I have built a dual 24V regulated power supply using two LM317 regulators. The transformer has two primary windings of 0-9V and 0-9V, so I connected the two windings in series to give an AC voltage of 18V, rectified by a full wave bridge (note: not half wave as used in Hicap) and filtered by a single 10000uF cap.
The Hicap does use full-wave rectification, but in the form of 'dual half wave' using a centre-tapped transformer.
For your transformer you will get a raw supply
at the full rated transformer load of: -
(18V x 1.4) - (2 x 0.7V) = 23.8V approx
(1)
The 2x 0.7V is the voltage drop in the two diodes of the bridge rectifier - here you see a potential advantage for the dual half wave circuit - only one diode (and its non-linearities) in series with supply.
Now at lower current drains this voltage will rise by an amount determined by the transformer regualtion figure, let's assume a 10% regulation figure and the raw voltage will be: -
23.8V + 10% = 26.1V approx
(2)
You need to measure the raw DC
under load to determine the accuracy of the estimates in (1) and (2).
All regulators have a
drop-out voltage - this is the minimum input - output differential for correct operation. This isn't a fixed figure either, it varies with current, temperature and manufacturing spread.
Assuming you wish to run something like an old preamp or CD player o/p stage you're in the region of 100mA maximum so using this conservative figure you can look to the manufacturers data sheet.
LM317 Data Sheet
Look at the graph on page 6 labelled 'Dropout Voltage', from this you can see 5 curves, plotting input-output differential against temperature.
We obviously need to look somewhere between the 20mA and 200mA load curves from this you can see at 25 deg C you need a differential of 1.5V
minimum for a 20mA load, rising to approx 1.7V
(3) for a 200mA load.
Good design practice calls for us to be conservative in our design practice - use next highest load line to that you are operating under and factor in the temperature extremes.
In your case you have a raw DC supply that you can't change without changing transformer, so we can work backwards to the acceptable regulated voltage you can achieve for correct operation, the normal route would be to design for a target regulated voltage, then choose an appropriate transformer.
Based on
(1) and
(3) we can see that the maximum regulated voltage one can achieve is: -
23.8V - 1.7V = 22.1V
using
(2) and
(3) it would be: -
26.1 - 1.7V = 24.4V
I suspect the truth lies between the two, but the above will allow you to take measurements and determine the correct values.
I doubt you will get 24V though.
Andy.
