Zener Voltage Regulator

Question: I have quite an extensive model train layout with many devices which operate by battery power.  As a result there is a need to use many types of batteries of all types and sizes. Very often I know what the voltage requirement of the device but do not know the current being drawn when the device is turned on. How can I measure the current being drawn?  I do not have an ammeter to use to make the measurement.  John, Brooklyn, N.Y.

Answer: You can measure the current that the batteries are drawing, performing the following steps:

1)    Determine what battery or batteries are presently used.

For example: 9 volt, 1.5 volt AA, 1.5 volt AAA

2)    If the device uses one 9 volt battery, one must remove the battery from the battery holder and make the following connection shown in the figure. One can use clip leads to attach the 9 volt battery positive (+) terminal to either side of a 1 ohm resistor. Then connect the other side of the 1 ohm resistor to the positive (+) terminal in the devices battery box. Then connect the negative (-) terminal of the battery box to the negative (-) terminal on the battery.

1)    Using the hook-up shown in the figure, use a digital voltmeter to measure the voltage across the one ohm resistor,then use Ohm’s law to calculate the current that the 9 volt device is drawing during its operation.

2)    Ohms Law states the Current = Voltage/Resistance

3)    For this example,  if the digital voltmeter, reads 20 millivolts, then the current = 20mv/1 ohm = 20 ma, then a 9.1 Zener Voltage Regulator can be built to supply the voltage in lieu of the 9 volt battery. Use a power supply that is at least 3 vdc higher than the Zener diode voltage.

Details using BZX79-C9V1 500 mw Zener Diode (Reference Internet for zener diode data sheet)

 Given the following:

1) Diode Voltage = 9.1 vdc +/- 0.1 vdc

 2) Test Current  = 5ma = Best operating Zener Current (available from data  sheet for the BZX79-C9V1 (9.1 vdc zener)

 3) Load  is the device with the original battery box which  requires 20ma based previous measurement on device.

This load is across the 9.1 vdc zener. See above figure.


 I total = I zener + I load                    Kirchoff’s Laws


I total = 5 ma + 20 ma = 25 ma.

 R1 resistance value = Voltage across R1/I total as per Ohms Law


 R1 = 12-9.1 = 2.9 vdc/25 ma = 116 ohms where (12-9.1)  is application of  Kirchoff’s Law

 9.1 volt Zener has a BZX79-C9V1 part number.

Data sheet indicates that the maximum power rating = 500 mw.

 Maximum Zener Current = Max Power/Zener voltage

                                       o r

0.5 watt/9.1 vdc = 55 ma  with load (Battery Box) disconnected.

 With Battery Box  removed, Zener power = 9.1vdc x 5ma = 45.5 milliwatts

 This choice of diode is perfectly safe even with a +/- 0.1 vdc variation of voltage tolerance and subsequent power change.

Notice that what we have achieved is the building of a  Zener Diode Voltage Regulator to replace the battery pack within the original battery box using the very basic Zener Diode Voltage Regualtor Circuit shown above.

The Battery voltage source to the Zener diode can actually be a variable or fixed voltage power pack  whose DC voltage is three to five volts greater than the Zener’s voltage.  In general, the following vendors are recommended for any of the above electronic components. :

Radio Shack – Local Store or on-line at

Futurelec at

If you wish to acquire a better understanding of Electronics Theory, I suggest you go to  the following  link: Electronics