Reading about PSU options to power my printer, I decided to use ATX PSU, more specifically a Haswell ready/compatible PSU. Non-Haswell PSU or regular ATX PSU problem is that they require load on 5v rail or they will not switch on at all or will swich off shortly after it's switched on. To use regular PSU-s, you will have to add some base load to 5v rail. Base load can be light bulbs, resistor, etc. But I didn't like the idea to have some resistor that would generate heat just to make PSU work.
So, monitoring week or so a local used PC part sellers forum, I found PSU I really liked - FSP HE-500. New one will cost about 50€, I got it for 15€. It's Haswell compatible, active PFC and has two 12v rails, both with max 18A output current. This PSU is going to power RAMPS 1.4 that has also 2 power inputs. One for stepper drivers and Extruder heater that requires 5A, and second input for Heated Bed that requires 11A. So, heated bed input that requires most current, will have 39% safety margin. So there should be no problems powering Prusa.
All i did next with my PSU is based on this blog post.
Testing
First, I tested my PSU. For that, I measured voltage between Power_OK (gray wire) and ground (black wire). On standby mode, that is if it's just connected to wall socket and powered, then you should get 0V between gray and black wires. Also, you can measure voltage between 5Vsb (purple wire) and ground. This should give you always 5V. Now, turn PSU on by shorting green wire to black one, fan should start and measuring voltage between gray ad black wire should give you 5V. If you have some problems with PSU, then now is time for warranty. Next steps will void your warranty.
12v rails
Yellow wires are for 12V. If you have single 12V rail PSU, then you will have only all yellow wires. In case your PSU has multiple 12V rails, then these yellow wires will have some stripes. Because I have two rails, then I have yellow wire with black stripes for rail 1 and yellow wire with no stripes for rail 2. And because both are rated at 20A max, then I do not need to choose which rail will be connected to hotbed that requires most current. Also, do not connect different 12V rails together. You would think that connecting 2 rails together you could use 20A+20A=40A, but in reality, if you are lucky, your PSU will not power on, or if you are unlucky, you will blow up your PSU. These rails are not meant to be wired together. Well, some of these can be wired together, but to be safe, just don't do it. Here is some interesting reading about multirails (and wiring rails together).
Removing wires
Next, I opened up PSU (disconnect it first!). Removed some dust as it was used PSU. And now it's time to remove some wires, and all connectors. I must say that I was cautious and kept too much wires as I didn't know what i will need in future. But now when I have printer basically working, I'm thinking of reopening PSU and cutting rest of wires. Currently I'm using 2 yellow with black strips, 2 yellow, 4 black, one purple and one green wire. I am using double wires to power RAMPS, meaning 2 yellow with black stripes are wired together and connected to RAMPS. Same with yellow wires and for both grounds. And wires that I left just in case, but are not needed: 11 black (ground), 5 yellow with and without stripes (12V), 2 red (5V), one brown (3.3V maybe?) and one gray (Power_OK) wire. So, total of 20 extra unused wires. Also, you may notice that some of wires are thicker and some are thinner, even if color is same. And these wires have lots of stuff written on, but also there is "18 AWG" or "20 AWG" or some other number with "AWG" written on. Basically, smaller number means thicker wire. So, you should always select thicker wires to use.
Conclusion
If possible, select Haswell PSU. It may or may not be multirail, does not really matter, but make sure your peak-current is at least 25% less PSU-s max current at 12V rail(s). You really need only some yellows, some blacks, one green and one purple wire. Rest of wires (that is over 20 wires) can be removed.
And now it's picture time.
And why not a testing video
So, monitoring week or so a local used PC part sellers forum, I found PSU I really liked - FSP HE-500. New one will cost about 50€, I got it for 15€. It's Haswell compatible, active PFC and has two 12v rails, both with max 18A output current. This PSU is going to power RAMPS 1.4 that has also 2 power inputs. One for stepper drivers and Extruder heater that requires 5A, and second input for Heated Bed that requires 11A. So, heated bed input that requires most current, will have 39% safety margin. So there should be no problems powering Prusa.
All i did next with my PSU is based on this blog post.
Testing
First, I tested my PSU. For that, I measured voltage between Power_OK (gray wire) and ground (black wire). On standby mode, that is if it's just connected to wall socket and powered, then you should get 0V between gray and black wires. Also, you can measure voltage between 5Vsb (purple wire) and ground. This should give you always 5V. Now, turn PSU on by shorting green wire to black one, fan should start and measuring voltage between gray ad black wire should give you 5V. If you have some problems with PSU, then now is time for warranty. Next steps will void your warranty.
12v rails
Yellow wires are for 12V. If you have single 12V rail PSU, then you will have only all yellow wires. In case your PSU has multiple 12V rails, then these yellow wires will have some stripes. Because I have two rails, then I have yellow wire with black stripes for rail 1 and yellow wire with no stripes for rail 2. And because both are rated at 20A max, then I do not need to choose which rail will be connected to hotbed that requires most current. Also, do not connect different 12V rails together. You would think that connecting 2 rails together you could use 20A+20A=40A, but in reality, if you are lucky, your PSU will not power on, or if you are unlucky, you will blow up your PSU. These rails are not meant to be wired together. Well, some of these can be wired together, but to be safe, just don't do it. Here is some interesting reading about multirails (and wiring rails together).
Removing wires
Next, I opened up PSU (disconnect it first!). Removed some dust as it was used PSU. And now it's time to remove some wires, and all connectors. I must say that I was cautious and kept too much wires as I didn't know what i will need in future. But now when I have printer basically working, I'm thinking of reopening PSU and cutting rest of wires. Currently I'm using 2 yellow with black strips, 2 yellow, 4 black, one purple and one green wire. I am using double wires to power RAMPS, meaning 2 yellow with black stripes are wired together and connected to RAMPS. Same with yellow wires and for both grounds. And wires that I left just in case, but are not needed: 11 black (ground), 5 yellow with and without stripes (12V), 2 red (5V), one brown (3.3V maybe?) and one gray (Power_OK) wire. So, total of 20 extra unused wires. Also, you may notice that some of wires are thicker and some are thinner, even if color is same. And these wires have lots of stuff written on, but also there is "18 AWG" or "20 AWG" or some other number with "AWG" written on. Basically, smaller number means thicker wire. So, you should always select thicker wires to use.
Conclusion
If possible, select Haswell PSU. It may or may not be multirail, does not really matter, but make sure your peak-current is at least 25% less PSU-s max current at 12V rail(s). You really need only some yellows, some blacks, one green and one purple wire. Rest of wires (that is over 20 wires) can be removed.
And now it's picture time.
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| Lots of unused wires from PSU. Also, big mess with wires on RAMPS, but I will fix that when I get my printer to work as I want it to. |
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| Close-up of main power |
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| Close-up of 5Vsb and PS_ON |
And why not a testing video
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