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Variable DC Power Supply

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Night Thrasher

New Member
I'm trying to make a battery charger for my NiCd battery and I search the internet a found Red Circuit design that has a variable voltage and current output. My question is how can i put a digital volt/current meter so I can set a precise output of both voltage and current?
 

k7elp60

Active Member
variable power supply

The best way to charge a ni-cad battery is with a constant current charger. If the charge current is set to .09 x the capacity of the cells and the output of the charger is capable of 1.5 V times the number of cells in series the charger can be left on with out danger of over charge.
The normal ni-cad batteries will have a full charge voltage very close to 1.5V per cell when chharged and still connected to the charger. A LM317T adjustable regulator with an appropiate resistor works fine. If you will give me the # of cells in series and the capacity of one cell in Ma, I will be glad to provide you with a value for the resistor and the required input voltage to the regulator.
 

niq_ro

New Member
I'm trying to make a battery charger for my NiCd battery and I search the internet a found Red Circuit design that has a variable voltage and current output. My question is how can i put a digital volt/current meter so I can set a precise output of both voltage and current?

see **broken link removed**

there are 3 examples with mathematichal fomulas

use A-meter in series with bateries to see current charger
 

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Night Thrasher

New Member
Re: variable power supply

k7elp60 said:
The best way to charge a ni-cad battery is with a constant current charger. If the charge current is set to .09 x the capacity of the cells and the output of the charger is capable of 1.5 V times the number of cells in series the charger can be left on with out danger of over charge.
The normal ni-cad batteries will have a full charge voltage very close to 1.5V per cell when chharged and still connected to the charger. A LM317T adjustable regulator with an appropiate resistor works fine. If you will give me the # of cells in series and the capacity of one cell in Ma, I will be glad to provide you with a value for the resistor and the required input voltage to the regulator.

The reason i want it to have a variable setting for the voltage and current is because i'm using different kinds of battery packs, from 8.4v to 12v and from 1500mah to 2400mah type and also having an adjustable current i can charge my battery to fast or slow charging time.

I've seen the LM317T but i cant find it here in the philippines.. i think it is not available here.. anyway i already built this circuit and my problem is to have the exact output volatge and curent thru a volt meter and want change it to a digital type to be more precise.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Re: variable power supply

Night Thrasher said:
The reason i want it to have a variable setting for the voltage and current is because i'm using different kinds of battery packs, from 8.4v to 12v and from 1500mah to 2400mah type and also having an adjustable current i can charge my battery to fast or slow charging time.

You only need to set the current, you can't possible have constant voltage and constant current at the same time.

I've seen the LM317T but i cant find it here in the philippines.. i think it is not available here.. anyway i already built this circuit and my problem is to have the exact output volatge and curent thru a volt meter and want change it to a digital type to be more precise.

The LM317T is a very standard part, it should be available anywhere, it's pretty well up there with 741's and 555's.

The voltage is irrelevent (as long as it's far enough higher than the batteries), you charge the batteries with current not voltage, niq_ro showed how to connect an ampmeter above.

But all you really need is a switched constant current source, there are loads of NICAD charger designs on the net - it's also a good idea to build one with a timer, over charging tends to shorten battery life.
 

k7elp60

Active Member
Night Thrasher said:
I'm trying to make a battery charger for my NiCd battery and I search the internet a found Red Circuit design that has a variable voltage and current output. My question is how can i put a digital volt/current meter so I can set a precise output of both voltage and current?
To answer your question about digital readings. If you put a digital voltmeter across the output terminals you will be able to adjust to a precision voltage. However the voltage will change as the output voltage is not regulated. P2 adjusts the voltage out. But each different load will cause a different output voltage. You can put a digital current meter inseries with the load and adjust P1, as it adjusts the output current limit.
Based on what you said about the nicad cells you want to charge here is my recommendation. If you limit the charge current to capacity x .09 the cells will never over charge. If the power supply will produce 15 volts on the output terminals it will charge either battery pack. By using a constant current charger with ample voltage the cells seek their on terminal voltage, and adjusting the voltage is not necessary.
But fast charging nicads that are not designed for fast charging can cause problems. If you fast charge they need to be timed and the circuit should have some protection built in like a temperature cutout in case the cells get to warm.
 

tincute

New Member
hi there.. im new at this site. anyways, i would just like to ask from those who might be able to help me. regarding the circuit of power supply shared by night trasher which he got from red.. what if instead of the specified transistors, i use replacements but having different ratings for Vce, and even Ic.. do you think it would still work as desired? that is, it would still be able to output up to 24V & current limiting up to 1 amp? thanks! i really need your ideas because i need a power supply that would output 28Vdc & 220mA. =)
 

audioguru

Well-Known Member
Most Helpful Member
tincute said:
regarding the circuit of power supply shared by night trasher which he got from red.. what if instead of the specified transistors, i use replacements but having different ratings for Vce, and even Ic.. do you think it would still work as desired? that is, it would still be able to output up to 24V & current limiting up to 1 amp?
Parts with different ratings? Do you like to blow-up stuff?
Millions of transistors have exactly the same ratings or better than those cheap and common ones.

Why bother making that power supply? It is cheap and simple and doesn't have a voltage reference like a zener diode as in real power supplies. I think its output voltage depends on the phase of the moon.
 
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