Aluminium anodising power supply revisited

[ceefna]

Hi all, in 2009 I got some invaluable help from this forum into building a power supply for anodising aluminium.
It has served me well but last week I had a flat battery on my car and used the power supply to start the car lots of smells of burning voltage regulators! I am going to change the design so I can anodise some larger parts.

Could you please have a look at a schematic I have found as a replacement.Do you think this would work? I need a max of 10A. Is the Led in the circuit just to show power is on?
I would like to add an led to show current limiting is on, how do I do that?
Also do the 2N3055's need to be insulated from the heat sink and does the BD243C need to go on the heat sink?
And lastly the LT10A04, can this be replaced for anything else as I can't seem to find them.

Thanks for your time again

Ceefna

 

[duffy]

This is one of those weird applications where you do not need to insulate the TO-3 from the heatsink because it is common collector. You should insulate the heatsink itself, though, because it will be charged to +40V. It's a good idea to heatsink the pre-driver because those old 3055's got lousy gain, so the BD243C is cranking out a lot of current. That 10 amp diode is not critical, any normal recovery rectifier rated for at least 10A and at least 50V will sub for it.

 

[ceefna]

Hi, thanks for the reply, will this diode be ok?
**broken link removed**

Also about the led on the schematic, what is it's purpose and how would I connect an Led to show current limiting?

Ceefna

 

[duffy]

That diode will work fine. The LED is just a power indicator.

Showing current limiting is a little tricky, "Current Sense" and "Current Limit" just lead to an internal transistor on the LM723. Putting an LED in series with either of these will throw off the current regulation because of the voltage drop. The voltage developed across the .1Ω emitter resistors on the 2N3055s isn't high enough to operate an LED even a transistor directly. The current out of the BD243C into the bases is too high to put one in series there. Try adding a red LED in series with a 10Ω resistor between pin 10 of the LM723 and Vout+. The two base-emitter drops will be at least 1.4V (higher in practice) and when you add in the .3V drop on the emitter resistors it looks like it could give you just enough to light one red LED when it is putting out 9-10A.

 

[ceefna]

Thanks for the info Duffy, I have built the circuit without a current limit LED as from what you said it was harder than I thought it was going to be.

Now I have a problem with the circuit. I didn't want to go to the expense of 10 turn pots so I used cheap 1 turn units. From max to min all the adjustment is within about 1mm section of the potentiometer. It's just too sensitive to use. Is there anything I can do about this problem of do I have to get 10 turn pots?

Ceefna

 

[duffy]

Just put a smaller-value pot in series with the existing one, connect it like a rheostat. I do this all the time instead of buying 10-turn pots. The big one gives you coarse adjustment, the small one gives you fine adjustment.

That quick n' dirty current limit LED I suggested by adding a red LED and 10Ω resistor between pin 10 of the LM723 and Vout+ might do what you want. Would only work near the 10A point - but it's so simple it seems worth a try. Maybe start with a 100Ω resistor first, to play it safe.

 

[ceefna]

Great idea for the pots Will try that tonight. Most of my anodising is done under 5A so the LED would never be on.

Ceefna

 

[ceefna]

Just put a smaller-value pot in series with the existing one, connect it like a rheostat. I do this all the time instead of buying 10-turn pots. The big one gives you coarse adjustment, the small one gives you fine adjustment.

Duffy, how do I wire in the second pot? i've just become brain dead!

Ceefna

 

[duffy]

Add it like this:

 

[ceefna]

Thanks for that A picture tells a thousand words. How about the voltage adjusting Pot? sorry for my ignorance

 

[ceefna]

Another silly question but does it matter where I connect it in to the Pin 3 line? does it have to be before the first 10w resistor?

Ceefna

 

[duffy]

Hook the other one up the same way, between that 1k fixed resistor and ground. You might use a larger value pot for that one, maybe 2k or so - but since we are dealing with the "feel" of the adjustment it's hard to guess what value is going to work best.

If you used good thick cable for that power-carrying lead to the clamp, it won't make any appreciable difference where you connect things on the pin 3 line. If you wanted to be ultra-precise, you could connect pin 3 and those two other sense lines to a separate wire and run that clear down to the clamp and attach it there (google "kelvin clips"). This would compensate for I*R drops in the power-carrying lead. Hardly ever necessary outside of a laboratory.

 

[ceefna]

Hi again. I have totally rebuilt the power supply today, please see attached schematic to see if I have done it right.

I used a 100R pot on the current adjust as you sugested but on the voltage adjust I only had a 4k7 pot and now It won't go above 5 volts. Is the pot too large and is my wiring ok on the schematic?
Have I fitted the voltage adjust pot the wrong way round?



Ceefna

 

[duffy]

4k7 is too SMALL - that's 4.7k, or 4700Ω. It should be 20k. You could even go with something a little larger, like 33k.

If you can't get a smaller one, reduce the 1.2k resistor it is connected to about 220Ω and change the "fine adjust" potentiometer down to 100-200Ω. You will still be well under a quarter-watt dissipation.

 

[ceefna]

got our wires crossed here, I have fitted a 20k pot as original schematic. I meant the additional pot i fitted to the voltage adjust was a 4k7.

Ceefna

 

[duffy]

You should be able to turn that down to ZERO ohms and get the same voltage output as you had before, because then it would be like it wasn't even in the circuit! Are you seeing that? If not, remove the pot and find out what else have you done differently before you proceed any further.

 

[ceefna]

Ok will do some more testing, did you have a look at the circuit not sure if I wired it correctly. Does low resistance on pot= more voltage?

Ceefna

 

[ceefna]

Problem solved, I think! I had marked the pots on the back with current and voltage so I didn't get them mixed up on the front panel but marked them up the wrong way round so the 20k was on the current adjust and the 1k on the voltage side. That was the problem when I built the first one and I transfered the error onto todays new build. I have corrected my error now and all seems to be working but I will be able to tell tomorrow when I anodise something.
I have just had a 12v halogen bulb connected to the output and adjustment seemed ok( got no voltage without current adjustment) I hope I will be able to set current and voltage independently as I need lots of voltage to anodise.
Thanks for all your help, I will let you know if it works.



Ceefna

 

[ceefna]

Help needed, Aluminium anodising power supply revisited is dead!

Hi all, I have a serious problem with the power supply. It has worked perfectly since march but now it is setting fire to the LM723's

I am on my forth now and that has just died. I have replaced all 4 transistors to see if it fixed the problem. When I switched back on the current adjust was working but it would not go over 3V before it set fire to the 723 again. I have tested the other components as best I can with my limited knowledge. The LM723's are still burning out without the supply connected to anything so it's not an external fault.
I run the circuit from a 12.5 amp 24v ac transformer through a bridge rectifier.

Please can you give me some guidence as to how to start testing/ what voltages at pins etc.
I am really stuck with this problem and am running out of 723's!!!!


Please let me know what other info you need.
Thanks for your time.

Ceefna

P.S could the 100nf cap off pin5 be the fault? I have not replaced this yet.