lord loh.

I am trying to build a current source that could deliver about 800mA current. It is meant for a battery Charger. I simulated the circuit for 70mA. Then I started to look for commercially available Current source ICs. Most ICs supplied not more than 10uA to 10mA. Is a current source of such high current capacity feseable?

And are current current mirroring circuits efficient? The theory says that the current in one branch is equal to the current in the other branch. So to get a 700mA constant current to the load, I have to drain down 700mA current. It seems a waste.

Please help.

Thank you.

__________________
Bharath Bhushan Lohray.
M.Sc. Electronics.

Nigel Goodwin

Current sources are perfectly feasible, but IC ones would be fairly pointless - try doing a google search, for a clue - how about an opamp/comparator and a power transistor!.

It would be pointless, why would you want a current mirror for a charger?.

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eng1

What about using a LM350? Adding one bjt and some resistors you build your charger.

lord loh.

Accident!!!

__________________
Bharath Bhushan Lohray.
M.Sc. Electronics.

lord loh.

I did enough of google search and discovered that charging a battery requires constant current.

Then I started looking for constant current circuits and found the simeple BJT circuit that I simulated and had good results for low loads. For High current of 700mA,

1. I would need a very high Vcc
2. Very low value emitter Resistor and high wattage.

Further searching lead me to current mirror circuits which I infered to be inefficient.

TO eng1:
And in the LM350 circuit, can I just choose a resistor as per my voltage needs and start charging? Where were you suggesting the power BJT?

Attached Images

	currentlimit.gif (5.3 KB, 360 views)
	current_690.gif (2.5 KB, 356 views)

__________________
Bharath Bhushan Lohray.
M.Sc. Electronics.

eng1

What kind of battery are you going to charge with that current?
If it is a lead/acid battery, a circuit with LM350 might be good (the bjt would be a general pourpose npn one, attached to the adj aterminal to limit the current through the battery).
If you're going to charge Ni-Cd / Ni-Mh or Li-on batteries you may need a charge controller... I learned something about charging stages with the LM3647 general description and datasheet. I hope it will be useful.

crowinu

check maxim-ic website or analog devices website and search in their application notes, there are some good info and some circuits about current sources.

regards
crowinu

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akg

a transistor's Beta is the current gain , so a 7ma current to the base(of a power trans: ) with gain 100 , should produce 700ma Ic , in theory.

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lord loh.

Can you suggest me a transistor that can handle 700mA Ic and also has a gain 100?

I am looking for transistors that can handle 1A - 3A - 5A current. Can anyone suggest me any? I have been looking at fairchild's pages

__________________
Bharath Bhushan Lohray.
M.Sc. Electronics.

Nigel Goodwin

The '100 gain' idea was really silly (sorry), you design circuits so the gain doesn't make any difference (within certain limits). As I suggested originally, an opamp and a transistor (or two) are all you need, look on discover circuits (or search these forums for the previous threads).

There are MANY transistors with suitable current ratings, try 2N3055, TIP41, BD131 - for just a few suggestions.

__________________
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akg

..
yeah i know , it is also expected to add a feedback ckt 8)

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"Give a man a fish, and he will eat for a day. Teach that man to fish, and he will eat for a lifetime."

FlyingDutchMan

I've found just using transistors and resistors a good way of making constant current sources. In the diagram I've attached, the current is determined by the R2 (I=0.7/R2), and R1 just needs to be large enough so as not to draw lots of current, but small enough to ensure the current gain through Q1 is sufficient. It is a very simple and stable circuit which doesn't rely on a specific voltage source. Q3 and Q4 do the current mirroring - these need to be the same type of transistor and preferably from the same batch - using different types here can lead to a wildly different current in the mirrored branch. I haved used both the current limiting and mirroring bits in a number of circuits without any problems. Now you just need to workout when the batteries are charged.

Attached Images

constcurrentandmirror.gif (4.1 KB, 207 views)

lord loh.

Thanks FlyingDutchMan, I had seen a similar circuit in a book (Art of Electronics?) once but never found it again...

__________________
Bharath Bhushan Lohray.
M.Sc. Electronics.