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Question re: heated bed build PSUs

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Ok, I have pored over these forums for awhile, and ordered what I think is everything I need for a heated print bed build with one exception. I just had a few quick questions regarding the suitability of particular PSUs.

Per the recommendation of several forum posts (as well as my own skills and familiarity), I have decided to go with a second PSU and a relay (ordered this one http://www.findchips.com/avail?part=ALFG1PF18 with a socket) to power my HPB. From what I've read, generally the greater the wattage, the faster the PCB will heat up. I've also heard some people having problems with lower wattage PSUs (like 180), and also with certain brands, so I was thinking a PSU that is 360 watts or above.

So now my question: I was wondering if anyone had a positive experience with a particular PSU that you could recommend to me. Any particular output you recommend? 12? 24? Variable? I understand the risks of 24V to be lower on the US 110v grid, but also am considering installing a fuse. Also, what output wires might I need for that particular PSU?

Thanks in advance,



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You should decide which HBP to buy and find out what power it consumes at what voltage.

Then buy a PSU that supplies that voltage and a wattage greater than that required.

The wires from the PSU to the HBP need to be thick and flexible.

Before you turn it on get an electrician or similar to check it over.


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First step is to know the resistance of the heated bed in ohms. Typically they are around 1 ohm. Then the wattage will be V*V/R (v squared over r). So if the voltage is 12V and resistance 1 ohm, 12V will give you 144 watts. To get 360 watts (that's a lot - I have 100 W which is slow but 200W I would think is enough) take the square root: 19V (if 1 ohm heated bed). If it's 2 ohm's heated bed then 12V gives you only 70 watts. 19V gives you 180 watts and 27V is needed to get 360 watts.

So it's good to understand ohms law and the power law. They are extremely simple formulas but really knowing subconsciously them is very helpful when doing anything with electricity.

If the manufacturer tells you the heated bed is "1 to 2 ohms" well that's a HUGE range. You really need to know this before you can pick the ideal power supply.

If you get a 2000Watt supply but it only puts out 12V and the bed is 1 ohm then you will only get 144 watts into that heated bed. The 2000Watt is a maximum, not a given.

This is frustrating when ordering a heated bed because you don't know how many ohms it is until you get it. Also the resistance changes when it gets hot. So you kind of need to know the resistance when it is at the extremes (room temperature and 70C) so you know if you will draw too much current (too much power) for your power supply.

For example if they tell you it's 2 ohms at 70C but don't mention that it is 1 ohm at room temp. And you buy a 400 W supply at 27V. When you hook it up it will try to draw 729 Watts and probably the supply will go into protection mode.


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Thank you both for your prompt responses!

Okkk, well the bed I was looking at is supposedly between 1.0 and 1.2 ohms resistance, according to the manufacturer, and can handle either 12V or 24V.

Thus for the 12V option, as far as I understand, assuming a mean resistance of 1.1 ohms, I would need a PSU that can handle 130 watts, and a minimum of somewhere around 10.9 amps.

Would this work for that? http://www.ebay.com/itm/12-Volts-DC-Power-Supply-Unit-LDA150W-12-Output-12-VDC-12-5-Amps-LDA-150W-12-/350833196006?pt=LH_DefaultDomain_0&hash=item51af49bbe6

For the 24V option, assuming the same mean resistance, I would need a PSU that could handle around 523 watts.

How about this? http://www.ebay.com/itm/Mean-Well-MW-AC-DC24V-50-100-350-400-600-1000W-1500W-Switching-Power-Supply-PSU-/111115520782?pt=LH_DefaultDomain_0&var=&hash=item19df004b0e

I am assuming I should us DC, but do not know if there are any advantages to AC, or if it would even work.


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It may be a bit slow at heating at 130W and a bit too much current at 523W. So a variable PS 12-24V at 350+ Watts would be best I think and adjust the voltage to set the wattage at around 300W DC is probably better at such high currents.


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Yes and no. Technically it's not designed for voltages in between but I'll get back to that...

These are the kind of specs that piss me off. Going here:


There is slightly more information. It turns out that it's really 2 resistors in series. You can put 24v across both of them (which would leave approx 12v at the center point) or you can ground the outer points and put 12V at the center point which takes twice as much current. This way you get same heating power and power draw either way. The problem is they don't specify the power draw nor each resistance.

So it's unclear if the whole thing is 1 ohm (both resistors added together) or if the whole thing is 4 ohms but is 1 ohm when you hook it up in 12V mode (both resistors in series). I would email them.

