Solar Relay

With extended periods of
bright sunshine and warm weather, even relatively large storage
batteries in solar-power systems can become rather warm. Consequently, a
circuit is usually connected in parallel with the storage battery to
either connect a high-power shunt (in order to dissipate the excess
solar power in the form of heat) or switch on a ventilation fan via a
power FET, whenever the voltage rises above
approximately 14.4 V. However, the latter option tends to oscillate,
since switching on a powerful 12-V fan motor causes the voltage to drop
below 14.4 V, causing the fan to be switched off.

In the absence of an external load, the battery voltage recovers
quickly, the terminal voltage rises above 14.4 V again and the switching
process starts once again, despite the built-in hysteresis. A solution
to this problem is provided by the circuit shown here, which switches on
the fan in response to the sweltering heat produced by the solar
irradiation instead of an excessively high voltage at the battery
terminals. Based on experience, the risk of battery overheating is only
present in the summer between 2 and 6 pm. The intensity of the sunlight
falling within the viewing angle of a suitably configured ‘sun probe’ is
especially high precisely during this interval.

This is the operating principle of the solar relay. The trick to
this apparently rather simple circuit consists of using a suitable
combination of components. Instead of a power FET,
it employs a special 12-V relay that can handle a large load in spite
of its small size. This relay must have a coil resistance of at least
600 Ω, rather than the usual value of 100-200 Ω. This requirement can be
met by several Schrack Components relays (available from, among others,
Conrad Electronics). Here we have used the least expensive model, a
type RYII 8-A printed circuit board relay. The
light probe is connected in series with the relay. It consists of two
BPW40 phototransistors wired in parallel.

Solar Relay Circuit

Solar Relay Circuit Diagram

The type number refers to the 40-degree acceptance angle for
incident light. In bright sunlight, the combined current generated by
the two phototransistors is sufficient to cause the relay to engage, in
this case without twitching. Every relay has a large hysteresis, so the
fan connected via the a/b contacts will run for many minutes, or even
until the probe no longer receives sufficient light. The NTC
thermistor connected in series performs two functions. First, it
compensates for changes in the resistance of the copper wire in the
coil, which increases by approximately 4 percent for every 10 ºC
increase in temperature, and second, it causes the relay to drop out
earlier than it otherwise would (the relay only drops out at a coil
voltage of 4 V).

Depending on the intended use, the 220-Ω resistance of the
thermistor can be modified by connecting a 100-Ω resistor in series or a
470-Ω resistor in parallel. If the phototransistors are fastened with
the axes of their incident-angle cones in parallel, the 40-degree
incident angle corresponds to 2 pm with suitable solar orientation. If
they are bent at a slight angle to each other, their incident angles
overlap to cover a wider angle, such as 70 degrees. With the tested
prototype circuit, the axes were oriented nearly parallel, and this
fully met our demands. The automatic switch-off occurs quite abruptly,
just like the switch-on, with no contact jitter.

This behaviour is also promoted by the NTC
thermistor, since its temperature coefficient is opposite to that of
the ‘PTC’ relay coil and approximately five times as large. This yields
exactly the desired effect for energising and de-energising the relay: a
large relay current for engagement and a small relay current for
disengagement. Building the circuit is actually straightforward, but you
must pay attention to one thing. The phototransistors resemble
colourless LEDs, so there is a tendency to think that their ‘pinning’ is the same as that of LEDs,
with the long lead being positive and the short lead negative. However,
with the BPW40 the situation is exactly the opposite; the short lead is
the collector lead. Naturally, the back-emf diode for the relay must
also be connected with the right polarity. The residual current on
cloudy days and at night is negligibly small.

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