small variable power supply?

[PaulM]

Any good sources for an inexpensive small power supply, say 0 to 12 volts up to 5 amps? I can run it off AC or battery if required.

Is it possible to build something that will work reasonably well? I have a lot of salvaged electronic components; is there any recommended schematics on how to put something together?

Thanks!

 

[Bobocop]

How small ?
If you can get your hands on an old computer (even a 486 or pentium 1, people throw them out all the time), you can use the power supply off of it.

They usually have dc outputs of 3.3, 5 and 12 volts to power fans, A drives, hard drives and cdroms, alot of them put out 20, 28, 30 etc. amps

 

[rkosiorek]

if you need 5A there are quite a few circuits based on the LM338 adjustable regulator chip. the dtasheet from National even includes one.

for up to 2A there is the L200 ic chip. and coming in at 1.5A there is the LM317. both of the L200 and LM317 can have higher outputs if a large pass transistor is added.

google will produce numerous schematics using any of these 3 chips.

rick

 

[PaulM]

I do like the computer power supplies, but is there any way to vary the voltages?

Thanks for the info Rick, I'll have to look into those a bit more.

 

[rkosiorek]

I do like the computer power supplies, but is there any way to vary the voltages?

Thanks for the info Rick, I'll have to look into those a bit more.

well sort of, all of the regulators require a power source. traditionaly a transformer/bridge rectifier/ filter cap combo would be used. now if you replace the traditional with a computer power supply, you would be using one of the regulator circuits to do the variable voltage thing.

your max variable voltage would be limited to 2 or 3V lower than the voltage you are feeding into it though.

i am sort of partial to the National Semiconductor designs. i have used a couple of LM338's in parallel on the output of a 36V switching power supply to make a 0-30V 10A adjustable supply. but unless you are using surplus or yardsale stuff, it may be cheaper to consider abuying a lab supply. Pyramid makes a few that deliver 0-30V at 5A that can be bought new for $150USD or less including delivery. check your local ebay listings.

for example, a couple of years ago i bought a circuit board for a commercial ower supply at a liquidators auction for $25.00 cheap enough for a 0- +/- 30V 10A regulator. at least i thought so until i finished getting a 60VCT 10A transformer, pots, knobs, switches, meters, chassis, fan, etc. easily came to the price of buying a good used supply. then add to that the time it took to assemble and i would have been much farther ahead just buying the HP 60V 10A bench supply that was next up at that same auction.

rick

 

[PaulM]

well sort of, all of the regulators require a power source. traditionaly a transformer/bridge rectifier/ filter cap combo would be used. now if you replace the traditional with a computer power supply, you would be using one of the regulator circuits to do the variable voltage thing.

That makes sense. So there is no simple way to vary the computer supply itself then.

I had a look through some of the stuff I have last night, no 338 but I did find an LM350 which appears to be a 3 amp version of the 338. I also found a 317. You mentioned boosting outputs if some transistors are used, I have several large audio transistors out of old stereos, would they be any use for this?

 

[rkosiorek]

oops, i forgot about the LM350. the difference in cost is so minimal compared to the LM338 that i would usually spend the extra cents to get the extra headroom.

the internal design of all 3 of these (LM317, LM338, LM350) is almost identical. big difference being in how much power they can dissipate and the max current, most times if an LM317 based circuit design you want to use won't deliver the current, just substitute in one if its larger brothers.

likely that you could use one of the other transistors. but without knowing the design you want to use or the particular transistor it would be hard to say.

rick

 

[PaulM]

Thanks for the help Rick, I set up a real quick and dirty circuit last night with the 350, and it worked great! I want to use this to vary the field voltage I am applying to my alternator-motor in the YZE thread.

 

[rkosiorek]

so what kind of alternator are you using? just curious really.

i once did a bench experiment using a delco 65A alternator i found in the trash. i powered it up using a Crystalyte pedal first 20A controller and a bench supply for the field coil. although they were no load tests i found i could get the best control of the field coil by setting the supply to a constant voltage of 5 or 6V and varying the current. didn't really spend enough time with this though other than to see that it could work.

rick

 

[PaulM]

It's a 120 amp one out of a tractor, also picked out of the scrap!

How do you hold the supply voltage constant, but vary the current, for what is essentially a constant resistance load?

 

[rkosiorek]

the effective resistance of the field coil is more than just the DC resistance of the winding. it is heavily influencd by the back EMF and that is RPM dependant.

at higher rpm you actually need to reduce the field current for the rpm to increase. at lower rpm you need higher current.

rick

 

[monster]

connect the green and black wires on a modern computer power supply and it will switch on

 

[PaulM]

Thanks for the help Rick, I set up a real quick and dirty circuit last night with the 350, and it worked great! I want to use this to vary the field voltage I am applying to my alternator-motor in the YZE thread.

Here's a couple pics of the "polished" product:View attachment 1
Note the sophisticated "overcurrent protection" :lol:

 

[rkosiorek]

KEWL!!!

nice looking project box. what is that from? was it built to hold the heatsink like that or were the standoffs adapted from something else.

rick

 

[PaulM]

KEWL!!!

nice looking project box. what is that from? was it built to hold the heatsink like that or were the standoffs adapted from something else.

rick

Thanks!

The box was originally a junction box in a wind turbine; it was being tossed so I snagged it. The heatsink with the standoffs came from an old battery charger that I salvaged from the scrap at work. Other components are all salvaged as well, the only "new" part of the whole thing is the hot melt glue.

 

[PaulM]

This little supply has been working great, but I've added a $5 multimeter to so I can easily monitor output current or voltage.