That to get 12.7 volts at the base for the output to be in regulation, the output can be no lower than 44.5V with the resistor values shown and the pot all the way to the top, i.e. the minimum regulated output voltage is 44.5V.

Am I calculating the output of the full bridge incorrectly? I was using 24VAC RMS from the transformer, multiply by 1.414 to get peak voltage = 34V, then subtract 0.6V for diode drop = 32.5.

How are you getting to > 40 volts (or, more specifically, what am I doing wrong)?

 

but you cannot have 44V output and base at 12.7V and potentiometer at maximum.

when pot is at max, MPSA will conduct, pulling base of darlington lower and output voltage...

 

Am I calculating the output of the full bridge incorrectly? I was using 24VAC RMS from the transformer, multiply by 1.414 to get peak voltage = 34V, then subtract 0.6V for diode drop = 32.5.

How are you getting to > 40 volts (or, more specifically, what am I doing wrong)?

that is correct... i don't know where 40+ volts came from

 

How are you getting to > 40 volts (or, more specifically, what am I doing wrong)?

There's nothing wrong with your calculation for determining the unregulated DC voltage.
But I'm not talking about that voltage, I'm referring to the output voltage.
My point is that the output can be no lower than 44.5V for a regulated output.
This means the unregulated input voltage to the regulator would need to be greater than that.
Thus the output will never be regulated without a change of the resistor values in series with the pot to allow a lower output voltage adjustment.

Make sense?

 

There's nothing wrong with your calculation for determining the unregulated DC voltage.
But I'm not talking about that voltage, I'm referring to the output voltage.
My point is that the output can be no lower than 44.5V for a regulated output.
This means the unregulated input voltage to the regulator would need to be greater than that.
Thus the output will never be regulated without a change of the resistor values in series with the pot to allow a lower output voltage adjustment.

Make sense?

Understood. Thanks.

 

sigh....facepalm...
my bad, i guess i need to start paying attention and reading values or comments... 10k is way too large (as mentioned in first reply)

 

The transformer has been changed to 24V secondary so the regulator cannot achieve its minimum 35V output.

you have forgotten to include transformer regulation. The 24v ac output is at full rated load. at no load, depending on transformer, it would be (typically) 12-25% higher.

 

you have forgotten to include transformer regulation. The 24v ac output is at full rated load. at no load, depending on transformer, it would be (typically) 12-25% higher.

So, unless the transformer is very oversized for the application, then 35V will not be achievable at full output current.

 

it always depends on the transformer. in general, big transformers have better regulation, so going large *may* not help at all. the real solution is to use a higher voltage transformer, but the op may be ok if he does not need much current at 35 voltage. check out a supplier like element14 or newark - they provide data sheets. regulation is not always expressed as a percentage - it can be like this - full load voltage 6v, open circuit voltage 6.9v (which works out to 15%). the output voltage roughy scales linearly for this example the output voltage at half load would be 6.45v