Current supplied by CCS

Kevinbd · 18th December 2005, 02:58 PM

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

lineup · 18th December 2005, 03:15 PM

Around (5.6-0.6)/1.8 mA

anatech · 18th December 2005, 03:16 PM

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

Ultima Thule · 18th December 2005, 06:06 PM

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

Kevinbd · 19th December 2005, 12:07 AM

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

anatech · 19th December 2005, 01:17 AM

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

Kevinbd · 19th December 2005, 01:25 AM

Hello Anatech,

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

Thanks anyway

Kevinbd · 19th December 2005, 01:34 AM

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

lineup · 19th December 2005, 01:38 AM

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

anatech · 19th December 2005, 01:54 AM

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

18th December 2005, 06:06 PM	#4
Ultima Thule diyAudio Member Join Date: Jan 2004 Location: Koskenkorva Land	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 __________________ "If transistors are blueberries and FETs are strawberries, then tubes must be.. pears" Michael 29th January 2010

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18th December 2005, 03:15 PM	#2
lineup diyAudio Member Join Date: Dec 2005 Location: the north	Around (5.6-0.6)/1.8 mA

18th December 2005, 03:16 PM	#3
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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

19th December 2005, 12:07 AM	#5
Kevinbd diyAudio Member Join Date: Apr 2002 Location: Swindon, U.K.	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

19th December 2005, 01:17 AM	#6
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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

19th December 2005, 01:25 AM	#7
Kevinbd diyAudio Member Join Date: Apr 2002 Location: Swindon, U.K.	Hello Anatech, Regretably the zener diode in quesion is not labelled in value. Thanks anyway

19th December 2005, 01:34 AM	#8
Kevinbd diyAudio Member Join Date: Apr 2002 Location: Swindon, U.K.	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

19th December 2005, 01:38 AM	#9
lineup diyAudio Member Join Date: Dec 2005 Location: the north	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

19th December 2005, 01:54 AM	#10
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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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