Request help with odd looking CCS fed shunt reg circuit and load current ability. - diyAudio
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Old 18th February 2013, 02:45 AM   #1
Ceglar is offline Ceglar  Australia
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Default Request help with odd looking CCS fed shunt reg circuit and load current ability.

Hi,

I understand the BF245A CCS, and without source resistor it runs at IDSS which will be 2 to 6.5mA, depending.

The circuit supplies 5VDC at 25mA.

Obviously not all of the load current comes via the CCS and I'm used to CCS fed shunt regs for tube circuits where the CCS supplies ALL of the current for both the shunt and the load.. so I'm a bit lost.

The question is, if I change the load to 5VDC at 1mA, will it still function without changing any other values?

This isnt for some mental exercise, I'm just needing to use some existing supplies for a different application that require considerably less current.


Thanks,
Shane
shunt-reg.jpg

Last edited by Ceglar; 18th February 2013 at 02:50 AM.
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Old 18th February 2013, 05:53 AM   #2
DUG is offline DUG  Canada
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The 1K resistor already draws 5mA.

You could plot the load regulation and decide then:

add 5K load, measure V

Repeat 24 times.

Have fun.
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Old 18th February 2013, 10:41 AM   #3
DF96 is offline DF96  England
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The Darlington pair needs to supply at least 2mA (or thereabouts) in order to switch on the second transistor. If not, the output voltage will rise by about 0.6-0.7V. Your 1k load is already taking 5mA, so all is fine. The circuit is overkill for supplying 1mA, but as you already have it it won't do any harm.
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Old 19th February 2013, 12:35 AM   #4
Ceglar is offline Ceglar  Australia
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Thanks a lot, much appreciated.

Shane
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Old 20th February 2013, 12:07 AM   #5
Ceglar is offline Ceglar  Australia
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Quote:
Originally Posted by DF96 View Post
The Darlington pair needs to supply at least 2mA (or thereabouts) in order to switch on the second transistor. If not, the output voltage will rise by about 0.6-0.7V. Your 1k load is already taking 5mA, so all is fine. The circuit is overkill for supplying 1mA, but as you already have it it won't do any harm.
How to determin the maximum current the supply can provide, assuming the transformer winding is not the limiting factor?.

Would 50mA seem ok?


Thanks,
Shane
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Old 20th February 2013, 10:12 AM   #6
DF96 is offline DF96  England
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Maximum current depends on transistor ratings and heatsinking, and minimum voltage due to ripple (which depends on the size of the main reservoir capacitor).
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Old 20th February 2013, 09:12 PM   #7
Ceglar is offline Ceglar  Australia
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I'll check the data sheet.. I can say that with 25ma the BD139 does get a bit warm.

Seems the idea with this scheme is to establish a voltage reference and then buffer its output with the darlington pair and avoiding the feedback inherent to a series ccs to shunt regulator circuit?

I wonder if theres much to be gained by using two inductors, one in the +ve rail after the ccs, and another in the ground cct between the zener and the 1k//cap, to help decouple it from the rest of the power supply and have better control of the current loops between the load and the final cap in the reg?

Application has 6 of these in a D/A converter cct.

Thanks,
Shane
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Old 20th February 2013, 10:12 PM   #8
DF96 is offline DF96  England
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Quote:
Originally Posted by Ceglar
Seems the idea with this scheme is to establish a voltage reference and then buffer its output with the darlington pair and avoiding the feedback inherent to a series ccs to shunt regulator circuit?
Yes, although there is heavy feedback in the follower.

I would not add inductors unless you have identified a problem they might solve.
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Old 21st February 2013, 01:59 AM   #9
Ceglar is offline Ceglar  Australia
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Quote:
Originally Posted by DF96 View Post
Yes, although there is heavy feedback in the follower.

I would not add inductors unless you have identified a problem they might solve.
Curious. Is the feedback associated with the follower of a different order than what would normally be associated with ccs fed shunt in the usual sense. (ie. where exactly is the benefit?).

Thanks again,
Shane
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Old 21st February 2013, 12:13 PM   #10
DF96 is offline DF96  England
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Feedback is feedback. However, within a single stage it may be easier to keep stable. Curiously, a follower is an exception to this: followers can oscillate at VHF/UHF frequencies if they see a small capacitive load. A big, or lossy, capacitive load such as an electrolytic is usually OK.
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