Hey, I came across this Low Volatage Shunt regulator on a Hobby RC forum, I am trying to figure out how the resistor Values are Calculated,


the one shown here, uses a TL431 regulator and a 2N700 transistor connected with Lithium Polymer batteries of value 11.1v or 7.4v, the circuit trips the alarm at a particular voltage in this case it is 9v when an 11.1v battery is used and 6v when a 7.4v battery is used. If anyone can help this would be super awesome thanks


2 Answers 2



R1 is chosen to set Vref on the TL431 to its nominal value of 2.495V. The output of the TL431 is dependent on the values of R1 and R2.

R4 is chosen to protect the Piezo from seeing more than 7V which is its limit. To figure out the value of R4 you have to consider the voltage drop of Vds on the FET and the current the piezo draws. From this calculate R4 to produce enough of a voltage drop to keep the voltage across the piezo to 7V or less.


1 ... Mark's answer is correct but may not fully answer your question. The following provides a formula for calculating R2 for a given Vbattery.

  • R2 = (Vb - 2.5) x R2 / 2.5

See below for details.

2 ... The circuit as shown is "dangerous" because it takes advantage of a "shortcoming" of the 2N7000 MOSFET which is not present in all MOSFETS.
Substitution of a FET with a lower turn on gate voltage (Vth or Vgs_th) may lead to an alarm that is always on or that draws some current at all times.

Unlike a bipolar transistor or MOSFET the TL431's Cathode (positive terminal) is NOT pulled to or even near ground voltage when the device is on. A typical specification sheet advises that anode voltage of a TL431 will fall to approximately no lower than it's gate voltage of 2.5V when turned on. In practice the voltage may be almost a diode drop below this (about 1.9V) but no data sheet ever advises this and it cannot be relied on.

With 2.5V applied to its gate a 2N7000 MOSFET is turned off. Typically 3V is required to cause it to conduct somewhat and 4V or more is needed for reasonable conduction.

Datasheet: http://bit.ly/DS_2N7000

The TL431 changes from non conducting to conducting when its gate is raised to or above its internal reference voltage of 2.5 Volt. Vb (Vbattery) is divided by RA & R2 and applied to the gate.

  • Vgate = R2/(R1+R2) x Vb

OR when the gate is JUST at the trip point

  • R2 = (Vb - 2.5) x R2 / 2.5

The writer has nominated 6V as the low battery point for 2 cells and at 9 volts when 3 cells are used. The "best" trip point depends somewhat on loading. Heavily loaded cells have a lower loaded voltage for a given state of discharge. I would personally suggest a slightly higher trip point unless cells are heavily loaded if maximum cell life is desired.

Sensibility check:

Plug the available data into the above formula to see if it gives his answers.

R1 = 2.49k, Vb = 6V,

  • R2 = (Vb- 2.5) x R2 / 2.5

  • = (6-2.5) x 2.49k / 2.5 = 3.49k

He has chosen the next highest standard value of 3.57k .

So results are the same. QED.

Note that the required tolerances are somewhat less than implied by the precision of the resistors used.


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