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ascanio

Heatbed realy and power supply questions

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I know many words have been said about adding a heatbed on the UM but and iI think I've read most of it but there are still a couple of things I'm not sure about becouse everybody seems to go in a slightly different way.

I have a mk2a pcb which should work at 12v 10 amp or 24v (still 10 amp??)

First of all: the relay.

The output from the UM is 19v but realys work at 12 or at 24 (almost all of them, (though I found a 18v relay on farnell http://uk.farnell.com/jsp/search/productdetail.jsp?SKU=1861593) so some people use a 12v relay (control pin) but it might burn, someone says it works ok and someone else says it burned. Others use a 24v relay but the guy at the store told me 19v might not be enough to activate it... So I guess the only solution is try the 24v and if it doesn't work pray for the 12v one not to burn right? About the other specs I guess they depend on the second power supply I'm gonna use for the mk2a

Fly back diodes

I couldn't understand if i need to put one (and how) becouse they should already have one in each of the 3 heater circuits right?

Second power supply

How many watt? Everybody use different solutions and the same solution seems to work with someone and not to work with others. For exemple if I use 12v to power up the mk2a it should draw 10 amp and therefore, as others suggested, I should get a 12v 12-15 amp power supply. But here it doesn't seem to be enough http://flashgamer.com/arduino/comments/mk1-heated-bed-and-different-power-supplies

So what do you suggest? I found different 12v power supply at higher wattage (360w) but is it a good solution? is it necessary? I was actually thinking of going with 24v on the mk2 but I couldn't find any suggestion for this configuration (amp drawn, watt ecc.) becouse I guess it is not the same as for the 12v solution... I mean since I increase the volt I should decrease the amp right?

Sorry if I wasn't clear but I know almost nothing of electronics, I am just trying to get it work and not burn everything....

 

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Hi Ascanio

I'm pretty sure I have that same PCB and it draws many more amps than that at 12V. At 24V you will double the current and it will be 4 X the power. Also at start up when the PCB is cold it has a lower resistance and draws more current. I have a 360W variable power supply which goes up to 24V, I turn it down all the way and it puts out about 13V. At this voltage it draws a lot of power and even more when the power supply's own fan kicks in.

I have a relay with a 12V coil that has never given a problem but I also think most 24V relays would probably operate at 19V.

The important thing for safety and reliability is to have the current rating of the power circuit to be ample. I suggest 40 amp rating for the relay switching contacts and thick flexible wires. I also suggest the 360W power supply at 12V. Mine uses up more power than it's rating for short periods but has handled it for many months OK. I wouldn't go for a lower power rating than this or higher voltage.

 

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You don't need flyback diodes as they are already built into the UM.

The mk2a acts mostly like a resistor. Read about ohms law on wikipedia. It is the basis for everything electronic. If you don't understand that then you should read it again.

The mk2a is specified to be 1 to 2 ohms but which is it? I guess it's closer to 1 ohm when cold and 2 ohms when hot? That's a huge difference. A factor of 2X. How annoying not to be more specific.

I=V/R (current = voltage over resistance). So if V is 12V and R is 1 ohm then you will get 12 amps.

If you double the voltage you get 24V/1ohm is 24 amps. 24 amps at 24 volts is 4X the power of 12 V at 12 Amps.

12V at 12 Amps is 144 watts.

 

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Not too sure if you already got things up and running but I used the same PCB. I measure about 1 ohm when its cold. I am using this power supply: http://www.newegg.com/Product/Product.aspx?Item=9SIA0U008P5041&Tpk=9SIA0U008P5041

It seems to work okay. My thermistor connected off the UM board (separate circuit for the bed) reads 77 degree C as the highest temperature I have gone too. It definetely gets too hot to touch comfortably. I am still testing my setup too. Hope that helps!

 

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My (bad) Experience with Relays and big Heatbed Current...

After I started using ABS as my preferred Filament, I had Problems with Warping and Curling of my Parts. I decided to add a heated Bed to my UM and use a 12V Powersupply for it. That worked for some Time, but after a While, the Relay of my Temperaturecontroller, a cheap one from ebay, began to make Trouble. The Relay didn't turned off because the Contacts burned togheter and the PCB overheated. The Clicking was also a bothering Thing...

As a Solution, I made my own Controller and changed the Powersupply from a 12V/240W to a 24V/350W. This works great and I adjusted it down to 20V, so I can use it later as a Replacement for the Stock one from UM. The Controller is shown at: http://www.thingiverse.com/thing:95994

It is currently in an early Beta State, but works great. I'm very happy with this Solution, because I can customize the Firmware and add some usefull Featrures to it like Auto Temp Down etc. It has currently 8 Profiles: 4 PLA and 4 ABS. There is no Relay anymore, just a Power N-MOSFET that doesn't get hot and that's great. My Heated Bed has an Insulation of Silicone underneath to conserve Temperature. By the Way, my PCB has about 3.8 Ohms and I have enough Reserve to use the Powersupply later with the UM Electronics.

 

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You don't need flyback diodes as they are already built into the UM.

 

In our case without a flyback diode on the controling wires that go from the bed powersupply to the ultimaker the LCD as would go crazy as you can see and read http://umforum.ultimaker.com/index.php?/topic/2470-strange-display-after-custom-firmware-upload/]here[/url].

Just for help building an KB around issues related with heated bed.

 

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Hi,

I recently finished my heated bed setup and also found that a MOSFET was a good way to go if you want to make a simple circuit youself. They don't suffer from the annoying clicking that normal relays do and are much quicker.

On mine I kept the setup totally seperate from the Ultimaker by using a small Arduino and and LCD.

I'm not sure what the others think but an SSR (FET based) also seems to be a really simple solution if you just want to plug something straight in (except for the resistor on the Ultimaker board)

The big advantage of SSR's is that they are generally optically isolated and can switch with 19v directly for 12V.

Cheers,

 

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