Adjustable voltage and current supply help

Discussion in 'General Electronics Chat' started by SteveDouglas, Apr 30, 2009.

1. SteveDouglas Thread Starter Active Member

Apr 30, 2009
42
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Hi All,

My first post here. I have built many circuits and I also program PIC's. I've etched, ironed on, and milled PCB's for my projects, ETC,ETC.

And I've built many power supplies using LM78xx and LM317's including a three output unit on my bench that has separate outputs for 5vdc,12vdc and adjustable 1.2 to 20 vdc. All of these are about 1.5 amps max with heatsinks and the adjustable one has digital LED displays for volt and current.

I say all this so you guys will know I'm not a complete n00b but I have never built a supply with adjustable current output so I'll explain my application below.

I need to build a supply for an electroplating setup with the following requirements...

Voltage adjustable from ~1.2 to ~10 VDC
Current adjustable from ~0 to ~6 Amps

This will never be used for any electronics applications. I am planning to get a high current car battery charger and use the transformer and other parts of it for this PS.

I expect to heatsink and use at least one fan as well and I'll have displays for V and A of course.

So my question (finally) is, can you please give me some advice on what IC I should use for the adjustable current and/or a schematic of one ?

I apologize for being long winded on all this, if I've left any info out, let me know and thanks for any help,

Steve

2. eblc1388 Senior Member

Nov 28, 2008
1,542
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You can not adjust the output current because it is determine solely by the load. This is related via Ohm's law: current = V/R.

Once the load impedance and the output voltage is fixed, the current is known. Whether the power supply can provide such current is another matter.

However, you can limit the output current of the power supply by dropping the output voltage, as is universally done by all power supplies that have a current limiter.

The current is measured by voltage drop developed across a low resistance resistor, with or without further amplification and if found to exceed set limit, a signal is fed to the voltage regulator to reduce output voltage.

3. SteveDouglas Thread Starter Active Member

Apr 30, 2009
42
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What about the PS pictured below ? How does it vary the current ?

Electroplating can require, say, 3 vdc at .5 amps for a small batch of parts being plated or say, 2.5 vdc at 5 amps for a larger quantity of parts being plated. The amperage needs to be varied depending on the surface area of metal parts in the plating tanks.

Steve

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4. Darren Holdstock Active Member

Feb 10, 2009
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eblc1388 is right. It sounds like the electroplating process you describe uses a constant voltage, while the current may vary a bit. The power supply you show is designed for a controlled voltage output, but with an adjustable current limit too. The power supply will reduce the voltage output to meet the current limit demands if necessary - in fact if the power supply is riding on the current limit, then it effectively becomes a constant current source. Not a very good one always it must be said, as the current limits are usually designed as a safety feature, not as a normal mode of operation.

5. Wendy Moderator

Mar 24, 2008
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There are transistor versions that may do what you want. From what you've said I'm assuming you're familiar with Ohm's Law as it applies here. You can set voltage, or you can set current, but one or the other overrides.

What you are describing is a current source, rather than a current sink. I've designed transistor version several times, they aren't overly complicated.

You've given us the basic specs. Are you wanting to set these values with a potentiometer, digitally, or with a control voltage? Many cases it works out to be the same thing.

This is an example of a quick and dirty power supply I whipped out a while back for another post. It is full of problems, but the section you are interested in is Q1, the current limiter section. It is an adjustable current limiter followed by an adjustable voltage regulator (all very low power, but the principle remains).

6. SteveDouglas Thread Starter Active Member

Apr 30, 2009
42
2
Thanks guys,

Bill, I do want to control it with pots and thanks for the schematic, I'll take a closer look at it. I'm looking to just build a very basic and simple as possible supply. But current and voltage each do need to be regulated.

Darren, I really need it to keep close to a set voltage and a set current. The voltage and current must be varied depending on what plating operation is being used at the time. So, copper plating may require 5v and 2 amps while the chrome plating bath may take 3v and 5 amps, ETC.

Please pass along any other ideas or suggestions on this and thanks again,

Steve

7. Wendy Moderator

Mar 24, 2008
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The root problem is the inevitable interaction between the two sections. It is possible to design them where they don't interact nearly so much, this schematic had a basic problem with the current from the voltage regulator leaking through, specifically the base current to Q2.

