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The LM317 Adjustable Voltage Power Supply kit

BillMC

Dec 2, 2022
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This kit has a US mains plug. Does this imply that it can only run on a 110V mains supply? There was no instructions for this kit, but on the advert for it - it said that the assembled kit can supply up ro 12V with a 220V mains supply and on to 6V with a 110V mains supply. So if this is true - could it safely run with a 240V mains supply?
 

BillMC

Dec 2, 2022
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Bluejets

Oct 5, 2014
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Get on to the supplier.
There should be some kind of instruction sheet provided or a link to download one.
 

danadak

Feb 19, 2021
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No instructions in any of the listings, other than simple assembly.


Regards, Dana.
 

kellys_eye

Jun 25, 2010
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Just an observation -

the transformer appears to be labelled as a 220V AC input, 12V AC output.

If built/used in the UK it (the transformer/rectifier part) will have around a 15V DC output feeding the LM317 and, with the internal volt-drop requirement of the LM317, would give an (approximate) maximum 12V DC output (variable from around 1.2V as a minimum).

The 220V AC transformer can be safely used on a 110V AC supply but the output will be HALF thereby the DC output after the LM317 will only vary between around 1.2V and 6V DC.

Junk the transformer. Fit a socket that accepts the DC supply from an old laptop power pack (commonly 19V DC output) and you'll end up with a power supply that varies from 1.2V to around 16V SAFELY.

Note - the DC output from the laptop PSU can feed directly into the power supply board where the AC (from the transformer) used to go (in which case polarity isn't an issue) OR you can remove the rectifiers (four diodes) and apply it straight to the smoothing capacitor but you MUST get the polarity right!

In my experience I wouldn't trust the spec or quality of the transformer in such kits - the rest of it is par-for-the-course but using a laptop PSU is far, far safer and gives you a much more reliable and wide ranging output.
 

BillMC

Dec 2, 2022
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Yes, when I read on its advert that 220V AC input gives a 12V AC output I assumed that it would be suitable for a UK mains input. Nevertheless, I was still dubious if it would be safe just to plug it into UK mains supply. Instead of junking the transforme and fitting a socket that accepts the DC supply from an old laptop power pack (commonly 19V DC output) as you suggested, could I just junk the transformer and plug it into this?
 

BillMC

Dec 2, 2022
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BTW Kellys-eye, I don't have a laptop power pack, but I do have an old power supply from a desktop - would that do instead?
 

Harald Kapp

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Nov 17, 2011
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could I just junk the transformer and plug it into this?
No. This one will output 110 V. Still lethal. A transformer will still be required.
I do have an old power supply from a desktop
That's perfect. Use the 12 V rail, remove the transformer and rectifier, connect the 12 V directly to the filter capacitor.
Note:
  1. most PC PSUs need the "power on" signal to be connected to GND to operate.
  2. Some PC PSUs need a minimum load for correct operation. Check this out for the one you have. A suitable resistor across from 12 V to GND is usually sufficient. When you use a lmap insted of a resistor, you get an ON/OFF indicator at no additional cost.
 

Bluejets

Oct 5, 2014
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Most pc power supplies are flat giving 12v dc.....then you must consider the output will be lower by the 317 requirements, so you might get 9v if you are lucky
 

kellys_eye

Jun 25, 2010
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BTW Kellys-eye, I don't have a laptop power pack, but I do have an old power supply from a desktop - would that do instead?
Not perfect but it will work. As above, expect 1.2 to 9V (ish) variable output from that input. Laptop PSU's are '10-a-penny' these days - lots of them are junked because the cable at the DC plug end often breaks internally, rendering it useless for the laptop but ideal for other purposes (since you can just cut the old DC output plug off anyway). Ask around - someone will have one! Sure you don't have an old DC output 'wall wart' or somesuch lying around? If you were UK-based I'd stick one in the post to you.
 

BillMC

Dec 2, 2022
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Problem solved, them. thanks, but I will look for an old laptop first.:)
 

Bluejets

Oct 5, 2014
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One thing to look out for there is, most laptop supplies are around the 19V DC mark and your 317, if used for 12V is going to have to dissipate that power as heat.

Better to have a supply that comes closer to your required output levels plus 317 drop requirements (usually a couple of volts)
 

kellys_eye

Jun 25, 2010
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is going to have to dissipate that power as heat.
Depends on the current consumption though..... even at 1A consumption that's only 7 watts and the heatsink shown with the kit should manage that for a short (long) enough time.
 
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