Hi,
I want to anodise some parts and need a constant power supply but needs to be adjustable as different parts require more or less amps.
Any suggestions on how to make this power supply adjustable?
Is it as simple as sticking in a pot. on the lm307?
Schematic below.
Hey Mat,
I’ll leave this open for others to take a look at, unfortunately, it’s a little over my head. I’ve listed a few other resources you can check out below:
Looks like that thing’s powered from mains voltage - I’d definitely be careful!
Hi Mat
What sort of voltage range are you after?
Cheers Bob
Hi,
Anything up to 30v maybe. Will most likely only require 20v or less. But 10amps for 60mins will be required. I believe these packs are set at 12v@20amps. Even that I can work with, just need to adjust it. I believe it is as easy as putting in a put but as previously stated this is a 240 unit and I just want to make sure it’s done properly. The hot side is easily identified.
But this thing has massive caps. I’ve been whacked a few times by smaller psu caps and it’s certainly reminds you to dissipate energy first.
Hi Mat.
OK. The regulator part has 21V input. Something a little less than that will be your maximum.
V6 is your output V sensing and initial control stage. R14 is SOT (select on test) resistor for setting output voltage, normally 13.8V. You will have to experiment but maybe you can have a pot (rheostat connected) in here to give some output V control. There is an 11V zener associated with the base circuit of V6 which suggests to me that the minimum voltage might be about 10V (21-11).
Current Limit: R30 is the current sensing resistor and will be a very low value, may even look like just a piece of wire.IC1 will amplify the small voltage developed across R30 and apply this to the base circuit of V6 so reducing the output voltage when current approaches the set point. R23 is a SOT resistor to set current limit. You may be able to replace this with a pot (rheostat connected) to get variable control.
Pots should be wire wound. NOT carbon.
I have no idea what sort of control range you will get by this method. That will have to be determined by experiment unless someone is familiar with this circuit and wants to try the maths.
As thing device is mains powered BE VERY CAREFUL. If possible power this via a 240/240 isolating transformer with sufficient current rating. Use the fuse size as a guide. Unfortunately not everyone has something like this available. I know I haven’t these days. A point to note along these lines a “Variac” although called a transformer is an autotransformer and DOES NOT isolate.
Cheers Bob
PS. When I was working AWA manufactured a similar power supply mainly for test bench use which had a pot available to vary the voltage from about 10V to something like 18V or so for workshop testing purposes.
So replaced both of the SOT resistors with 5k linear pots. Volts adjusts from 11.6 to 13.8v with fairly nice control of the pot. Amps I’m not so sure about. Full adjust was achieved within the first quarter turn of the dial and was difficult to fine tune.
I used a 75ah battery that was down to 11.6v as the load. I could only read a max input of just under 9amps at 13v. I couldn’t get it to load more.
Not sure if the battery was the best option but it’s what I had.
Any thoughts on testing or getting better amp adjustment. I mean, a quarter of a turn might be able to adjust 0 - 8 amps and a full turn may do 0-20amps but there wasn’t a sufficient load to require a full turn adjustment???
Can see in the pic a 3.9k on the current standoff (R23) and a 5.6k (R14) for the volts.
And FYI, 13v @ 8.5amps for 30mins and the heat sink wasn’t even warm. This thing was just idling.
Hi Matt
Light globes. A good old stand by.
At 13V a 12V stop lamp (21W) will draw about 1.6A. Wire a few in parallel If you have sockets it is much easier as you can then plug lamps in and out to adjust the load. When you get the current limit working measure what the pot range is for that result and change the pot value to get a better range of pot travel.
I am assuming you connected the pot as a rheostat, ie; use one end and the slider, ignore the other end or connect it to the slider. You need to finish up with a variable resistor. I think you would have done this correctly to get the result you did.
When the output voltage was 11.6V was the resistance of the pot zero or max. I would have expected about 10V but I think the reason for the 11.6 to 13.8 was the load being a battery. The minimum would not go below the battery voltage.You will have to use a resistive load (12V globes). You also should remove the SOT resistors and replace them with the pots. The voltage SOT resistor at the moment is 5k6Ω which has set the supply for 13.8V so if you want a few more volts you will have to increase the pot value. That is why the adjustment range was 11.8 (battery V) to 13.8 (5k SOT).
You will have to experiment a bit but I think you are on the right track. It now remains to be seen if that power supply will supply the current you need.
Cheers Bob
ps: When adjusting the current limit on a bench supply with that function it is normally set with the output short circuit. This supply would probably be the same procedure. At 13V the battery probably would not take any more than 9A. You will have to work the voltage adjustment (do this first to get it finalised). You should not need a load for this. Then I think work on current limit with a short circuit output. If you are nervous start with a load. 24V globes may be better as the current per globe will be more and you won’t need as many to reach 20A. Another bit of insurance would be to reduce the size of the fuse but have a few spares on hand.
You can’t adjust the amps. The current draw in any particular circumstance will be determined by the voltage and the load. The PSU will have a maximum amperage that it can deliver - above that point it will ether shut down or blow up.
If you increase the voltage then you will get a higher current for a given load, but you won’t change the maximum that the unit can deliver.
To adapt the unit to handle a higher current rating will involve many components including the mains transformer and the power transistors - it’s not practical.
Hi Jeff.
True, the current is determined by the voltage and load but you CAN set the maximum current to be delivered. That is what is done in this case with the current SOT resistor and which Matt is trying to make variable. Like a current limited bench supply. It does this by reducing the voltage when preset maximum current starts to exceed setting. So you are right of course, current is still being controlled by the voltage but including the ability to do this automatically is not always made available. In this case it is.
It would appear that the normal operation for this supply is 13.8V and current limit set for 20A. I think that the name Codan suggests a supply for a Transceiver so the current limit is in case of mis alignment of the transmitter causing excessive current consumption and causing supply damage.
I think in the preceding posts there has been a terminology problem where the term 'current adjust" has been used instead of “current limit”.
Cheers Bob
Yes. That’s the point I was making. OP is interested in current adjust “as different parts require more or less amps” (emphasis added). So all the references to current limit are irrelevant to the issue.
Hi Matt. Refer my reply on 06/06/21.
URGENT. Delete all reference to setting up current limit by short circuiting the output. This is the procedure with a bench supply that is adjustable down to zero volts. With this supply I think you will have a minimum voltage of about 10V so the short circuit procedure won’t be possible. You will need a load. But whatever, deal with the voltage adjust first. You won’t need a load for this.
Cheers Bob
Yeah, I’m not a big fan of short circuit anything that’s pumping out 20amps and doesn’t have a fuse on it. I didn’t take any notice of that one.
The volts range was 11.6v to 13.8vish. That’s no load using the 5k pot.
And I will start using the term current limit. This is set to 20amps so I’m only looking to run 20amps or limit down to say 1amp.
Hi Matt
The short circuit current limit adjustment is the method of setting this for a bench top supply prior to connection to other equipment. These supplies will adjust voltage and current down to zero and are designed to do this. As I said above the name Codan suggests a supply for a Codan transceiver where a wide adjustment range is not required. The normal voltage would be 13.6V to 13.8V.
I was not surprised you did not get a greater voltage range with a 5k pot as the SOT resistor was 5k6. I think you need a bit more here if you want a bit higher voltage. The lower limit is not surprising as there is a 11V Zener in the base circuit to transistor V8. 11V plus 0.6V for the base emitter junction = 11.6V.
Current limit. I don’t know how low you will get here. Experiment required. The lower volt limit of 11.6V may have an effect on this. I am not familiar with that supply so I don’t know. You may get down to 1A but experiment will tell.
Cheers Bob
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