Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Current Source and current mirrror

Status
Not open for further replies.

lord loh.

Member
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.
 
lord loh. said:
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?

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!.

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.

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

Attachments

  • current_690.gif
    current_690.gif
    2.5 KB · Views: 705
  • currentlimit.gif
    currentlimit.gif
    5.3 KB · Views: 706
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.
 
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
 
lord loh. said:
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.
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.
 
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 **broken link removed**
 
lord loh. said:
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 **broken link removed**

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.
 
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.
 

Attachments

  • constcurrentandmirror.gif
    constcurrentandmirror.gif
    4.1 KB · Views: 450
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top