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current limiting 3

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kenre

kenre

Mechanical
May 23, 2005
300
I once had a circuit diagram for adjustable current limiting using a ua 741 and 2n3055 pass transistors.
No longer to be found. Anyone have a similar one? its to limit up to 30 amp.


Was told it couldnt be done, so
i went and did it!
 
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I'm okay with keeping it simple, but you stated that an SCR would produce less heat than a FET, and I was asking how you came to that conclusion.

Dan - Owner
Footwell%20Animation%20Tiny.gif
 
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Would this be the type of thing you are looking for?


You have to adjust the components to get a higher current. Stay within the limits of the transistor. In the end, you may end up with a 4A or 5A current limit per transistor or some such thing. Then, just parallel the circuits up to get your total current limit.

If you want to go further then build an adjustable regulated supply for the base that is capable of supplying the base current for multiple transistors and use it to drive all the transistors in parallel.

If you haven't got it figured out yet, the current limit is the base voltage minus 0.7V divided by the emitter resistor.

I think Operahouse is referring to using a phase controlled rectifier via SCR's. If so, the phase control part is not simple and it's hard to filter the output for a constant DC. However, if a lot of ripple voltage is OK in this application then it could be done.
 
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Operahouse's circuit is great, but i need to have linear adjustment, not just a few steps. my old, now lost circuit used a pot to do this.

With the voltage regulator, using nichrome wire to load it, at 18 amps, and 30 volts yes the 4 of TO3's and heatsink were too hot to touch, even with fan cooling.
Im all open to ideas, one mate mentioned using a 555 and mosfet as a switching supply. Im not sure how smooth the DC output would be, i need very little to no ripple.

Im far from an EE, but enjoy building things myself as its the best way to learn more about it.





Was told it couldnt be done, so
i went and did it!
 
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You can expand it up, but it will still produce the same heat. A 2:1 pot could be added wasting more heat in the resistors.

Using about the same parts it could be converted to PWM. See Nationals AN181 fig 9. Increase C3 to operate with a suitable inductor, R4=0, put an opamp at the bottom of R5 with a cap to ground, and current sense in the common return leg. Problem is getting an inductor to handle 30A. I found some big inductors in some sodium lamps that could be paralleled. You're between standard look up designs.
 
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If you are serious about doing plating... You need to pitch this "make it yourself" thought. Get a robust reliable powersupply that has already dealt with the packaging, heat management, safety, indication, distribution, and adjustability, and focus on the plating,(the ultimate point!). The path you are going down is like a guy learning to fly a plane so he can pick up some parts without paying UPS.

To wit here is what I see on the first two eBay pages I looked at.

There is no way you can come close to competing with the logic of buying one of these verse making one. You can't come close.







Keith Cress
Flamin Systems, Inc.-
 
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JimKirk - "...limiter will dissipate 1,200 watts..."

I understand why I've never seen an external current limiter at this sort of power level. The advantage of an internal one is that it can turn down the voltage. Thus it would be far easier to add the current limiting circuit inside the existing power supply than to build an external limiter.

Of course, it would be even easier to buy a cheap PS with the feature already built in.
 
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Yep, that was sort of my point. To throw around that kind of power, a switch mode supply, or something with an SCR pre-regulator is a huge advantage. In any event, none of the possibilities are for the faint hearted or inexperienced. I built a current limiter that handles up to 20 amps or 50 volts, using 10 parallel FETs heat sinked for about 100 watts continuous dissipation. It's powered by a 9 volt battery. Scaling that up for a dissipation 12 time as much is certainly doable, but it'll heat up a room pretty fast.

I agree with Keith, buy something designed for the purpose.
 
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To buy would be a nobrainer, but being in Australia its very slim pickings! USA voltages and Frequency, by the time i added a stepdown transformer, and assuming the supply will handle 50Hz instead of 60Hz.

Is there anything wrong with homebrewing ?
I do realize there is going to be a heat issue, we are used to heat down under hehe.

hmmmm, what do to.






Was told it couldnt be done, so
i went and did it!
 