Finally found my drawings in my albums for the current source by itself.

8. leftyretro Active Member

Nov 25, 2008
394
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I think in electroplating it's the current that is the key variable to be controlled. As other said, one way or another the voltage would have to be manipulated to achieve the desired 'setpoint' current. Controlled current mode is what I think you are after, which is not the same as current limiting control.

Lefty

9. Wendy Moderator

Mar 24, 2008
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What's the difference? Serious question, I'm trying to get the nominclatures down. Controlled current mode mentally translates as constant current, and it is what the current control on the HP power supplies I used to work on did. So what would be current limiting control? Current foldback?

10. SteveDouglas Thread Starter Active Member

Apr 30, 2009
42
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I appreciate you taking time to find that schematic Bill. I'm not sure how to implement it, would I just run the output of a LM317 into it ?

I'm still learning and have yet to actually plate anything. Here's some info on plating current requirements from another site:

"Nickel & Copper plating require 1 amp per 16 square inches of your part(s)"

"Chrome plating requires 1 amp per square inch area of your part(s)"

This may actually be more involved than I first thought, I was thinking I could basically use something like a 317 setup and add TO3's for high current handling to vary the current. Evidently it's much more complicated than that or just building a variable voltage supply. Working with electronic circuits all these years, high current or controlled or limiting it was never important and now it is in this application.

That said, I still think there must be some way to use an inexpensive car battery charger to modify for plating use. The "real" plating supplies cost hundreds of dollars and I just don't think that they can have that much "extra" stuff in them, so to speak.

11. Wendy Moderator

Mar 24, 2008
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The base schematic is it, feed it DC voltage, out pops current whatever you set it at. Being fundamentally analog it will require serious heatsinking. I don't see anything wrong with using a battery charger, or whatever else you get your hands on. The schematic I showed was just a current regulator (ie, constant current source).

The parts are basic, so you can experiment with it. If you're interested we'll go through the design exercise and I'll explain it in detail, it is a basic transistor configuration.

Being so basic, the current is basically up to whoever designs it.

12. leftyretro Active Member

Nov 25, 2008
394
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Controlled current is an adjustable 'constant current source' that will raise or lower the output voltage as required for load resistance changes.

Current limiting control is a mode where the voltage will be lowered to the load only if and when the current reaches it's adjusted setpoint value. If the load resistance increases then the current limit changes nothing.

These are two very different modes of operation that require different circuitry in the power supply.

Lefty

13. Wendy Moderator

Mar 24, 2008
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OK, thanks. I learned them as Constant Current and Current Foldback. The LM723 (a goodie but oldie chippie) has current foldback as part of it's design. For the OP, this was used by the power supply in the event of a dead short, where the power supply shut down, but sent enough current through the load to sense when the short was removed.

14. Pich Active Member

Mar 11, 2008
119
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Steve take a look at this constant current power supply. Depending on the ATX power supply you use it can regulate up to 20 amps. I use it for electroplating through holes on PCB's, work well.

15. Wendy Moderator

Mar 24, 2008
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Kinda forgot a link there, hey?

16. Pich Active Member

Mar 11, 2008
119
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The image was too large for upload, I uploaded the file as an attachment. I will try again

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17. SteveDouglas Thread Starter Active Member

Apr 30, 2009
42
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Thanks guys, I started a schematic in Eagle based on Bill's circuit and a 317 circuit I've used and will post it when I'm done and see what you think about it.

Pich, that looks good, thanks for posting that. I didn't see where the voltage can be controlled too on that. How would that be added to that circuit ?

Steve

18. Wendy Moderator

Mar 24, 2008
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Before you start designing a PCB I'd strongly suggest designing a circuit and posting it here.

19. SteveDouglas Thread Starter Active Member

Apr 30, 2009
42
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Hi Bill,

That's exactly what I said I would do in my previous post. I'm definitely not going to do a PCB before the circuit is figured out. I'm only doing the schematic in Eagle at this time.

Thanks

20. Wendy Moderator

Mar 24, 2008
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Ah. What is root power supply going to be?