So from their description it appears to be either a 523W heater (1.1 ohm total, quarter ohm in 12V mode) or a 130W heater (4.4 ohm total, 1 ohm in 12V mode). I'm going to guess it's 4 to 4.4 ohms total because at 523W or more it's hard to beleive it would take 10 minutes to heat to 100C. So I suspect you can wire it two ways:

4 to 4.4 ohms


1 to 1.1 ohms

BUT THEY DON'T SAY THAT ANYWHERE! It's a guess but a very good guess.

Having said that, if you put 24V across where the 12V is meant to go the resistor should be okay (hopefully won't melt) but you might blow out the LED's. But that's not a big deal - you can skip the LEDs. But I'm sure you could up the voltage somewhat - to maybe 15V (in 12V mode hookup) where you would get 205 to 225 watts. I wouldn't go higher than 15V though. Not without playing with it.

If it were me I wouldn't order the power supply until I received the MK3. I would then hook up 12V to it and look at the power draw. Use that to determine the resistance at different temperatures. I would then try to create higher voltages and see how hot the thing gets (don't want to burn anything). Only then would I order a power supply for permanent use with the UM.

You can get (almost) free high current 12V power supplies out of any PC. My junkyard has all I could ever want. I just have to drive over, take an old junker PC, bring it home and remove the supply. Then short the green wire to a black wire and power comes on (more on wikipedia about the green wire thing).

Then later you can graduate to a larger supply by ordering one online.


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I saw that, too, but should then this heated bed http://reprap.me/Alu-Heatbed-MK3 be able to operate anywhere between 12V and 24V, and not just at those individual temps?


It's designed only for those 2 voltages and no other. It will work fine at

lower voltages. At higher voltages - who knows - there are 2 possibilities:

1) You can blow up the LEDs. This isn't a problem if you don't install


2) You can heat it so much that something starts to melt or burn. That's

probably pretty damn hot - like at least 200C would be my guess. Maybe even

300C before there's a problem. But I don't truly know. I suspect it can

take at least 50% more wattage than it is specified for but I don't know.

Personally I would go for it and see what happens. If you smell smoke be

ready to turn it off right away. But I probably wouldn't go over 15V.


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Ok, the vendor finally got back to me and said "The resistance is double for the 24V," thus indicating (I think) that at the 24V hookups, the resistance would be 2.0-2.4 ohms. Thus, unless I am missing something, power draw *should* be (24^2 / 2.2) = ~260 watts at 24V.

The vendor had previously told me that 180W would be sufficient for the 12V hookup, meaning (I think) he meant that the entire board's resistance would be 1.0-1.2 ohms at for 12V, yielding power draw of (12^2/1.1) = ~ 130 watts.

Thus, if I go with a 24V PSU having a draw > 260 watts, I should be OK. I still plan on holding off on purchasing the PSU until I get the board, but does that sound correct?


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Ah. Great. This makes sense. You can probably order the power supply now.

You should probably get a 24V supply > 260 watts. In fact since the resistance could be as low as 2 ohms then it needs to be > 288 watts. In fact I would get at least 350 watts to be safe (don't want the PS to burn out on a hot day or if you leave it under a towel or something and it over heats).


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So much text and in the end it was so simple. ;) But on the other hand I really understood that you want to go the safe way.


I'm using the MK2b PCB at 12V. I think its pretty the same thing but without a aluminium core. I just uses my multimeter to verify the vendors answer. The resistance amounts to 1.1 ohms.


Before I installed it in my UM I tested the current when I'm using just one ground block (12V at nr. 1 & ground at nr. 2 OR 3). The result was something between 6 and 6.5 amps. My multimeter only is fused up to 10 amps so I couldn't verify the whole current - but to take it twice I thought was the safe way.


I fused the PSU to relay connection with an 15 amps fuse. I simple using an old 350 watts PC PSU that can supply up to 18 amps. I had no desire to wait around 4 weeks until the PSU from China arrived. And the same PSU here directly from Germany has a price that is five times higher. For the future I will buy a China PSU on ebay.


But what bothers me a little is the relay what you are bought. I had a short look at the tech. specifics and the coil has it nominal voltage at 18 volts. However the screw terminals on the UM board which you are using to steer the relais delivers around 19 to 20 volts. That means that your relay coil is permanently under overvoltage. There are definetly a security reserve but maybe one day the coil dies a heroic death. ;)


I am using a simple 24 volts car relay. The switch point is at around 15 volts. But the coil can handle up to 24 volts plus security reserve. For around 2,50 € no other part can beat this. And it's absolutely easy to install.


Just my two cents.


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