1 Topic by BFresh

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Topic: PCB heated bed questions

I recently had problems with the standard heated bed not getting to temperature. Turns out it had overcooked itself.

So i bought a Reprap style PCB heated bed, made all the necessary modifications and upgrades (power supply, added jumper leads to the sanguinololu board).

I have run some tests and it is taking even longer for the bed to heat up now. 40 minutes to get to 90 degrees.

Why could this be?

Note: I have used the old silicone heat pad between the alu bed and the new heat bed to leave space for the thermistor.
          I use a 5mm thick mirror tile on top of the heat bed for the printing surface
          Thermistor is placed on the centre hole with thermal heat paste for heat transfer
           I followed Lawsy's guide on this forum

Can you use 3D printing parts to mount the heat bed, i.e. touching it? will these not melt?

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2 Reply by elmoret

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Re: PCB heated bed questions

What bed?

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3 Reply by BFresh

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Re: PCB heated bed questions

This one. http://www.reprap.org/wiki/PCB_Heatbed_MK2

I have a Solidoodle 3

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4 Reply by elmoret

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Re: PCB heated bed questions

Where did you get it from? That link is a wiki page, not a supplier. Very possible the bed has too high of a resistance.

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5 Reply by BFresh

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Re: PCB heated bed questions

It was this one from this store http://www.emakershop.com/browse/listing?l=126

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6 Reply by elmoret

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Re: PCB heated bed questions

"old silicone heat pad between the alu bed"

This is probably not a good idea. More insulation the heat has to work its way through. Usually PCB beds replace the aluminum.

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7 Reply by BFresh (edited by BFresh 2013-11-23 20:55:22)

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Re: PCB heated bed questions

elmoret wrote:

"old silicone heat pad between the alu bed"

This is probably not a good idea. More insulation the heat has to work its way through. Usually PCB beds replace the aluminum.

The old heat mat is on top of the alu bed, then the new heat bed, then glass printing surface. The old heat mat is there as a spacer between the alu bed and the new heat bed and has a cutout in it for the thermistor and cable.

Just to make it clear, the old heat mat is not between the new heat bed and printing surface so does not insulate the printing surface.

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8 Reply by elmoret

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Re: PCB heated bed questions

Ah gotcha. Then its insufficient power or excessive bed resistance.

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9 Reply by BFresh

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Re: PCB heated bed questions

I'm pretty certain it's not insufficient power as I tested it with a bench supply which would supply 12V up to a maximum of 40 Amps. I also used a 500w ATX power supply which supplies 12 volts up to 20 amps and got the same result.

If it's resistance, how do I get around that?

Thinking about it, I may have wired the terminals the wrong way round on the bed. In theory it shouldn't make a difference so I didn't worry about it too much at the time, but could it?

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10 Reply by elmoret

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Re: PCB heated bed questions

Terminals wrong way around wouldn't be a problem, copper traces don't have polarity.

If it is resistance and you used sufficiently sized wire, then the bed is defective.

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11 Reply by BFresh

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Re: PCB heated bed questions

I used the wire from the original silicone heat pad. Would this cause a problem?

Are there any software or firmware settings I should check?

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12 Reply by Briggs

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Re: PCB heated bed questions

Put an ohm meter on it. What does it read? 1.2? Higher?

13 Reply by BFresh

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Re: PCB heated bed questions

As it happens I bought 2 of those heated beds.

Tested them both with a volt meter and both have a resistance of 1.5 ohms

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14 Reply by Briggs

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Re: PCB heated bed questions

No surprise there, it should read .9 to 1 ohm for decent performance, and even lower for max performance. You can easily draw 15 amps, however. Better have an awesome psu.

15 Reply by BFresh

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Re: PCB heated bed questions

Are you suggesting that everything is working as it should and it's just down to the resistance of the bed?

Is the bed faulty?

Is there anything I can do to improve performance?

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16 Reply by elmoret

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Re: PCB heated bed questions

Briggs wrote:

No surprise there, it should read .9 to 1 ohm for decent performance, and even lower for max performance. You can easily draw 15 amps, however. Better have an awesome psu.

Depends on how well calibrated the ohmmeter is. What's the resistance shown when you touch the probes to each other, BFresh?

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17 Reply by BFresh

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Re: PCB heated bed questions

0.5

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18 Reply by elmoret

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Re: PCB heated bed questions

bingo. 1.5-0.5=1.0.

I=V/R, so it'll draw 12 amps at 12 volts. That's plenty. Either the wire is too small, or the power supply/traces on the control board are causing voltage drop.

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19 Reply by BFresh

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Re: PCB heated bed questions

I noticed the existing wire has a large gauge one that connects to the heat pad, which is connected to a smaller wire to fit the little terminal connector.

I just a volt meter on the standard wire. The thinner part that goes into the connector measure 0.5, and the thicker part measures 0.4 ohms.

I have got various replacement wire to use of different gauges, all bigger than the standard wire, but I'm confused....
Do I need to use the thicker gauge wire all the way to the terminal?

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20 Reply by ronsii

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Re: PCB heated bed questions

Thicker wire is better for lower current loos on the wire itself, if you use a thinner wire to the heater the wire will heat up and not conduct the power to the heater.