current limiting 3

[kenre]

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.

http://www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

 

[VEBill]

Is this to be part of a power supply where the 2N3055 transistors are already part of the voltage regulating circuit, or an external stand-alone series current limiter?

If the former, then the ARRL Handbooks normally have more than one low voltage, high current power supply circuit that includes current limiting using the pass transistors with one more op-amp and a couple of transistors to control the current limit by means of the pass transitors.

Personally, I don't recall ever seeing a stand-alone current limiter.

 

[itsmoked]

Doesn't use either but how about figure 8?

http://www.national.com/an/AN/AN-181.pdf#page=3

Generally with an opamp and a 3055 in the same sentence you mean 'current booster'. Are you sure that's not what you mean?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[OperaHouse]

Do you want current limit or current foldback?

 

[kenre]

The exact terminology in not sure on. I have a 40V powersupply being used for anodising. amperage must be held to a constant value. I have a voltage regulator atm, using a LM317 and 4 of mj2955 transistors. unfortunatly i have to constantly monitor the amps being drawn as resistance in the bath changes.

I dont need to regulate the voltage as that will be set by the resistance, so a seperate circuit to replace the voltage reg will be needed.

http://www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

 

[OperaHouse]

This should be listed in the constant current notes for the LM315. Basically the four PNP transistors are in parallel. The input pin of the regulator is fed power through a resistor that also supplies the base current for the transistors. The out pin and collectors are connected to a current sense resistor sized to give you regulator voltage at the desired current. The regulator common pin is connected to the other side of the resistor and id also the power supplies output. Transistors should have a small load sharing resistor in each emitter. Actual design values based on desired current.

 

[VEBill]

Maybe you could swap power supplies with 'JohnBreen'.

http://www.eng-tips.com/viewthread.cfm?qid=210823

He has some huge 7.5V 300A power supplies unsuited for his 13.8v purposes, and it appears that your power supply is missing a key feature (adjustable current limit) for your purposes.

 

[kenre]

300 Amps!! sounds like a large welder!! My electroplating and anodzising setup doesnt warant such capacity.
Im in Australia so not really suitable, even tho i do have 3phase (415v)
I built my supply from scratch, rewound microwave transformer.
I had a good look at the 317 data sheets, and might go this way! Thanks OperaHouse. Myself being very low on the ladder of electronic design could you point me to a circuit diagram?

http://www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

 

[OperaHouse]

Well here it is, only because you got me feeling sorry because you have to eat kangaroo jerky and probably run this on solar panels. I have a friend who tried to import that Billybong stuff and ran into all sorts of problems with alegator regulations. You can keep switching in resistors in a binary fashion to get any current. Use at least 5W wirewound resistors.

 

[itsmoked]

I gave you one. Figure 8.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[kenre]

Thankyou OperaHouse!

Kangaroo is acutally rather nice, much better than possum soup.

I take it i cant just change the resistors to a Pot for variable adjustment?

itsmoked, thankyou for your contribution, but i wouldnt know where to wire in any extra pass transistors.

http://www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

 

[OperaHouse]

Where are you going to find a pot that will take 15A and is under an ohm! Current is defined as 1.2/R. The values I gave you result in about half the current listed. More than 20 years since I used a LM317, thought it was about 1.7V and not 1.2V. I could add a pot but there are many downsides to it. You can add additional switches, but don't go below .5 ohm on each switch.

 

[richs]

Hiya-

If this is a "one off" type and you don't feel comfortable with doing a large amount of engineering, you can have several automotive type lamps/headlamps wired in parallel with switches to do the current regulation and act as loads for you.

If you search the web for anodizing, you will run across one of these circuits. Just a bunch of lamps and the current is adjusted (with the aid of a meter) by turning off and on various lamps. Since you are running around 40V, you might want to string a couple of lamps in series for each load as you don't want to accidentially burn out a 12V lamp with 40V across it.......

If you want some more "robust" types of loads, one might want to salvage the reistance wires from old toasters, electric wall heaters, etc. Note that you will want to consider compression type connections for the wire used. Again, these can be switched in or out via parallel circuits.

Also, out on the web, there is a rough calculation for the current/surface area of the part being anodized. I don't have immediate access to the info. It had something to do with anodizing telescope parts. Was a pretty good article.

I have a back burner project doing anodizing but will be using a car battery and a large wire wound variable resistor for my efforts. Since mine is just about a one-shot affair, it will be crude, but hopefully effective doing small parts.

Doggone it. I couldn't find the link on google directly.

Maybe a wander over to:

http://groups.yahoo.com/group/anodizing101/

Might find some additional input.

Best of luck on your efforts.

Cheers,

Rich S.

 

[kenre]

This supply wil be used for Anodising, type 2 and 3, and also hard chrome plating, so it will need to be robust and reliable! If only i could locate the circuit i once had.

http://www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

 

[OperaHouse]

You said you wanted up to 30A. To be more reliable, I would use more than 4 transistors. What is your open circuit voltage and normal electrode voltage. At 10V across the transistors that would be pushing 80 watts on each transistor. 300W is a lot to dissipate. The LM317 has a lot of protective features. This circuit is simple and robust, you are just pushing it if you go over 15A.

 

[jimkirk]

You don't say what the resistance of your load is, but keep in mind, with a 40 volt supply and a 30 amp limit, there's a chance that your limiter will dissipate 1,200 watts, mostly in the power transistors. In that case, with an absolute maximum dissipation of 115 watts for the 2N2955 (in a TO-3 case), you'll need 10 or more devices with lots of heat sinking (hopefully with a fan) to scale up OperaHouse's circuit.

If there's no chance of the full voltage appearing across the current limiter, you can reduce the number of transistors, but you need to consider the base drive current.

The DC current gain of 2N2955 transistors can be a minimum of 20 @ 4 amps and 5 @ 10 amps. That means that with 8 parallel transistors each handling about 4 amps, the LM317 will need to supply a total of 1.5 amps to the bases. The TO-3 package LM317 has that as a minimum spec, but at a maximum voltage across the device of 15 volts.

With only 4 PNPs, each will have up to 7.5 amps of collector current, the current gain could be as low as 10, so you need 3 amps to supply the bases. So with 8 pass transistors, the design is marginal for 30 amps, with 4 transistors you're depending on luck that you have 2N2955s in the top end of the hFE range, and the power dissipation could be a killer. Consider an extra transistor to drive the parallel bases in a Darlington configuration.

You might also think about using FETs as the pass devices. They parallel fairly easily, and require minuscule gate drive.

 

[MacGyverS2000]

I'm also going to suggest FETs instead of BJTs (or at least IGBTs)... paralleling BJTs is a good recipe for thermal runaway.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[OperaHouse]

All that stuff still wastes heat. You could do better with just an old SCR. If you are going to have a FET at least include a switching regulator. I gave him just what he asked for, a LM317 and four PNP MJ2955 transistors. Obviously this worked in the past for him because he knew the components.

 

[MacGyverS2000]

And how, exactly, is an SCR going to waste less heat than a FET?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[OperaHouse]

The discussion turned to "using FETs as the pass devices." I have an old adjustable SCR power supply that puts out 30A over a wide voltage range that produces little heat. Old technology, but it worked. Using a FET in a passive role only complicates everything for this guy who is having problems with a couple resistors.