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barbar
Joined: 08 Dec 2003
Posts: 25
Location: Israel
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 27 Jul 2004 13:34 simple current limit |
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| Is any one know about simple current limit for battery-resistor tester?
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batdin
Joined: 03 Jun 2004
Posts: 181
Helped: 3
Location: Bulgaria
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| 28 Jul 2004 7:48 Re: simple current limit |
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Sorry about my being so ignorant but what is battery-resistor tester?
Generally,the simplest form of current limiting is one resistor.Suppose in your case this is too simple a solution.
So another simple limiter (having a low voltage loss) consists of one transistor,two resistors and one diode.
You can also use LM317 but put up with its inherent voltage loss (about 3-4V)
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barbar
Joined: 08 Dec 2003
Posts: 25
Location: Israel
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| 28 Jul 2004 11:12 Re: simple current limit |
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Thanks fr the replay.
I am intersting in the trn. sulotion. Can you tell me where I can fined athat design?
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aslak
Joined: 04 Nov 2001
Posts: 29
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| 30 Jul 2004 19:34 Re: simple current limit |
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Hello,
Simple currrent limiter is in attached picture. You can adjust limiting current with R1. Values shown are for 1A, scale up or down as needed. V1 and R3 are not part of the limiter but represent source and load. Voltage drop across the circuit is about 1V when current is below limiting knee.
Rgds,
Aslak
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E-design
Joined: 01 Jun 2002
Posts: 785
Helped: 58
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| 31 Jul 2004 3:45 Re: simple current limit |
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| aslak wrote: |
| Values shown are for 1A |
Should be 0.1A
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E-design
Joined: 01 Jun 2002
Posts: 785
Helped: 58
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| 31 Jul 2004 5:26 Re: simple current limit |
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Working of cct:
On low load current levels, bias current for the transistor is supplied through R2. As the load demand more current, more bias current will flow into the base resulting in a much bigger current between collector and emitter (due to current gain in the transistor) to the load. D1,D2 will not conduct until the total voltage drop (Vbe + VR1) is around 1.2v. This will occur with 0.1A flowing in R1 (assuming 0.6v drop per diode and b-e junction)
If the load now demands more current any additional current through R2 will be shunted away by D1,D2 from the base, thus preventing the current through the transistor to increase. The total load current will only increase by a very small amount (excess base current shunted into load) with any further decrease in load resistance.
An added bonus is that the diodes will also provide temp compensation. When the Vbe voltage drops with a increase in temp, voltage over D1,D2 will also drop to prevent more current flowing into the base.
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