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The BEAM Circuits Collection is a BEAM Reference Library site.

The Zener-based solar engine
Using a Zener diode as a voltage sensor

The Zener solar engine is, as its name implies, a simple type 1 solar engine based on a Zener diode. This is the original solar engine design, by Mark Tilden, no less!


How it works (simplified)
The capacitor charges until the PNP transistor (here shown as a 2N3906, but you could also use a BC327) receives base current through the Zener and turns on. Then the NPN transistor (here shown as a 2N3904, but you could also use a BC337) turns on and the capacitor is discharged through the motor. As the NPN turns on the 2.2K resistor starts to supply base current to the PNP and the circuit snaps on. When the capacitor voltage drops below about 1V, the the PNP turns off, the NPN turns off and disconnects the motor from the capacitor which starts to charge up again.
The voltage across the capacitor rises slowly as it is charging from the output of a solar cell. This voltage also appears across the Zener in series with the PNP base emitter junction. The 2.2 KOhm resistor is connected  in series with the motor to the cap and both are in parallel with the base of the PNP. When the voltage across the Zener rises above the Zener voltage, it starts to conduct.  Now a trickle of current passes through the 2.2K resistor and the motor and until the current rises to 250uA, the voltage drop at the base of the PNP is less than 0.6V. At that point the PNP base voltage is high enough for the PNP to start to turn on. This applies current to the base of the NPN transistor , which then provides a direct motor current path. As the NPN collector voltage drops to 0V, the current through the  2.2K resistor reverses and starts to supply the base current for the PNP, taking the Zener diode essentially out of the circuit. The motor draws current until the voltage on the storage capacitor is down to about 1V.

Note that you can replace the Zener diode with one or two diodes in series (trip voltage = 0.5V times number of diodes), or with LEDs in series (trip voltage = 1.4V times number of LEDs):


Wilf Rigter's comments on this SE:
The problem with this simple SE design is that it only works with just the right components, the most important of these being the motor. If the motor is too big or inefficient it will not work at all. If the motor is just slightly out of range of the required parameter you may be able to get it to work here by replacing the 2.2K resistor with a 10K pot and adjusting it to get reliable operation. Once set up the pot can be replaced with an equal fixed value resistor. The circuit as shown generally works OK with a 30 Ohm motor over a narrow range of light conditions.

SE designs using FLEDs work better, and using a 1381 [see the 1381-based, "Miller," and VTSE designs -- Ed.] makes the solar engine much more tolerant to motor types.


Wilf also provided a much more detailed explanation of just how this SE design functions:

Without feedback these simple SE circuits would really be simple. But with feedback and despite the simple design, the SE circuit operation is really quite complex. Let's walk through the circuit and build up a mental model of the operation of the Zener SE circuit to understand just what those electrons are up to. There are two kinds of feedback, negative and positive in the SE circuit and as you might guess, they tend to fight each other. Let's change the circuit slightly to separate the two kinds of feedback and discuss them one at a time.


Assume the 2.2K resistor is not in the circuit (i.e. no connection):

As long as both transistors are off the cap keeps charging up from the current of the solar cell toward the maximum available voltage. When the voltage on the cap rises to the reverse breakdown voltage of the Zener in series with the PNP base / emitter junction, then the PNP "starts" to come on.

Now without the 2.2K resistor the circuit will sit on the delicate balance of negative feedback.

The PNP collector amplifies the base current from the Zener by about 50 times. This 50x amplified Zener current starts to flow in the NPN base which then amplifies the Zener current in the NPN collector by about 50 times for a total amplification gain from PNP base current to motor of 50 x 50 = 2500. When the NPN turns on it "starts" to discharge the capacitor; as this happens, the voltage on the capacitor drops, which lowers the Zener current, which lowers the PNP collector current, which lowers the NPN collector current. This is the stabilizing or balancing effect of negative feedback. In fact, the circuit smoothly reaches equilibrium when the NPN comes on just enough to dump 98% of the short-circuit current (Isc) of the solar cell; the remaining 2% flows through the PNP and Zener.

So without the 2.2K resistor, the effect of the Zener conduction would stop there: the NPN collector current through the motor would be exactly equal to Isc. If that Isc is less than the minimum current required to turn the motor, nothing else happens except for the small voltage drop across the stalled motor winding from the NPN collector current. This circuit is used in other applications as a Shunt Voltage Regulator, as it regulates the voltage across the cap.



With the 2.2K resistor connected to the NPN collector the circuit behavior is more complicated and first we look at how it affects the SE triggering process.