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If you really want it adjustable a 1.5V battery can be added to subtract out the 1.2V reference. Connect a 2.2K resistor to the battery - and the other to a 10K pot. The wiper now connects to the LM317 common pin. Te other end of the pot connects to the battery + and to where the common pin of the LM317 used to connect to, the output of the regulator. This will be adjustable to the upper limit of the current sense resistor selected. A relay can be used to turn on the NEGATIVE lead of the battery. A dead battery results in maximum current. Battery life should be quite ling.
 
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Adding a current limiter (foldback of the voltage) into an existing power supply would only require a handful of components. And the only added component that would experience the full current would be the low value series resistor (which can be made with a suitable chunk of wire). Everything else would be low power op-amps and transistors and similar. If you can match your existing power supply architecture against one with a current limit, then the design would be almost cut-and-paste.

 
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VE1BLL,
I thought all current limiters work on the principle of reducing voltage. But i dont know much. I would like to see this circuit.

Operahouse, would this modification still have the 3055's making lots of heat?


Was told it couldnt be done, so
i went and did it!
 
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Which modification? Converting the LM317 to PWM operation will greatly reduce the heat. Any analog solution will produce the same heat. The number of transistors will have to increase to 7 (one in darlington config)regardless and there will be design issues you will likely be unable to work out with PWM. So many questions from someone who won't give a star.
 
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"I thought all current limiters work on the principle of reducing voltage."

An EXTERNAL current limiter would essentially DROP the voltage to limit the current and thus generate huge quatities of heat.

INTERNAL current foldback circuits just "reach over" and tweak the voltage down using the existing voltage regulator circuit in a manner that is virtually no different than adjusting the voltage.

That's why external current limiters are a non-starter for high power systems. But internal current limiters are very common on power supplies.
 
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I imagine your current power supply for this is just a transformer, rectifier and capacitor. Still, you are asking to build a 1KW supply with minimal parts. A 30A tank doesn't seem like a hobby application. Can you break it up in multiple smaller tanks?
 
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Another idea...

If your existing power supply has remote sensing terminals, then you could fiddle with that signal to foldback the output voltage without modifying the internals of the power supply.

In other words, you sense the current in the usual manner with a low R resistor, run that signal through an op-amp or two, and then use it to control the power supply via the remote sensing terminals.

Of course, you'd have to consider all the circumstances, and maybe mix in some consideration of the output voltage as well.

 
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I think VE1BLL has come up with something that sounds like it will work a treat, Internal . Maybe my original circuit worked this way, with the UA 731. What are remote sensing terminals? I can modify the internals as much as i need to as i built it from scratch.

25 amps minimum needed to hard chrome plate @ 3Amps/Sq in.

Ok a star for you Operahouse.
Yes it is transormer, rectifier and capacitor, with the voltage ajustment atm.

I dont need the voltage adjustment.


Was told it couldnt be done, so
i went and did it!
 
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You are not understanding the basics. In any analog regulator circuit heat will be produced. It is the difference between the input voltage and the electrode voltage. That difference times the output current will be the amount of heat produced. Having a large surface area to get rid of that heat and distributing that heat in multiple transistors will make it reliable.

You have a perfectly good circuit now that with additional transistors could handle the current. Your application appears to require a large differential voltage that will waste a lot of heat. That will turn any circuit you likely will find to toast in a short period. There is nothing out there that will magically solve your heat problem except using a Pulse Width Modulator.

I am all for experimentation, but there is a big difference in building a PWM circuit that supplies 3A and one that will provide 390A. A lot of people have built little PWM regulators and think they work great. On closer look they have problems because of layout and component selection. I don't believe you have the equipment and knowledge to develop PWM ans support quickly wains here as things get really technical.
 
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I misunderstood the starting point. I didn't realize that the existing power supply was completely home brew (including the regulator circuit design). I was making an assumption that the existing circuit already included some key features, and that those features were of mature design (capable of dealing with the worst-case heat for example).

At these power levels an analog solution is impractical, and a switching solution is non-trivial and fraught with hidden difficulties.

So the simplest solution would be e-bay; even in Australian there should be something available. Or look to the local Amateur 'Ham' Radio clubs for cheap surplus power supplies. At commercial PS meeting your specs might be US$100-$250 on the surplus Ham radio market.

 
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