Aluminum Heat Bed High Resistance... is this normal?

I have a 200mm x 200mm MK3 / MK2b Aluminum Heatbed and resistance measures a high 1.9-2ohms ( this is after subtracting the resistance in my cheapo meter )

Is this normal for the aluminum beds or is it possible mine is defective?

That will do about 75 watts at 12 volts . . . what problem are you having?

- Tim

Hi Tim, thanks for the reply.

The problem is the common one of limited temperature and speed of heat up.

Details are:

200mm x 200mm MK3 / MK2b Aluminum Heatbed, wired to Ramps 1.4 with 14awg wire to board and to power supply.
Power Supply is the standard 12v 30A switching type, commonly called an LED power supply.
Ramps has a heatsink on the Mosfet. It's being powered with 14.7 volts. The D1 diode has been removed and the Mega 2560 is powered via a buck module, voltage converter. Othewise, the Ramps board is a stock, cheap one.
Approximately 13.9+ volts are moving from the ramps 1.4 into the bed.
The bed is insulated on the bottom, which did improve temperature and heat up time a bit.

I can achieve only about 90 degrees at full power, and this takes a good 20+ minutes. With a magazine on top, I have achieved 110 degrees. ( but drops quickly when I remove it )
I can get it up to PLA temps in about 6 to 7 minutes, and it can hold there fine. I can print with PLA. I'm just trying to get it up to higher temps comfortably, and so far, it's not working well.

So, I'm trying to determine my next course of action and also figure out if this is normal for these aluminum heat beds.

I am thinking about:
Replacing the aluminum bed with a PCB type and using glass on top.
Replacing the power supply.
Replacing the ramps board or perhaps using a relay.

Any thoughts or advice is greatly appreciated.

-Marc

Have you wired it correctly? For 12V one of the wires must bridge two of the pads (which I think puts the elements in parallel), 24V you just use two pads.

That would be a higher resistance that normally desired. According to the wiki 1.6 ohms is the high end of where it should be. That's definitely you're problem as it's lower powered plus the addition of a giant aluminum heat sink.

Yup, wired correctly.

cdru, thanks for confirming what I thought may be the case.

I really like the aluminum, but honestly, now I'm worried that if I fork out for another, it will have the same issue.

I metered the Amperage from the Ramps board to the heatbed. At approx 14 volts, it was only drawing 6.5 amps max.

It should be pulling 10 Amps, correct? Although with the voltage higher, that would affect the Amps, I think.

I imagine this could be the high resistance, ramps board ( mosfet? ) limiting amps, or perhaps the power supply itself.

R=V/I = 14 Volts / 6.5 Amps = 2.15 ohms. Add in a bit of resistance for wires and connectors and that jives with what you measured in your first post a little under 2 ohms. If the resistance was the 1.4-1.6 ohms that the wiki says it should be, than it would draw between 10 and 8.75 amps respectively.

How much power is drawn by your board can be theoretically computed based on it's resistance and voltage. This eliminates your RAMPS board, mosfets, fuses, and your power supply. If you had measured your board with a resistance of 1.4 ohms but were still having heating problems or you measured only 6.5 amps of current being drawn with a resistance of 1.4 ohms, then you would want to look more into all those things. But you measured your resistance, voltage, and actual current being used, and everything matches up like how Ohm's Law says it would.

You're bed's resistance is the problem. You can't force it to draw more current than it does. You have the choice of sticking with the bed you have, but never being able to get high temperatures and having longer heating up times. Or you can get a replacement board. Whether you get a refund/replacement from your original source, or you go with a different site is up to you.

I had the same problem. Instead of turning up the 12v voltage and risking damaging the Arduino, I fitted a second 24v PSU.
My aluminium bed now heats up to 110 deg C in about 2 mins.

cdru, wow, thanks, very enlightening information. I'm a rookie when it comes to electronic calculations, such as Ohm's Law, but I'm slowly learning more about micro-electronics in general.

I have PCB type heatbed on order, and I already have the glass, so I'm going to see how that goes next. I may try another aluminum bed in the future, but only if the dealer can assure me of a proper resistance. I'm not sure if the seller will give me any sort of refund on this high-resistance unit. Might be too late for that, but I will definitely let them know.

dave3d, I have a voltage buck module / converter to limit the power going into my Mega and I've disabled the D1 diode on the Ramps board, so there is no way I'll damage it. I have been looking at the 24v power supplies, and I may get one. But I really do not want to run dual power supplies. Since I altered the Ramps, I could almost run 24v now, although I may need to change some fuses ( maybe something else? ) and I would have to replace the heater cartridge on the hotend for 24v.