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Old 18th December 2005, 02:58 PM   #1
Kevinbd is offline Kevinbd  United Kingdom
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Default Current supplied by CCS

A quick one. With a -35 volt rail, what current is this supplying ?.

It feeds some 100R degeneration resitors for an NPN LTP.

Special help required! LOL

Kevin
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Old 18th December 2005, 03:15 PM   #2
lineup is offline lineup  Sweden
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Around (5.6-0.6)/1.8 mA
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Old 18th December 2005, 03:16 PM   #3
anatech is offline anatech  Canada
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Hi Kevin,
It wants to sink about 2.8 mA. 5.6 V - 0.6 V = 5v V. 5 V / 1800 ohms is around 2.8 mA.

That's if the other parts of the circuit are working.

-Chris
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Old 18th December 2005, 06:06 PM   #4
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Kevin,

do you know the value of the other zener to the right?

Hopefully it's greater than 5,6V to make that circuit work sufficiently, because if equal to ZD1 the current through ZD1 will be very small and it means the zener voltage may be even up to 1 volt lower or so than the marked "5,6V" making this circuit a bit unpredictable of the theoretically calculated current and you have to meassure then, this is because Zener voltage vary a lot depending on the zener current.

Say we need a zener current of 1 mA, that means we have a voltage fall of 2,2 Volts over R10, meaning the other Zener diode should be at least5,6 + 2,2 = 7,8, rounding up to closest standard value 8,2 V.

I assume actually the other zener being of quite low value due to the power marking R11 being 1W, eg. it seem to have been choosen to work with "high" voltage.
With the other zener being 8,2 V the current alone through the zener would be approx 12 mA, which corresponds to around 330 mW in R11.

Cheers Michael
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Old 19th December 2005, 12:07 AM   #5
Kevinbd is offline Kevinbd  United Kingdom
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Ultima Thule,

The other zener to the right was the same. I presume 5.6V. Unusual conifguration for a CCS I must admit.

I guess it doesn't really matter whether the CCS is 2mA or 3mA.

Thanks for your help

Kevin
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Old 19th December 2005, 01:17 AM   #6
anatech is offline anatech  Canada
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Hi Kevin,
The other zener must be greater than 5.6V or the circuit will not operate. The base current is not high, so most of the current through R10 will flow through ZD1. The circuit isn't that unusual really.

-Chris
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Old 19th December 2005, 01:25 AM   #7
Kevinbd is offline Kevinbd  United Kingdom
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Hello Anatech,

Regretably the zener diode in quesion is not labelled in value.

Thanks anyway
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Old 19th December 2005, 01:34 AM   #8
Kevinbd is offline Kevinbd  United Kingdom
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Relooking at the shemactic again, the Zener actaully is labelled.

ZD2 is 10v. ZD1 is 5.6v

Sorry I made a mistake Anatech and to the other people.

Kevin
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Old 19th December 2005, 01:38 AM   #9
lineup is offline lineup  Sweden
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Isnt it possible to measure with circuit in operation?

Current passing a resistor is always Voltage/R
Constant current in this case is Voltage/1.8k
where Voltage is voltage over resistor

Zenerdiode voltage can also be measured

I guess you can not run your circuit.
Alternative then is to demount components and measure
or run them in test circuits
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Old 19th December 2005, 01:54 AM   #10
anatech is offline anatech  Canada
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Hi lineup,
Correct, the voltage across R7 is the value this will sink. That will be 5.6 V - about 0.6 V which gives you around 5 V across 1K8.

Hi Kevin,
10V is a good value, this tells us that the ZD1 current is close to 2 mA. Therefore the current through ZD1 is stable and the circuit will operate properly.

-Chris
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