When the Zener starts to turn on the Zener current flows through the 2.2 K resistor and the motor winding and generates a voltage at the base of the NPN equal to V = I x R (ohm's law). But no current can flow through the PNP base until the base / emitter voltage is about 0.55 V. That means you need a minimum current of I = E / R or about 0.25 mA through the 2.2 K resistor before any current even starts to flow in the PNP base.

If the solar cell can't deliver this 2.5 mA current through the 2.2 K resistor, the circuit operation stalls there. This problem is often traceable to the fact that the current that the solar cell can deliver drops off rapidly when the voltage approaches the maximum solar cell voltage. So the solar cell must have both the right current and voltage for a specific SE. A larger resistor (i.e. 10 KOhm) may help here but at a price as noted later. An alternative solution is to add a diode in series with the 2.2 K resistor with the anode connected to the NPN collector. Yet another way to improve the initial turn on is to add a small (.22 uF) cap across the 2.2 K resistor to amplify the switching voltage without interfering with the DC characteristics.

If the solar cell can generate enough current to overcome the first obstacle of the voltage drop across the 2.2 K resistor, the PNP starts to turn on and provide base current for the NPN and the NPN collector current starts flowing through the motor winding which causes a voltage drop.

Now comes the magic of positive feedback.

When the NPN collector voltage starts to drop from the voltage across the motor winding, that reduces the voltage and current in the 2.2 K resistor which causes more of the Zener current to flow into the PNP base which increases the NPN current and lowers the collector voltage even more. This causes a rapid escalation in current flowing through NPN and the motor winding. All of this occurs rapidly but is not instantaneously since the switching process is a race between positive feedback and negative feedback, as the additional current in the NPN causes a voltage drop in the photocell voltage which then reduces the Zener current, etc.

So we need a bit more than just 0.25 mA to overcome the negative feedback part. So if things don't stall in the previous step, the positive feedback takes over as the NPN collector voltage drops below the Zener voltage, the voltage and current in the 2.2 K resistor actually reverse (so instead of draining current away from the PNP base it starts to supply extra PNP base current). This results in even more NPN current but also starts to drop the voltage on the capacitor since it now needs to supply most of the current flowing through the motor.

You can see now the importance of a capacitor that has a low internal resistance and can supply current quickly without much voltage drop.

It is also important that there is sufficient NPN base current supplied to cause the NPN collector voltage to drop to saturation (low voltage drop) and operate in the nonlinear region. This makes the SE more efficient and reduces instability during triggering. This means that the 2.2 K resistor must be optimized depending on the gain of the transistors and the motor load current. A 10 K potentiometer can be used to adjust this base current for best operation, being replaced with a fixed resistor after the correct resistance is found.

Since the motor is an inductor and inductors are electromagnetic devices which resist rapid change in current, this helps speed up the positive feedback and switching of the SE, since the voltage on the NPN collector can drop rapidly without an instantaneous change in collector current.

If all is well and the positive feedback won the race, the SE is "latched on" like an SCR until the voltage on the capacitor drops below two base / emitter voltages (1.2 V) at which point the PNP and NPN base currents approach zero.

Positive feedback is also required to successfully reset the SE and start a new cycle and this is the second obstacle that a successful SE design must overcome. Similar to the race between positive and negative feedback above, it is important that the transistors turn off rapidly but several factors tend to prevent this.

Ironically, if the solar cell short circuit current is high there may be enough base current to keep the transistors on and prevent the SE from resetting. The symptoms are initial voltage rise on the cap, then trigger and then the voltage on the cap remains below the trigger voltage until the charging current is interrupted, i.e. by blocking the light.

When the transistor base currents drop the NPN will come out of saturation and the rising collector voltage will reduce the current through the 2.2 K resistor which turns off the PNP which turns off the NPN. A cap across the motor can reduce the rate at which this happens, and interfere with the SE reset.

One more comment -- sometimes a capacitor is placed in parallel with the motor winding. Remember that an inductor in parallel with a capacitor forms a resonant circuit in which at one frequency the signal losses approach zero and small voltage oscillations build up to become large oscillations. Add some transistors and feedback and there is a tendency for the circuit operation to stabilize around this particular instability, generating acoustic noise and vibration in the motor windings instead of motor rotation.

It should be pointed out that stray capacitances have the same effect and that as long as the oscillation is short duration and leads to SE trigger or reset it can be advantageous to the circuit operation. Although Zener type SE circuits have problems with capacitors across the motor winding, FLED SE circuits seem to prefer it and 1381-based SEs [see the 1381-based, "Miller," and VTSE designs -- Ed.] generally don't care.

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Page authors: Wilf Rigter and Eric Seale
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