Request help with odd looking CCS fed shunt reg circuit and load current ability.

[Ceglar]

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

 

[DUG]

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.

 

[DF96]

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.

 

[Ceglar]

Thanks a lot, much appreciated.

Shane

 

[Ceglar]

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

 

[DF96]

Maximum current depends on transistor ratings and heatsinking, and minimum voltage due to ripple (which depends on the size of the main reservoir capacitor).

 

[Ceglar]

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

 

[DF96]

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.

 

[Ceglar]

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

 

[DF96]

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.

 

[Ceglar]

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.

Would you consider 47uF/6.3V BG NX Hi-Q to be big and/or lossy enough?

 

[Robert F]

That doesn't look like a shunt reg to me. The CCS just supplies the voltage reference.

 

[DF96]

Most electrolytics will be big enough and lossy enough. Low ESR ones perhaps not.

 

[Ceglar]

Most electrolytics will be big enough and lossy enough. Low ESR ones perhaps not.

Thanks DF, have to wait and see I guess. Appreciated the help by the way, thanks a lot for that.

Regards,
Shane

 

[Ceglar]

That doesn't look like a shunt reg to me. The CCS just supplies the voltage reference.

What sort of regulator would you suggest that it is?

Shane

 

[Robert F]

What sort of regulator would you suggest that it is?

Shane

It's a series reg.

 

[Robert F]

It's a series reg.

Sorry that was accurate but not very helpful. In a shunt regulator the regulating device is in parallel with the load. The regulator you show the pass transistor is in series with the load, it is just drawn in a strange way.