Heated bed doesnt work

I've been following the instructions here to make the heated bed..

So far I've managed to setup the nichrome wires connected in parallel across the whole aluminium plate. But I'm only getting 0.3Ohms of resistance, I have around 13-14 wires on each ladder of nichrome wires. Have I done something wrong already?? Ive also checked there is no short between the nichrome wires and the aluminium plate.

Then I'm lost with the rest of the electronics.. eg i'm using RAMPS 1.3, so do I still need a capacitor and MOSFET ..etc?? I thought it would work simply by connecting the two ends of the heated plate cables into the D10 terminals similar to getting a PCB hot plate. When the power was on (set the heated bed to eg 55C) the terminals read 0V... However when I unplugged the wires from the heated bed, the D10 terminals went back to 12V...

Also I'm unable to measure the Amp as every time I connected my multimeter leads to the terminals, it created a short circuit with a spark, then the PSU switched off... But my meter can take readings up to 20A..

Can someone tell me whats going on? Sorry but my electronics knowledge is only stuck at A-Levels 7 years ago..

many thanks.

From the wiki:

Quote
Check the heater resistance. If you wired it as two parallel circuits as I did, each half should be 3 ohms. If you have done it as a single circuit it should be 1.5 ohms. Connect up the heater.

Only 0.3Ohms indicate something is wrong and you have a short somewhere. A picture of your layout may help diagnose the problem.

misan

Photo attached.

I've been measuring the two ends as I added the wires. Originally, there was 1.8Ohms, then it dropped to 0.9 > 0.6 > 0.4 > 0.3 etc as more nichrome wires were added.

To check for short, I placed one lead of my multimeter to either end of the wire, and the other lead to the aluminium board. The buzzer did not sound, and when I placed the multimeter leads to the + / - terminals of the heater wires, the buzzer went off...

this is the layout I have..

It seems that the side away from the connector goes over the central wire causing a short at the connector. Or is that wire insulated? Sorry I can't tell from the picture.

And what is the resistance of four inches of your nichrome wire? (if you can measure 10 inches, or more, even better but tell us the length you've measured too).

misan

for 12 inches, I measured 2.6 Ohms

Maybe your nichrome is a lot thicker than Adrian's.

You measure current by connecting your multimeter in series with the load, NOT across the PSU output.

---

[www.hydraraptor.blogspot.com]

Ok, that means every inch is 0.4ohm. According to your drawing you have 22 legs of four inches aprox (1.6ohms each) in parallel. That gives a total of 0.07ohm. The 0.3ohm your meter shows is likely the resistance of the probes.

You need to have something around 1-3ohm total. So you can try just a loop of 10 inches. Unfortunately it wil be kind of small. Or you can put two loops of 20 inches in parallel. Or you can use other model of nichrome with higher resistivity... but forget about the wiring you have now as it is not going to work.

misan

Edited 1 time(s). Last edit at 01/08/2012 02:14PM by misan.

nophead:
Thanks for reminding me.. school boy error..


misan:
The brown and blue wires were insulated at the point you circled.

Is that the right way to wire up the nichrome wires? So if I get some thinner nichrome wire, the circuit will work.. as long as the total resistance is around 1-3 Ohms? Is that right?

Do I need any additional items if I have RAMPS 1.3 board?

Many thanks.

Ok, then I think your wiring work is correct.

The circuit does not work because your wire has much lower resistance than required here.

Depending on how you have "measured" the current, a MOSFET might be dead in your RAMPS board (if you measured in the heated) or not (if you measured on the power supply).

You did not mention what power supply ... do not forget that heated bed will require from 8 to 12 more Amps from your supply. It has to be up to the job (or a second supply will be needed).

misan

my PSU has a 500W output, 30A with 12V as it says in the output table. so it should be beefy enough.

How can I check if the MOSFET is dead? When I unplugged the heated bed from D10, the 12V reading came back does it mean it is still working?

I found this resistance wire online, it 60.6Ω per yard, is this any good??

[www.ebay.co.uk]

Edited 1 time(s). Last edit at 01/08/2012 04:08PM by ahbtsang.

With this wire you can connect the power for the heated bed to the two legs of the sides (not to the center and one side). Disconnect the two sides connected by the red wire and connect leave the central (blue) wire disconnected.

misan

i see. thanks very much for your help.

however, can someone explain why there wasn't any voltage across when I connected the heated bed? but when I unplugged it, it read 12V at D10?

does it also mean if I want to test if it is working, all I need is to cut a piece of nichrome wire to about 1ohms, then connect to the D10, and it should get hot.. right??

Edited 1 time(s). Last edit at 01/08/2012 05:40PM by ahbtsang.

If the resistance is too low the power supply will not be able to handle such a high current and may shutdown (or deliver a much lower voltage instead).

Yes, if you connect 1 or 2 ohms of nichrome to D10 it should get very hot (be careful with that, as without thermistor it will not be switched off after reaching any temperature).

misan

just tested by connecting a 1.4 ohm nichrome to the D10 terminals, it was glowing RED hot like a bulb filament!!! Now I'm a bit frightened whether it will be like this when it is stuck to the heated bed.. will that even melt the kapton tape???

Its glowing red because you just generated 102 watts of heat in a small length of wire suspended in air. The print bed acts as a heat sink and keeps the wire from getting that hot - just be sure that the whole length of nichrome is in contact with the bed.

Also, shouldn't the heated bed be on the D8 terminals?

And the bed thermistor will close the regulation loop meaning that nichrome resistor is not on all the tim. Once the desired temperature is met the resistor is switch off for a while. This contributes to the resistor not getting red hot too.

misan

I've just built my HBP using nichrome which sounds to be the same as you were using.
13.7 ohm / metre, = 0.35 ohm / inch. Works fine setup as below, the spacing seems to give reasonably consistent temperature distribution.

Laying the nichrome out within an 18cm square on the 22.5cm square bed,
with 6 runs, worked out near enough the right resistance.
In the layout below, the horizontal lines are solid core copper wire,
the vertical lines are nichrome (except obviously for the leads off to the board - a Sanguinololu in my case.
All joints crimped.

18cm = 2.5 ohm, each block of 3 in parallel gives 0.8 ohm,
then the two blocks in series gives 1.6 ohm. Near enough for government work.
With 12v, this will draw about just under 8 Amps, a bit over 90 watts.
It heats the plate up to 55 deg in a few minutes.

           Thermistor
                ||
     to FET     ||      12v 
         |      ||      |
    +----+----+ || +----+----+
    |    |    | || |    |    |
    |    |    | || |    |    |
    |    |    | || |    |    |
    |    |    | || |    |    |
    |    |    | O  |    |    |
    |    |    |    |    |    |
    |    |    |    |    |    |
    |    |    |    |    |    |
    +----+----+----+----+----+

Edited 1 time(s). Last edit at 01/09/2012 03:44AM by nb99.

Finally got it to work... with some calculations, my nichrome wire has 8.6 Ohms/m and i calculated that I needed 2 wires of 349mm connected in parallel in order to achieve 1.5ohms of total resistance.. and now its working and can heat up the bed to 60C in about 7-10 mins..

thanks very much for all the help guys.
Adrian