Using an extruder controller for a heated bed

Hi! I'm planning on setting up a heated bed for my mendel (once the parts arrive, anyway), and I will have a spare extruder controller in addition to the one controlling my extruder, so I'm thinking of using that for the heated bed.

According to this page:
[reprap.org]
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"The first electrical question that arises is obviously: how much power is needed to keep the bed at the right temperature? I run my bed at 55oC (though see below), and that seems to need a little under 100 W at a duty cycle of about 70%.

If you are going to run the bed at 12 volts, then this implies a bed resistance of 122/100, or about 1.5 ohms. This further implies a total current of 8A."
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And this page:
[reprap.org]
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3 x MOSFET drivers for controlling up to 14A @ 12V. Perfect for heaters, fans, solenoids, etc.
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So it appears the extruder controller can handle it if I just connect the bed heater to the extruder controller where the extruder heater connection would normally be. (Let me know if I'm mistaken!)

My problem after that is how to manage the software and the firmware. The latest reprap host software seems to have heated bed options, but I don't know how to set up the firm ware for this kind of configuration. configuration.h under the extruder firmware and under the motherboard firmware aren't entirely clear.

The NIF5003 MOSFETs have that rating, but only when given adequate heat sinking. There is no heat sinking on the extruder controller so you probably wont get much more than 2A before they get hot and shut down unless you add some heat sinks and / or a fan.

Also I doubt the PCB tracks and the wiring will handle 8A.

Edited 1 time(s). Last edit at 12/17/2010 10:49AM by nophead.

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[www.hydraraptor.blogspot.com]

You can use the Heated WolfBed solution from Stoffel15.
This solution is also being sold completely assembled by GRRF.
In fact, nophead was the one that gave Stoffel15 the idea!
Seems to work great and you use the A6 output on the extruder board.
No power problems since it only drives a relay that turns the AC to the silicon mat on/off.

Edited 1 time(s). Last edit at 12/18/2010 09:55AM by rhmorrison.

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

You shouldn't need two extruder controllers. One controller should have three mosfets for controlling up to three 12V devices. Configure your firmware/software to use one for the extruder heater and a second one for the bed heater.

It might be worth noting that the heater shown above was designed to run on 220V. I don't know many people who have 220V outlets handy for something like this. Maybe other countries are different, but in the US, a typical outlet is 120V only. A typical house might have two, maybe three 220V outlets and they are buried behind bulky appliances. Is there a 120V version of this kit?

If it is just a resistance load then you could probably run it on 120V but you would get roughly half as much heat. It should be about the same as running a 12V light with a 9V battery - it still lights up, just not as bright. If it uses induction, I'm not sure how the lower voltage would affect it.

Edited 2 time(s). Last edit at 12/17/2010 05:11PM by dazed.dnc.

In Europe all the sockets in a house are the same voltage, about 220V. If you run it from 120V it will be more like 1/4 of the power (P = V^2/R). You would need a 120V version of the heater in the US.

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[www.hydraraptor.blogspot.com]

Quote
nophead
You would need a 120V version of the heater in the US.

Of course, you need a heater match for your normal household voltage.

Edited 1 time(s). Last edit at 12/18/2010 04:59AM by rhmorrison.

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

nophead Wrote:
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> In Europe all the sockets in a house are the same
> voltage, about 220V. If you run it from 120V it
> will be more like 1/4 of the power (P = V^2/R).
> You would need a 120V version of the heater in the
> US.

Oops. I meant to figure the wattage but I had current on my mind instead. I forgot to account for P = IV.

Edited 1 time(s). Last edit at 12/17/2010 07:41PM by dazed.dnc.

This is a good picture:

[upload.wikimedia.org]

Showing the delightful range of voltage in the various countries around the world. I have a different problem. NZ household sockets tend to be rated for 10 Amps where as UK sockets are 13 Amps which can effect the usage of some workshop tools and appliances, that I might have brought in to the country

nophead Wrote:
-------------------------------------------------------
> The NIF5003 MOSFETs have that rating, but only
> when given adequate heat sinking. There is no heat
> sinking on the extruder controller so you probably
> wont get much more than 2A before they get hot and
> shut down unless you add some heat sinks and / or
> a fan.
>
> Also I doubt the PCB tracks and the wiring will
> handle 8A.

Thanks nophead! That's the kind of expert advice I was looking for. Too bad it's not going to be as plug and play as I thought.

rhmorrison Wrote:
-------------------------------------------------------
> You can use the Heated WolfBed solution from
> Stoffel15.
> This solution is also being sold completely
> assembled by GRRF.
> In fact, nophead was the one that gave Stoffel15
> the idea!
> Seems to work great and you use the A6 output on
> the extruder board.
> No power problems since it only drives a relay
> that turns the AC to the silicon mat on/off.

Thank you for the suggestion rhmorrison. What do you guys think of this for the heating pad and the relay for north american 110VAC?

[www.amazon.com] (Unfortunately it's a little bit skinny for the mendel build platform.. if I use this hopefully the cool spots won't cause a problem)

[www.opto22.com] (Please bare with me. As of yet I know very little about electronics. I just looked for a relay that handled 120VAC and enough current.. I don't know if there's anything else I have to consider. I'd be happy to read educational materials if anyone can point me at good ones)

Can anyone forsee any considerations, suggest any alternative materials, or give me any advice?

That relay uses zero-cross switching. I'm not sure if that is a good or bad thing, but mine does too. When your controller switches it on/off, there will be a small delay in response because the AC voltage has to hit the 0V part of the curve before the relay will actually switch. This will throw off the response of your temperature control a bit. However, I have seen people say this helps with reducing EMI. I think that applies more to inductive loads though. So for this application, I'm not sure if it helps, hurts, or makes negligible difference either way.

Also, 500W is quite a lot. My heater is closer to 240W. It works ok, but I think something in the range of 150 to 200W would have been better.

A higher wattage heater can be controlled more accurately with PID, but getting it adjusted can be tricky. I can't speak for what PWM will do to your solid state relay either. There are some that are advertised for use with PWM applications, but I don't know if this is something that all solid state relays can do or not.

If you try to use Bang-Bang control with an over powered heater, you will get more temperature swing because of the delay between the over shoot and the controller responding to it. A lower power heater will take longer to heat up, which helps to smooth out the temperature oscillation.

Thanks for the advice. I'll find a lower powered heater. Apparently there's a pretty wide selection of engine heater pads, although they're a bit pricy.. cheapest I could find that would suit my needs is about $50USD .. but it's self adhesive and has probably been thoroughly tested for fire safety so that's not so bad.

I google translated the heated wolfbed link above, and it seems that connecting this up would be very straightforward. This diagram seems to show me where to connect:

[reprap.org]

But it's highlighting three connections?

I'm also looking into how to modify the software.. configuration.h has some settings that look relevant?

#define BED_TEMP_PIN 6
#define BED_TEMP 6
#define BED_OUTPUT 15

The comment above BED_OUTPUT says 'MOSFET driver' .. not really sure what that means.

Oh wait.. I think maybe A6 means analog input maybe? For the bed thermistor?

Sorry I'm really new at electronics.. I'm a software developer by trade, and it's hard trying to figure out that page with google translate..

Yes, the A6 is for the bed thermistor input, the Mosfet on the extruder is used to control the relay (as opposed to nichrome wire or a heater resistor since the power required would be too much for the Mosfet).

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