Heatbed Heater Options

Ok so after far too long, I finally have a new heatbed and SSR. The problem was with DHL, they ended up having it in their posession since December 28th, and it only arrived yesterday. Wonderful.

I'm now making a start on the long journey to doing all the upgrades. The first thing to tackle is the ATX power supply. I'm making a start now on tidying up all the wiring, removing excess wires etc. I know there is a chance that I'm going to have to put a load across the +5V, so I've a few questions with wiring this up:

1. IF I test it and I find that I do need a load resistor, could I use a 12V DC fan? I know that at 5V, it wont be running at full power, but it's ok, I'll find a use for it. If not, I've a tonne of 0.25W resistors. Would they suffice, and if so, what value should I be looking at?

2. I'm going to have to use my old heatbed to print a bunch of new parts for mounting the new heatbed. Thing is, it's currently mounted with 4 x 2R2 8W aluminium clad power resistors. The datasheet for them is here: http://uk.farnell.com/te-connectivity-cgs/ths152r2j/resistor-al-clad-15w-2r2-5/dp/1259346 I can't seem to find a Maximum Temperature for them. Originally, the heatbed had 4 x 4R7 15W aluminium clad power resistors, and I still have those somewhere. I'm trying to establish if I can get away with the 2R2 resistors for a short while to get the parts printed. I know they only heat in spots, but it's only a very temporary solution.

Ok interesting development. I had a look at the original Felix 1.5 instruction manual, cos that used an ATX supply too. In that one, the Green wire in the ATX was soldered to one of the Ground wires. The Green one in that PSU is labled "PS/On" on the actual circuit board for the power supply. Now when I looked at the circuit board for this one, what do i spot? A green wire with PS/On where it hits the PCB. My instinct is to do the exact same with this one. Green > Ground so it's on once it's plugged in. Makes sense, right?

I don't think a 12V fan would provide sufficient load on the 5V rail. I think you need a load of several hundred mA. RepRapPro used to supply an ATX PSU with the Ormerod kit. Their ATX power distribution board has 2 resistors, a 30 ohm 5W one and a 22 ohm one that I guess is about 2W. But I can't easily tell which outputs they are loading. My guess (based on the power ratings) is that the 30 ohm resistor is across the 12V rail, and the 22 ohm resistor is on the 5V rail.

One easy solution would be to connect a 12V rear light auto bulb across the 12V supply (AFAIR these bulbs are about 5W), and another across the 5V output.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Crap.

The fans I have are 0.96W, and I don't have any power resistors remotely large enough. According to this site (http://makezine.com/projects/computer-power-supply-to-bench-power-supply-adapter/), a 10R resistor at 10W across the +5V and Ground is enough for most PSUs. But it looks like I can also test it with the fan in there first to see if it does generate enough of a load. If it shuts off, at least I wont be damaging the PSU, right??

The next thing I've just noticed is that the wires coming off the new heatbed are quite thin. I measured the resistance, and it's a solid 1.5R, so by my calculations, that's 8A. Based on my earlier diagram of using the SSR, I'm wondering if those thin wires from the head bed will be thick enough to take 8A. This is them compared to some of the ones I've trimmed from the PSU:


The black/yellow are the PCB, the purple/grey were cut from the PSU.

Ah I think I've figured it out now. This is on the side of the ATX case:



So that means I'm going to need resistors across Ground and 1 x Red Wire (+5V), 1 x Orange Wire (+3.3V) and 1 x Purple Wire (5V StandBy), so that they reach minimum loads, am I correct?

If so, then by my calculations, these power resistors should do it:
27R @ 1W across the 3.3V (122mA)
47R @ 1W across the 5V (106mA)
47R @ 1W across the 5V (106mA)

1W resistors I should be able to get this week easily and cheaply, but before I do, I just thought I'd check to make sure that I'm correct in my calculations!

Bit of a mediocre day shopping for parts yesterday. The closest I could get was 2 x 33R and 2 x 39R power resistors. I'm fairly sure they're 3W and 5W judging by their size. The plan is to use the 2 x 33R on the 2 x 5V connections, and the 2 x 39R in parallel across the 3.3V. These are the resistors in question, I put a standard 1/4W resistor in the middle for size comparison:



I'm still not sure about the wires coming from the heatbed. Should I replace them with something thicker?