I am a great admirer of the above mentioned types of ICs simply because they are easy to understand easy to configure and require bare minimum number of components, and are also relatively cheap to build.
However in areas where input voltages can be higher than 35 or 40 volts, things become difficult with these ICs.
While designing a solar controller for panels which produces in excess of 40 volts, I searched a lot over the net for some circuit that would control the 40+ volts from the panel to the desired output levels, say to 14V, but was quite disappointed as I couldn’t find a single circuit which could fulfill the required specifications.
All I could find was a 2N3055 regulator circuit which couldn’t supply even 1 amp current.
Failing to find a suitable match I had to advise the customer to go for a panel that would not generate anything above 30 volts…that’s the compromise the customer had to make using a LM338 charger regulator.
However after some thinking I could finally come up with a design which is able to tackle high input voltages (DC) and is a lot better than the LM338/LM317 counterparts.
Let’s try to understand my design in details with the following points:
Referring to the circuit diagram, the IC 741 becomes the heart of the entire regulator circuit.
Basically it has been set up as a comparator.
Pin#2 is provided with t a fixed reference voltage, decided by the value of the zener diode.
Pin#3 is clamped with a potential divider network which is appropriately calculated for sensing the voltages exceeding the specified output limit of the circuit.
Initially when the power is switched ON, R1 triggers the mosfet which tries to transfer the voltage at its source (input voltage) across the other side of its drain pin.
The moment voltage hits the Rb/Rc network, it senses the rising voltage conditions and within a fraction of a second the situation triggers the IC whose output instantaneously goes high, switching off the mosfet.
This instantly tends to switch OFF the voltage at the output reducing the voltage across Rb/Rc, prompting the IC output to go low again, turning ON the mosfet so that the cycle locks in and repeats, initiating an output level that’s just exactly equal to the desired value set by the user.
The values of the unspecified components in the circuit may be calculated by the following formulas and the desired output voltages may be fixed and set up:
Ra = 0.2 * Rb (k Ohms)
Rb = (Output V – Z2 voltage) * 1k Ohm
Rc = Z2 voltage * 1k Ohm.
The mosfet is a P-channel, should be suitably selected which can handle the required high voltage, high current in order to regulate and convert the input source to desired levels.
The maximum output voltage should not be set above 20 volts if a 741 IC is used. With 1/4 IC 324, the maximum output voltage can be exceeded up to 30 volts.