above 10A... How can I make sure the leads coming from my PSU is giving enough amps??

Many thanks.
Adrian

At room temperature the thermistor should be over 100k.
1.8 Ohms is a short (as you suspected).

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Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
BLOG - PHOTOS - Thingiverse

I have had the the thermistor spike to 375C also. It must be something with the thermistor because if you jiggle it around a little it should go back to reading the correct temperature.

oh.. 357C actually... I switched the pin heads for the headed bed thermistor with hot end thermistor and it reads the correct temperature, so I'm sure it is not the pins setting or thermistor table error, so must be a short somewhere touching the aluminium board.

As with the PSU, I am using a 450W PC power supply, so I'm sure that is powerful enough. However, if it is drawing so much current from the PSU, Adrian suggested to use FAT wires to connect to the heated bed, is it necessary to replace the thinner wires coming from the PSU to fatter one too?

Thanks

You could do, or use 3 or four yellow and black wires tied together. I think the wires from PC PSU's should be 36/0.2 but on a cheap one I am using they are only 36/0.15. A big difference when you realise it is a square law!

Also any voltage drop in the wiring has a big effect on the heat because that is also a square law.

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[www.hydraraptor.blogspot.com]

The 12V wires from a PC PSU are 18 AWG ( e.g 24/0.2) officially rated 6A per conductor, or 72W. So using two should be ok up to 140W. They seem a bit light but should be OK for short runs.

The current ratings for wires seem to be mainly rated to the temperature rise, and are designed to prevent the insulation overheating. If you have wires packed in a bundle in a conduit in a high ambient then you need to derate the current rating a lot. Conversely, with a single wire in free air you can get away with exceeding the nominal rating.

For single wire maybe go up to 1.5 mm^2, e.g. 32/0.25. Either way, best to keep the wires short as possible. You either waste power or money.