Can I use Zener diod in Series?

[polashd]

I want to use Zener diode in series (like the design in the attached pic).

My intention is to get Vout= Vz1+Vz2

Is it possible?

If possible, what would be the power rating of the Zener diodes and current limiting Resistor.

can you give me formulas for this circuit to calculate Watt, Vout, I, etc.

For example:
Vs=32v
Vout=28v
Imax=20mA
D1=15v
D2=13v

 

[Gary B]

Yes you can but, it is a crappy way to do it. The zeners have to drop the difference between what the power supply can deliver and what the load can use. The usual way is to use the zener to establish a fixed voltage to be applied to the base emitter junction of a transistor. Its collector drives a power transistor that does the serious power dropping. Your spec does not specify the possible range of input voltages and the range of output currents.

For instance, you say that the maximum output current is .020 amps. If your load needs .020 amps, then the zeners need only drop enough power to remain in their operating range (you did not specify the current/power range of the zeners you intend to use). If you turn off the load then the zeners have to dissipate all of the power the load would have or .56 watts. If I recall correctly, the most common zener power specification for generally available units is ¼ watt. The 15 volt device doesn’t stand a chance if the load disconnects and the 13 volt will also kick the bucket; it will just take a little longer to do it.

I’m sure some of the other guys will give me a hard time for even admitting that it can be done that way. This is really a bad idea; it makes more sense to use a 7824, 78M24 or 78L24 3-pin fixed output voltage regulator. They all take an input between 27 and 40 volts and will provide 1A, 0.5A or 0.1A output current. All that is needed is a couple of filter caps, one on the input and one on the output.