Convert 0-30V 3A PSU to 5A or more

[audioguru2]

Hi Kain,
Your PCB is going to be really small. Allowing space for mounting bolts and a 15W resistor for R7, where are the rest of the parts going to go? ;D

 

[Kain]

I would use 2 monting bolts x 3mm so this is not a big deal. The real thing is that I took the 15W resistor off the board - it will be way better if it's attached on the box and link with short wires to the board since it will allow it to cool better i believe. This is why the board can be that small ;D

 

[audioguru2]

Hi Kain,
Good, you'll have a nice and small, cool PCB. ;D
Are you going to use an aluminum-cased and small-finned resistor with a 25W rating?

 

[Kain]

Yup. The resistor I will be using will be the one i posted about earlier - aluminium cased with small fins as you said.

 

[ante1]

Hi Kain,

Just make wire connections for R7 on the pcb and bolt the resistor to the chassis instead. Some of these resistors can only take one third of the wattage without any heatsink. At 5A the resistor will dissipate almost 7W and you don

 

[Dazza]

Thank you Kain, for taking on the job of designing the PCB . Double layer board, that will be a new challenge for me I'll be looking forward to giving that a go ;D. I was planning on putting this project together some time after Christmas, to add to the original PSU that I put together, with a couple of modifications and works very well 8).

 

[Dazza]

Do I understand this right, I can use this power supply to recharge rechargeable batteries, by setting the voltage and current limit to suit the requirements of the battery to be charged. Is it possible to add a circuit that can be adjusted that will sense when the battery is fully charged, then disconnect the power or maybe shut down the PSU.

If it can be done I think it would be very handy .

Sorry if this isn't the appropriate place to ask this question.
I was thinking it may be a handy optional add-on for this project.

 

[Kain]

Dazza, I think the application you are talking about can be achieved pretty easy - I mean the self turn off. In fact, you almost have it built in already ;D. If you noticed the LED that is used for indication would light up if the current limiter is working. However, as far as I understand the current is still there, but cannot exceed the given value. So, if you battery is the load in the circuit, while it's being charged it would become lighter load thus the circuit protection would stop working and the LED would turn off. If you invert this signal for the LED you can have a circuit that goes on when the LED turns off, such as something that breaks the circuit and so on, such as thyristor in combination with reley or something else. Use your imagination ;D. If I am right your shut off is as easy adjustable as the curent limiting is. I hope I'm right thoough hehe

 

[audioguru2]

Hi Guys,
A battery that is charged from a regulated current supply does not become a lighter load.
It will continue to draw the same amount of regulated current that you have set for maximum, but after it is fully charged will convert the entire charging power to heat and gasses. If the charging current is high the heat will damage the battery or the gasses cause it to vent or explode. If the battery is a lithium rechargeable, it will probably catch fire.

Depending on the battery's chemistry type and temperature, you can charge it through a current-limiting resistor (or current regulator) for a fixed amount of time. If the charging current is high, a high temperature cutoff circuit should also be used in case the charging battery was not completely discharged.

IC manufacturers make battery charger IC's for each chemistry type.
Good explanations about charging batteries are in our Articles section.

 

[Dazza]

Hi Kain, are you trying to make me blow up my batteries , just kidding I don't have any yet ;D ;D ;D. Lucky ;D.

Hi audioguru, from what you have explained, it won't be easy to allow for different battery types to be charged.
Maybe I should just try for battery types that I will mostly use .

 

[Kain]

Dazza, I almost got you though, huh? ;D Just kidding. The blow off thing hasn't happened to me before but for I always use resistor to limit the curent down as audioguru suggested. Audioguru, doesn't the current protection limit the current down so it will serve the same purpose as resistor in this case?

 

[audioguru2]

Hi Dazza,
I wouldn't use a supply with adjustable voltage and current controls to charge a battery. A kid comes along and fiddles with it or you accidently bump it and you have a very dangerous situation.

The charging characteristics of each battery type is different, although a Ni-Cad and a Ni-MH are similar. Aren't you using a good old lead-acid battery? At room temperature, just limit the current to a reasonable amount with a resistor and limit the voltage to 13.8V for a 12V battery. When it becomes fully charged, it won't over-charge like the other types will.

Hi Kain,
That's good that you use current limiting resistors to avoid explosions! You are correct, current regulation serves the same purpose as a current limiting resistor, except it automatically changes its resistance to keep the current at the same value when the load voltage or source voltage changes.

 

[ante1]

Hi Audioguru,

This is not entirely true, I have charged many batterys in this way and never caused any damage. If you just do it right there is no problems as I see it. For example to charge a car battery, set the voltage to 13.75Volts then connect the battery and set any current up to 5% of the capacity. For a 60Ah battery this will be 3A, and leave it. There will be no destruction or gassing or explosion, what will happen is the voltage is now lower (current foldback) than the 13.75V set but the current will stay at 3A. Slowly the voltage will rise to 13.75V but not more, then the current will start to fall. Gassing will only occur in a lead acid cell if the voltage exceeds 2.38Volts (14.28V) so there is no gassing here. The voltage set at the start is the maximum for the charge that

 

[audioguru2]

Hi Ante,
You charge your batteries at a very low safe current and use voltage limiting.
I was cautioning Kain because he didn't understand that a current regulator will increase its output voltage to maintain the set current.
He also misunderstood that a fully charged battery would automatically reduce its charging current which certainly is not true if the charger's voltage is not limited.

Did you notice that car amps are rated with a 14.4V supply?
It must be the battery's gasses that create the extra output power!

 

[Dazza]

OK great, I know how I can make this now, and safely 8). Good safety point audioguru .

Thank you.

 

[ante1]

Audioguru,

Well, the question was

 

[Kain]

Alrighty. Here is the double sided PCB for the project as I promised. It took me a while - I have other things to do too you know ;D. I included the separated Top, Bottom layers - they are all ready for print meaning you don't need to mirror anything. The only thing you will need to do is scale them corectly. I would scale them by the IC pins - they are very good indicator if you have scaled correctly. When you print out both layers on separate paper when you put both printed sides toward each other, the vias should match. I included the schematic, the component guide as well as the view of the board when both layers are matched as I said before. If somebody please can check if I didn't mess up the schematic since there have been too many posts here about the mods to this project. Keep in mind that the 50000uF cap and the 15W resistor are off the board. There are vias for them but they will be mounted off board. If you find any errors please let me know. I'll be doing this one myself soon ;D

PSU_5A.pdf

 

[ante1]

Hi Kain,

Very nice job!
Don

 

[Kain]

Well this is a good idea about the boards and the wires. I think I will use photo-exposure on a large sheet so there will be probly about 5 boards per sheet or so - I need to calculate. I still have one question though - the main power nets I did with width of 3mm. Since I lost my chart I was hoping if somebody can tell me if this is enough to take 5A at 30V? If this is fine then I believe the design should be good.

 

[Kain]

Ok guys. It seems that nobody is feeling like working enough to check the board I just posted. Well you better check other people's job if you plan to use it because I messed it up a bit - the footprint for the op amps was kind a wrong - distance between pins was way too high so I had to correct this. It's good at least that I checked it ;D. The one I'm posting now is all good. I also made little corrections so whenever you have a bridge from one layer to the other it has larger pads 8). Have fun, and please check it again

PSU.pdf