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using LTC3780 and ATX PSU to create Variable Lab Supply

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emmsee

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I started out building a Bench PSU (fixed voltages) from an ATX PSU, but have recently watched several videos on the LTC3780 Buck Boost controller and want to now create a variable bench power supply.
I watched videos to achieve the variability but using a LM317 voltage regulator.
I've seen one YT video using a PSU with the LTC3780, however keeping the ATX separate to the output case.
Not much else out there but I feel it's because the LTC3780 is a recent addition to the mix. It's pedigree is great and it gets top positive feedback from the DIY crowd.
I would de-solder the current and voltage pots and wire in 2 case face potentiometers.
Also insert 2 x voltage/current displays if I go variable and fixed output.
I have a 700 watt (240 volt) Thermaltake PSU which has a very small case (probably too small to fit the LTC 3780 and posts etc).
This is a new (and new style) PSU with +5v, +3.3v,+12v1, +12v2,-12v,+5vsb as well as the usual - voltages.
The 12v1 has highest current at 30 amps, with the second 12v at 22 amps,The +5vsb has 2.5 amps,+5v has 15A, plus 3.3v has 24A
I've ordered a larger all metal case into which I propose to attach the Thermaltake case plus the LTC and all the attachments.
1/ is this method (using an LTC3780) of achieving the variable output feasible?
I'll be using it to charge a balanced 7S4P 18650 battery pack among many others.
2/ should I screw or glue the Thermaltake to the new case (thinking of the grounding)?
3/ Use +12v1 or +12v2 for primary power to the LTC3780?
4/ Use the +5vsb attached to a usb for 5v charging?
5/ use the unused +12v for a fixed 12v output?
6/ The PSU has 23 x black, 4 x +12v2 and 7 x +12v1 wires; how many black should I group for the LTC ground - does it matter?
7/ use all 7 of the +12v1 (30 amp) for the LTC positive, if not how many?
There are many mentions in You Tube videos of cutting off unused wires, but none that mention precisely what to do with them and how short to cut them. I assume (silly of me?) each unused cut wire should be isolated with either electrical tape or liquid electrical tape then combined in tape and hidden out of the way. Any advice would be much appreciated.

The new case has plastic surrounds that isolate the face/back from the top/bottom/sides.
8/ Should I strap connect all surfaces?

Any other advice/comments would be welcome - no trolling welcome.
cheers
Mike
 

throbscottle

Well-Known Member
Welcome to ETO!
1/ I'm not familiar with this device, but it looks like you should have no problem. Just make sure you have an adequate heatsink for the worst-case efficiency situation. First thing I found which may help you: https://www.instructables.com/id/What-Is-LTC3780/
2/ Screw! Glue is guaranteed to come unstuck eventually. Go to www.formfactors.org to find the standards for the cut out and screw holes you would find on a PC case, which you can use for your PSU
3/ It probably doesn't make any difference. The 2 12v supplies have separate internal current limiting. It depends on what you plan to do with the other one, but either way you are unlikely to use all the amps! Looking at point 7/ I would use the 22 amp one
4/ Just use the normal +5v for USB charging, unless you particularly want it to work when the PSU is turned off
5/ Sounds like a good idea
6/ The ground wires are pretty hefty, combined together they can carry the full rated current of all the PSU outputs, plus some overhead for safety. Bear in mind that in boost mode, the input current to the LTC3780 will be much higher than the output current, and in buck mode it will be lower.
7/ I built an ATX conversion and connected multiple output terminals to each voltage. You could use one of the 12v channels exclusively for it and connect the other one to several terminals. My (not adjustable) version is here https://www.instructables.com/id/Yet-Another-ATX-to-Bench-PSU-Conversion/
If you cut any wires there is no need to do anything special with them unless you think there's a chance of them shorting. They are simply connected to the same place on the PCB inside the supply as the other wires. It's better though to simply double up (or more) on wires connecting to the outputs. Don't connect 12v1 and 12v2 together.
8/ If the parts of the case are isolated from each other, yes you should.
I hope you will adopt my switch design! https://www.instructables.com/id/Latching-Momentary-Switch-for-ATX-PSU-Conversion/
 

emmsee

New Member
Welcome to ETO!
Go to www.formfactors.org to find the standards for the cut out and screw holes you would find on a PC case, which you can use for your PSU - That website has been substantially altered and I could not find the cut outs and screw holes.
3/ It probably doesn't make any difference. The 2 12v supplies have separate internal current limiting. It depends on what you plan to do with the other one, but either way you are unlikely to use all the amps! Looking at point 7/ I would use the 22 amp one - I'm inclined to use the larger current for the Buck Boost converter to improve the efficiency.
4/ Just use the normal +5v for USB charging, unless you particularly want it to work when the PSU is turned off. As the 5V stand by has a 2.5 amp output current it seems to me to be ideal for charging smartphones etc, even when the front power is off
6/ The ground wires are pretty hefty, combined together they can carry the full rated current of all the PSU outputs, plus some overhead for safety. Bear in mind that in boost mode, the input current to the LTC3780 will be much higher than the output current, and in buck mode it will be lower. Good point
7/ I built an ATX conversion and connected multiple output terminals to each voltage. You could use one of the 12v channels exclusively for it and connect the other one to several terminals. My (not adjustable) version is here https://www.instructables.com/id/Yet-Another-ATX-to-Bench-PSU-Conversion/ Love your work throbscottle, the format for your instructable is easy to follow and helped me immensely
8/ If the parts of the case are isolated from each other, yes you should. They will be because of the plastic/silicone frames for the project box so I will ground all sides
I hope you will adopt my switch design! https://www.instructables.com/id/Latching-Momentary-Switch-for-ATX-PSU-Conversion/ I don't feel I need to go that far with the switch, but like what you've done with it
Thanks for your advice thobscottle (now there's a new word to add to my dictionary!!
 

throbscottle

Well-Known Member
You are most welcome. It's throbscottle because I couldn't spell frobscottle, by the way, which is the giant's favourite drink in The BFG.
If you go on formfactors.org, follow archived documents link at the bottom of the left hand pane. All of the ATX12V documents that actually exist (there's a few dead links) contain a mounting hole plan on one of the high-numbered pages. Take your pick - I think they all have the same plan though.
Can't remember but I think if you want smartphones etc to charge at higher than 500mA you need more than just the power connection, you have to do something with the data connection too. For normal (up to 500mA) charging you need to connect the D+ and D- wires together. Chargers often then have a pull up or pull down resistor, but it's not in the specification.
Good luck!
 
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