There are many fixed-voltage IC regulators available such as those with 5V, 6V 8V, 9V, 12V & 15V outputs. But what if you want a voltage output that does not fit into one of the standard ranges or if you want to be able to easily adjust this output voltage?
The MiniReg is the answer: it can be set to provide the exact voltage you require. It’s based on an LM317T 3-terminal regulator. The PCB has only a few other components: three diodes, three capacitors, two resistors and a trimpot to set the output voltage from the regulator.
Fig.1: the circuit diagram of the MiniReg
Fig.1 shows the circuit details. The LM317T adjustable regulator provides a nominal 1.25V between its OUT and ADJ (adjust) terminals. We say it is a “nominal 1.25V” because, depending on the device, it can be anywhere between 1.2V and 1.3V. This doesn’t really matter though, because we can adjust the output voltage to the required level using the trimpot VR1.
The output voltage from REG1 is set by the 110Ω resistor (R1) between the OUT and ADJ terminals and by the resistance between the ADJ terminal and ground.
This works as follows: by using a 110Ω resistor and assuming an exact 1.25V reference, the current flow is set at 11.36mA. This is calculated by dividing the voltage between the OUT and ADJ terminals (1.25V) by the 110Ω resistor. This current also flows through trimpot VR1.
This means that if VR1 is set to a value of 1kΩ , then the voltage across this resistor will be 1kΩ x 11.36mA or 11.36V. This voltage is then added to the 1.25V reference to derive the output voltage – in this case 12.61V.
In practice, the current flow out of the ADJ terminal also contributes slightly to the final output voltage. This current is of the order of 100μA. So if VR1 is set to 1kΩ , this can add 0.1V to the output, ie, we get 12.71V.
If you are interested in the output voltage equation, then it is:
VOUT = VREF(1 + R1/R2) + IADJ x R2
where VOUT is the output voltage, VREF is the voltage between the OUT and ADJ terminals and IADJ is the current out of the ADJ terminal (typically 50μA but as high as 100μA).
R1 is the resistance between the OUT and ADJ terminals, while R2 is the resistance between the ADJ terminal and ground.
Diode D1 in series with the input provides reverse polarity protection. This means that if you connect the supply voltage around the wrong way, you cannot do any damage.
Diode D2 protects the regulator if the input becomes shorted to ground while it is powered up. Without D2, current would attempt to flow back from the output capacitor through the regulator to the shorted input and that could kill it. But D2 becomes forward biased and conducts, effectively preventing any reverse current flow through REG1.
Diode D3 is also included to protect REG1. It does this by clamping the voltage between the ADJ terminal and the OUT & IN terminals in the event that one of the latter is shorted to ground.
Finally, capacitors C1 & C2 reduce ripple and noise by bypassing the IN (input) and ADJ terminals respectively. C3 prevents regulator oscillation by swamping any low-value capacitance that may be connected to this output.