zener
I want to design a simple voltage regulator circuit using a zener diode along with a resistor to protect the diode from excess current. This is the classic zener circuit you see everywhere.
The diodes are unmarked and I don't have a spec to reference. But I know how to find the forward and breakdown voltages of the zener using a power supply and a multimeter.
Now I just need to determine the operating range of current for the diode so I can properly size the resistor. How would I do this?
It's just the basic operating principle of a diode. An ideal diode would allow current to flow in one direction but block current flow in the other direction. This is based on how it's made, with a p-type region, an n-type region, and a depletion zone in between. Like the bottom diode in this picture:
When you apply a some voltage, in your case 0.9V then the p-type holes and the n-type electrons move into the depletion region because they are repelled by their respective battery terminal. With enough voltage (0.9V in your case) the free electrons in the depletion region get moving and current begins to flow like this:
Now in the ideal case if you were to reverse that battery the opposite will happen and you'll get no current flowing:
In the real world though you can only apply so much reverse voltage or push before you hit the breakdown voltage and current begins to flow freely in the reverse direction. Zener diodes take advantage of this fact and are constructed to breakdown at lower voltages such as your 3.3V.
Sources:
You can read more about how zeners are made here
Or see the article I got all the pictures from here
How to choose
The first part is consideration for the instantaneous power dissipation: \$24 * 80mA\$ As long as your Zener can handle that it will not die immediatly.
The next is the total energy.
The Inductance is key here & to a lesser degree its resistance.
The use of a zener ensure a constant voltage during decay as oppose to an exponential if it was to naturally decay via a freewheel diode & the coils own resistance.
\$V = L\frac{\Delta i}{\Delta t}\$ V = Zener Voltage L = coil inductance di = 80mA
Leaving dt
Now you know the energy stored in the coil: \$E = \frac{1}{2}Li^2\$. This energy needs to be transferred from the coil to the Zener.
With the time required from the previous calculations you now know the total power that needs to be dissipated \$P = \frac{E}{\Delta t}\$
With the power for one dissipation known you can then start short listing Zeners
Finally ... duty. Do you want the zener to be chosen such it can just handle one discharge event and then takes time to cool or do you need to tolerate a minimum enable-disable duty. Equally how many short succession enables-disables.
This increases the rms power that the zener needs to tolerate
Best Answer
Trial and error. Start by finding the more-or-less zener voltage by using a power supply AND A 100 OHM RESISTOR. Now measure the dimensions of the zener. Go to a website such as Digikey.com and do a product search on zener diodes of the same voltage as yours. Call up the data sheets for various wattage zeners until you find one which matches yours in terms of package size, and figure that this is probably the wattage rating for your diode.
Now examine data sheets for various zeners of your voltage and power rating and determine a characteristic operating current. Now determine the range of possible load currents which your regulated voltage must supply. If the current range is comparable to the nominal operating current you are in luck. If not, you may be in trouble.
Find the nominal voltage which will be dropped by the resistor, which is the power supply voltage minus the zener voltage. Size the resistor so that it will nominally provide the maximum load current plus the nominal zener current at the specified voltage. If the load current range is large, do the same thing but be sure to determine the worst-case power dissipation for both the zener and the resistor. It is entirely possible that your zener is not adequate to the job.
The key is to realize that, since the dropping resistor is fixed, the zener must adjust its current to keep the total current (zener plus load) constant, so that the voltage across the resistor stays pretty much the same.
Example 1- A 12 volt supply and a 5 volt load. Load current is in the range of 20 to 40 mA. The zener is 5.1 volts with a package compatible with 500 mW, and a standard example would be a 1N5231B, with a nominal current of 20 mA. At maximum load, total current is 60 mA, so the resistor value will be $$R = \frac{V}{i} = \frac{7}{.06} = 117 \text{ ohms} $$ Resistor power is $$P = i^2R = (.06)^2\times117 = .42 \text{ watts} $$ At minimum load the resistor current must be the same (since the voltage drop is the same) but now the zener draws 40 mA, and the zener power is $$P = Vi = 5\times.04 = .2 \text{ watts} $$ which is well under 500 mW. The resistor should be a 1/2 watt unit.
Example 2 - Same power supply and zener, but the load current is now 20 mA to 100 mA. At max load current the resistor current is 120 mA, so $$R = \frac{V}{i} = \frac{7}{.12} = 58 \text{ ohms} $$ and $$P = i^2R = (.12)^2\times58 = .84 \text{ watts} $$ while at minimum load current the zener current is 120 mA, and $$P = Vi = 5\times.12 = .6 \text{ watts} $$ and the power levels are too high to use your zener.