12 volt 6 amp power supply

Lightfoot

I constructed this circuit on a PCB for a benchtop power supply (also an electric motor supply for my bidirectional PWM fan driver). Do you think its a good design?



▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Well well, I'm seeing things, three of them.

-Stanley Blystone

LoopyByteloose

Generally, the 78XX series voltage regulators CANNOT be used in parallel to create more power.

Apparently they begin to destructively oscillate.· There are other designs that use a power transistor following the regulation that are quite good and actually less components.· Take a look as the suggested circuits on the 78XX series PDFs from the manufacturers.

With all good power supplies, you should start out with fusing and spike rejection from the AC line, then a step-down transformer, then good ripple rejection, and then your regulation.·


▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

Post Edited (Kramer) : 6/10/2006 8:37:08 AM GMT

PJAllen

No, Sorry --
78xx regulators in parallel don't/won't share the current as you expect; one will end up with (much) more than the yeoman's duty, a lot like "current hogging" with LEDs in parallel.

Jim Richey

The general idea is that 78xx regulators should not be used in parallel.
But with luck,it will sometimes work out,as I have found.One circuit I
have in extreme conditions does fine with 2 paralled.

Here's a statement that sums it up:

Unfortunately, you can't rely on this circuit to work every time. If the regulators are very well matched (same batch) it might work. If they have significantly different output voltages, one will volunteer more current than the other and you will find it shuts down well before the circuit gives the X2 maximum current. It is a perennial problem with regulated supplies, if you want to parallel them, you need some sort of active current sharing circuitry as well.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Thanks, Parallax!

LoopyByteloose

Some 78XXs have thermal shut-down and I suspect that is the source of Jim Richey's success. The distructive mode is suposedly a thermal run-away. It can't guarentee stabily though. I cannot imagine six being more stable, probably much less stable.

You could have six independent 1 amp outputs instead or even more. It only becomes a problem when you want to drive over the 1 amp. While they claim to make 3 amp 78XXs, it seems that they are near impossible to locate and it is just easier to use a 3055 that is controlled by the 78xx.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

Lightfoot

I found a circuit that looks even better from thr 78xx datasheet. I have all the components, however those resistors. What wattage should they be? That formula they give me for 6 amps I assume that they want me to use a 2 ohm resistor. Also, why do they show a wire going across the emmiter and collector leads of the transistors?

Thanks [noparse]:)[/noparse]



▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Well well, I'm seeing things, three of them.

-Stanley Blystone

Robert Kubichek

Three of Them said...
I found a circuit that looks even better from thr 78xx datasheet. I have all the components, however those resistors. What wattage should they be? That formula they give me for 6 amps I assume that they want me to use a 2 ohm resistor. Also, why do they show a wire going across the emmiter and collector leads of the transistors?

Thanks [noparse]:)[/noparse]

Here is a link to a 5 amp regulator LM3138 / LM338 that can do what you want.

cache.national.com/ds/LM/LM138.pdf

Towards the bottom are examples of circuits, that can handle higher amperage..

Bob N9LVU

Lightfoot

Here is my new design using an LM338K. It is a modified version (for a fixed 12 volts) of the variable regulator found on page 6 of the datasheet (www.national.com/pf/LM/LM138.html). This better?



▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Well well, I'm seeing things, three of them.

-Stanley Blystone

Robert Kubichek

Three of Them said...
Here is my new design using an LM338K. It is a modified version (for a fixed 12 volts) of the variable regulator found on page 6 of the datasheet (www.national.com/pf/LM/LM138.html). This better?

You've got diode 2 in the wrong place!!!!! Check the page again...
Here is the image as an attachment.

Bob N9LVU

Post Edited (Robert Kubichek) : 6/11/2006 1:45:42 AM GMT

Lightfoot

I already built the thing on a breadboard and it works, however I drew this schematic on the fly so thats why the error is there. Now assuming the schematic is right, you are free to answer my question.

Thanks [noparse]:)[/noparse]

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Well well, I'm seeing things, three of them.

-Stanley Blystone

Robert Kubichek

Three of Them said...
I already built the thing on a breadboard and it works, however I drew this schematic on the fly so thats why the error is there. Now assuming the schematic is right, you are free to answer my question.

Thanks [noparse]:)[/noparse]

Ok, you forgot one critical safety item " fuse the ac side of the circuit so if it overloads it kills the ac power....
And I don't know if I would combine the circuit dc ground to the case/ac ground, but in that, I could be wrong...

Bob N9LVU

Lightfoot

I had a fuse in there all along. If you want a revised schematic just say so.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Well well, I'm seeing things, three of them.

-Stanley Blystone

aalegado

Why build a bench power supply? ...a PSU from a PC is so much easier to use and offer incredible current capacity.

http://forums.parallax.com/forums/default.aspx?f=15&m=129331

...I realize that building your own is a Good Thing but I had an ATX PSU laying around taking up space so I put it to work. [noparse]:)[/noparse]

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
I wouldn't connect that if I were you...

Lightfoot

This is not only a bench power supply, its one for electric motors. I already made one with an LM338K. I performed a power resistor test (3 amp current draw) on it and it held.

Thank you all for your help [noparse]:)[/noparse]

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Well well, I'm seeing things, three of them.

-Stanley Blystone

John Bond

Question…
My experience with switch mode PSUs (which is what PCs use) is that·one has to draw·some minimum current on one circuit for the others to work. Is this the case with PC power supplies? Which circuit needs the current?
Please, any PC or switch mode gurus, your comments
Also, I would place independent quick blow fuses on each voltage, I have·shorted a PC supply into a spectacular display of·pyrotechnics. Maybe they're better these days but it's not an experience for us timid souls.

PJAllen

Yes, John Bond, you are correct.· It's the 5V output that governs the operation of the whole, minimum load is usually at least 10% of rated output.· Some special units have active current-sensing and switch a "pre-load" in/out as necessary.

John Bond

Thanks PJ Allen

I've avoided using PC supplies for general use since that incident. In fact, I've avoided switch mode units generally. It would be useful to have +5V -5V +12V -12V and +3.2V as a single unit right on my bench. I'll put a dummy load, say 0.2 Ohm 10 watt across the 5 Volt.

Obviously, my ancient variable supply and the half dozen LM317 and 7805, 7809 PSUs will give much smoother power but my bench is getting very crowded.

John