Variable regulating power supply circuit

[reckerd]

Hello, I need to build an adjustable regulating power supply circuit. I currently have a Power One 28VDC 3.6A power supply. I need to build a regulating circuit for the output that maxes at 24VDC and has a minimum output of 6VDC. I figure the Power One will regulate the current so I am not concerned with that except for making sure my regulating circuit can handle its max. Any ideas or schematics will be appreciated.
Thank you in advance.
RE

[Robert Reed]

Try this site.

www.national.com/pf/LM/LM317.html - 81k

When you get there, download the data sheet. It has a schematic for high current out put.

[reckerd]

Wow! its uses are almost limitless. I think this circuit will work. "5A Constant Voltage/Constant Current Regulator"
Do you think this will need special protection since I am using it in a capacitor welding circuit. Right now I am using a seperate power supply to charge the capacitors and an SCR to discharge through. I am concerned the rapid discharge of the capacitor will damage the charging circuit. If I use a diode how big of a diode do I use?[/img]

[Robert Reed]

You might want a low value,high wattage series resitor in the charging circuit to ease the burden on the supply - it depends on the charge/discharge rate and the value of the capacitor - not enough information.

[MrAl]

Hi there,


I'd like to add some additional information...


The LM317 is a pretty nice device, it's current rating isnt really up to
this job (3.6 amps) though without using an extra external pass device
(transistor) and associated parts. A better device might be the LM1085,
which handles up to 3.0 amps without any additional parts (except
the adjustment resistors of course) but if 3.0 isnt enough then there is
the LM338, which handles up to 5.0 amps (maybe even up to 7.0 amps)
and that should do the job. These other two IC's are the same as
the LM317 pretty much except they handle more current by themselves.

There is a secondary problem however, and that is the one about
power dissipation. With the output adjusted to 6vdc and the input
at 28vdc and the current at the max of 3.6 amps (if that combo is
possible of course with the load being used) that leaves us with
almost 80 watts of power to dissipate in the heat sink. This would
be true no matter what type of linear regulator you decide to use
such as one of those above). 80 watts is a *lot* of power, not only
for the heatsink but it's also a big waste of energy in today's energy
minded world. It also takes a *very* big heatsink to dissipate all
that energy so the IC itself does not overheat.

A better choice would be to use a switching regulator. These alternate
regulators have been around for years now and are very reliable too.
What's the big difference? The linear would be using up 80 watts of
power and getting pretty hot, while the switcher would use maybe
2 watts or maybe even less and not get hot at all. This is a big big
difference.
Because switchers have become so desirable over the years several
companies have come out with IC's that work pretty simple, and even
one company even calls them that, "Simple Switchers". That's National
Semiconductor, which makes a huge number of IC's for various apps.

There are IC's that work almost the same as these linears (except
they need an inductor too) and they come in various sizes to match
your application. Since this app requires 3.6 amps a 5 amp device
would do nicely. One device that can do the job is the LM2677.

The difference in heat sink size would be drastic too, from a 4x4x4 inch
heat sink for the linear circuit to a small 2x1x0.25 inch for the switcher.
For a real life tried and proven example, i regularly use switchers with
NO heatsink at all for 1 or 2 amps output. The T0220 package seems
to work ok by itself sometimes, depending on the power level.


This is only one of the many IC's out there for switching regulators:

http://www.national.com/pf/LM/LM2677.html


One thing you will note is that the design process for these regulators
is a little more complex than for a linear because you need to select
an inductor and some large capacitors, but with the app notes today
this isnt too bad either and believe me, it's worth the extra work!

[reckerd]

Thank you for the responses. Since I don’t know how to draw out a schematic that can be uploaded to this listing I am going to attempt to spell it out.

I have two 2farad car audio capacitors connected in parallel. I connected a large stud type SCR from one side of the cap to a copper pencil electrode and the other side of the cap to another copper pencil electrode.

I charge the caps using a Tenma laboratory DC power supply. While holding the electrodes on a battery tab to the battery I discharge the caps through the SCR. This welds the strips to the battery terminal.

What I want to do is make a more self contained device that can be adjustable for the varying thicknesses of the battery tabs.

The capacitor show a surge of 24vdc in rating and that is why I said the charging circuit needs a max at 24vdc instead of 28vdc which is what the power supply I have has an output for. The 6vdc is an arbitrary number and could be higher or lower.

Please let me know if you would like more info. I thank you for your help.
RE

[MrAl]

Hi again,

Oh ok, so you are saying that you only use it for one or two connections
and then you dont use it anymore that day, or do you use it for one or
two connections (welds) and then use it again right away?

I ask this because the way you use this affects the total average
power dissipation and if it's only used intermittently then that relaxes
the circuit requirements (like the size of the heat sink) considerably.
For example, if you only do two connections every 10 minutes then
a much smaller heat sink can be used, and not as much total power
wasted either even with the linear circuits.

[reckerd]

Its frequency of use will vary. Sometimes we will be welding 8 AA batteries together which equates to around 16 discharges.
This is of course provided we got a good tack. If not then that number will increase. After that the unit may sit around for a couple of day before we need to use it again.
At that time it may be two batteries being welded or up to eight.

[Robert Reed]

As MrAl points out, it all comes down to duty cycle - i.e.- one cap discharge for a fraction of a second and then a couple of seconds to orient for the next 'spot' weld would be a very low duty cycle and the power supply is only doing work (and dissapating heat during this charging period. If this is the case, a series charging resistor would be desirable, possibly 2 to 4 ohms. However this may take 10 seconds or longer to fully charge a completely empty 2 farad condenser. Is that wait time permissable between 'spots'?
Also , and again if this scenario is correct, you could probably weld all day long with no ill effects.

[MrAl]

Hi again,

Yes, and i'd just like to add that to find out if the linear circuit was going
to work ok in the long run, the user could try doing a few welds (10
maybe) and feel the heat sink to see if the device is getting very hot.
If not, it's good to go. Since the duty cycle is low i would think you
could get by like this with a relatively small heat sink (2x2 inches with
a little thickness like 1/2 inch or so).

The only thing i have to add now is that sometimes circuit use changes
with time, as you start to realize the value of your circuit and you may
tend to use it more and more, which of course raises the duty cycle.
This would mean changing to a larger heat sink or a switching regulator
in the future. Just something to keep in mind.

[reckerd]

Thank you so much for the great responses. I am looking foward to building it.