Power supply question

I am just starting to collect all the bits for a Mendel and I have a question about the power supply. I will probably want to add a heated bed fairly soon after building it. Will a 400w ATX power supply run the machine and still have enough left over to run a 12v heater for the bed?

I haven't even done any planning on the heater so I don't have any estimated power consumption figures. I don't even know what temp I would want the bed at. I am just looking for a gut feel from some of the more experienced builders. If I should be buying a 650w power supply it's cheaper and easier to do it now.

Any thoughts on this?

400w should be heaps. Just remember you need a decent load on the 5v line for the 12v to remain stable at high currents. I use a 12v dichroic globe on the 5v line, with the double advantage of a worklight and a visual indication of when the motors are able to move.

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Wooden Mendel
Teacup Firmware

Thanks triffid_hunter, I had no idea that I needed a load on the 5v line.

It's the electronics part of this that worries me the most. I will buy the boards from Makerbot but if I have any problems getting them to work I have no idea how to troubleshoot them. I guess I will figure it out as I go!

The recommended NEMA 17 motors for Mendel are 1.7 A @ 2.8 V per winding. With a 12V supply that is about 2 * 1.7 * (2.8 / 12) = 0.8A, say 1A allowing for losses.

So we have 4 motors @ 1A and a heater @ ~ 2A, so 6A @ 12V to run the basic Mendel.

A heated bed needs about 1kW / m^2 to maintain 100C for ABS. The Mendel build area is 200 x 200 I think, which is 0.04m^2 so 40W. I would use at least twice that to get a reasonable warm up time, so 80W, call it 100W for some margin = 8.3A.

So the important PSU spec would be a 12V rail with more than 14.3A. The other rails have negligible loads so as TH says you need to load them otherwise the PSU is totally out of balance and will give a 12V rail that is a few volts lower than it should be.

The more power you can spare for the heater, the faster the warm-up time will be.

Edited 1 time(s). Last edit at 02/05/2010 07:15PM by nophead.

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[www.hydraraptor.blogspot.com]

not all the other rails, just the 5v rail. Basically, a computer power supply provides a regulated 5v, and all the other rails are unregulated but closely linked to the 5v due to winding ratios in the transformer. Because of this, if you draw many amps from 12v (or any other rail) but leave 5v unloaded, you will get unacceptable voltage drop. However, if you load the 5v, the other rails remain far more stable.

It's exactly the same principle as attaching a high power light or heater to a generator so it doesn't choke when the fridge or other inductive load kicks in.

As far as I know, the 3.3v rail undergoes secondary regulation and is sourced from 5v or 12v so it's basically out of this part of the equation.

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Wooden Mendel
Teacup Firmware

Quote

So the important PSU spec would be a 12V rail with more than 14.3A.

So all you really need is 172W? Plus whatever you run off the 5V rail.

I personally don't run anything from the 5v rail- I have a 7805 hanging from the 12v so I retain the design-goal option to run it from a 12v battery. as far as I know all the official electronics have their own 5v regulator too.

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Wooden Mendel
Teacup Firmware

Yes 172W plus the minimum load on the 5V rail would be the minimum spec for the Mendel plus a heated bed. A good idea to get something higher if you can afford it to allow for future additions or faster heating of the bed. It also not a good idea to fully load a PC PSU as many have fraudulent ratings and will go pop.

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[www.hydraraptor.blogspot.com]

Note that most PSU ratings are calculated by finding the maximum load of each rail with minimum necessary load on other rails, then adding all the rails up, totally ignoring the fact that the power supply can't run all the rails at maximum simultaneously.

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Wooden Mendel
Teacup Firmware

Yes but safety regulations require the maximum load to be stated correctly, so these are fraudulent. They rely on the fact that PC's don't take the load they state under normal circumstances.

If it is a 450W PSU you should be able to load it up to 450W, keeping each rail within its current limits and run it continuously. In fact most cheap ones blow up if you do.

Edited 1 time(s). Last edit at 02/07/2010 08:38AM by nophead.

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[www.hydraraptor.blogspot.com]

nophead Wrote:
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> Yes but safety regulations require the maximum
> load to be stated correctly, so these are
> fraudulent. They rely on the fact that PC's don't
> take the load they state under normal
> circumstances.
>
> If it is a 450W PSU you should be able to load it
> up to 450W, keeping each rail within its current
> limits and run it continuously. In fact most cheap
> ones blow up if you do.

The main problem with cheap PC PSU's is temperature, they are correct in stating it can handle 450w, but it's usually 450w at 20degC, but under continuous load they warm up and due to impedances varying the capacity of the power supply invariably drops, so if the user is still pulling peak current as always the system fails at the weakest point.

When you send a PSU for safety approvals in the UK the test will apply the max rated load until thermal equilibrium is reached. If they go pop they fail. Not the same in China it would seem!

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[www.hydraraptor.blogspot.com]