Oct 10, 2012

High Voltage, High Current DC Regulator Circuit Using IC 741

We all are pretty familiar with the 78XX voltage regulator ICs or the adjustable types such as LM317, LM338 etc. Though these regulators are outstanding with their specified functioning and reliability, these regulators have one big disadvantage.... they won't control anything above 35V. The circuit presented in the following article introduces a DC regulator design which effectively counters the above issue and is able handle voltages as high as 100V.
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.

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  1. Great design. Is there any calculation to determine the max output current given the output voltage?
    For instance, I have a 80W FV panel that has a 45V Voc, and I want to lower its voltage before plugging it into the charge regulator, and I was wondering whether this design is suitable under the max sustainable power standpoint.

    1. Thanks!!

      The output current will simply depend on the mosfet rating, so you can select the appropriate one as per your application and get current according to your needs.

      I haven't checked the circuit practically yet but I believe the concept would work as specified.

  2. Sir,

    I need a good and stable ckt which can take input of 42V @ 20amps to charge a battery bank of 12V 400ah. Plz help me in this regards,


    1. kadimi You can try the following circuit:


  3. Tx for the reply., but the issue is the battery bank is only of 12V not 48v. The input would be from 25 to 35v but the battery bank is only 12v. Hope to get a viable one.

    1. OK, then you can try the second circuit from this link:


    2. ...sorry the above link won't be suitable....you will need a BUCK converter circuit for this.

      Please Google "buck converter circuit" you find plenty of them, use the one suits your application the most

  4. Hi I would like to build voltage amplifier as well as regulator using lm741. The input would be 50 mv to 50 volt. The input pulses per min would be 1-30000. The output pulse should be regulated 5 volt at all frequencies and voltage inputs. There would be only +12 volt source for powering the op-amp & not the -12v. i.e the pin #4 will be grounded. the output from pin#6 would go to the micro-controller as it requires 5 volt. Plz if you could build the schematics for me.

    1. you can try the first circuit as given in the following article:


      replace the MIC points with the input frequency and replace C2 with a 7805 IC.

  5. I was looking for a motorbike regulator design and came across this. Has anyone got this circuit to work because I can't see how the op-amp can drive the mosfet gate voltage high enough to the source to turn it off so it would just seem to constantly conduct/pass. A p-channel enhanced mode mosfet typically has a source gate threshold of a 2-4 volts so if the op-amp can't drive the gate to nearly the input voltage, it cant shut it off and the op-amp is powered from a much smaller voltage so it can't get there. There needs to be a pull up resistor to pull the gate to the source voltage and the op-amp needs to drag that down which gives circuit start up issues so it'll never start-up.

    I might have got this wrong but reading about a p-channel mosfet characteristic's on the internet I can't see how this can work at anything other than when the input voltage is very close to the desired output voltage - otherwise it will runaway and always pass the input voltage out to the output

  6. I might be missing something, but after searching the internet for how a p-channel mosfet works I can't see how this circuit could work except when the input voltage is very close to the output voltage (like within 1-3 volts). A p-channel mosfet needs gate to to raise up to the source to shut it off and this design holds it too low to ever stop it from conducting. The op-amp is driven from a much lower source voltage than the mosfet source so it can't shut it off - this design will just uncontrollably leak power from the input to the output. Also the Source/drain assignment compared to the chip drawing is backwards for a P-channel mosfet. Has anyone got this to work?

    1. yes, it's rather a silly mistake in the diagram...the mosfet S/D needs to be reversed, with its source connected with the input supply.

      I have discussed the concept elaborately in the following post, you can refer to it for an in-depth study:


  7. Hello sir help me I m 48 volt DC convert to 18 volt DC and 1am