Only a handful of components are needed to build an instrument which is indispensable in any RF workshop: the grid dip meter or ‘dipper’. The main function of this clever piece of test equipment is to determine the resonance frequency of unknown tuned circuits within a certain range (typically, 1.5 to 80 MHz). Apart from this, the dipper doubles as an RF signal generator and an absorption frequency meter. Some amateurs even use it to repair radios or tune short-wave antennas!

The operation of the grid dip meter is based on the principle of energy absorption. An oscillator produces RF energy through an inductor. When this inductor is brought in the vicinity of another inductor with the same resonance frequency, the latter ‘draws’ energy via inductive coupling. Because the oscillator is equipped with an RF output level indicator, the energy loss is easily detected as a ‘dip’ when the instrument is tuned, and hits upon the resonance frequency of the unknown inductor. The name ‘grid dipper’ is historic, and a remnant of the days when this instrument was built using valves (electrically, the nearest equivalent of valve grid is the gate of a FET).

The oscillator in the grid dipper may be modulated with a fixed tone to make its signal easily identifiable on a short-wave tuning scale (where chaos may reign).

Basic Circuit and Operation

The heart of the circuit is an RF oscillator. The oscillator is usually a Colpitts design. In other words, the tuned circuit has a capacitive tap. Other oscillators, for example, the Hartley oscillator, use a tap on the inductive element. A capacitive tap is made by connecting two capacitors in series, and then in parallel with an inductor. The junction of the two capacitors forms the capacitive tap.

The capacitive part of the tuned circuit is formed by two series- connected halves of variable-capacitance diode (‘varicap’). Each coil gives a different frequency range, and is plugged into a socket. The tuning in each range is accomplished by varicap.

The adjustable voltage required for the tuning of the varicap is supplied by a potentiometer. So, turning this pot tunes the grid dipper across the relevant frequency range.

Unfortunately, the sun may not shine just when you need electrical energy. The reverse is also true: energy may not always be required when the sun supplies plenty of it! In addition to the solar cell array, a stand-alone system requires another important component: an electrical energy storage device.

The first device that comes to mind for this function is the battery, which is available in many different shapes and structures. Apart from special battery types, including chloride-zinc, iron-sulphide, lithium, nickel-iron, silver-zinc and sodium-sulphur, a number of which are still under development, familiar types such as the lead (gel) acid, NiCd and NiMH batteries are widely used for this purpose. These batteries feature a high and fairly constant capacity, nearly loss-free current acceptance and delivery, and extended durability despite many charging/discharging cycles. Lastly, they are almost maintenance free.

The lead plates of special batteries for solar systems have selenium or calcium doping instead of antimony as used in car batteries. These special batteries are marked by high cycle repeatability, excellent charge efficiency, low self-discharging, high immunity against deep discharging and overcharging, and, unfortunately, a high price! Of course, there are exceptions. Some online stores sell top products for low prices. Check before you buy!

Just as with solar cells and modules, batteries may be connected in parallel or in series. When doing so, it is essential to use batteries of the same type, with the same capacity, nominal voltage and charge condition. You can also connect the batteries in parallel and in series (few batteries in series and then few such lines in parallel). But this increases voltage and current that is needed to charge them.

Reverse Current Protection

The reverse current protection diode prevents the battery from discharging itself via the solar cell. Irrespective of its operating principle, the control circuit avoids overcharging of the battery. A deep-discharging protection should be integrated in all cases.

A single crystalline solar cell supplies a no-load voltage of about 0.6 V, independent of its size. A cell made from amorphous silicon produces a slightly higher voltage of about 0.8 V. Under normal circumstances, i.e., assuming a normal cell size of 10×19 cm, the power output is relatively low at 1.2 to 1.4 Watt. Consequently, cells have to be joined into solar panels (or modules) before usable currents and/or voltages become available. As with batteries in a torchlight, cells are connected in series to obtain a higher output voltage.

Conventional solar modules supply a no-load voltage of between 15 V and 22 V, which indicates that they consist of up to 40 series-connected solar cells.

To raise their output voltage, solar cells may be connected in series. Similarly, parallel connection may be used to raise the output current. In many modules, cells are connected in parallel as well as in series.

The size of the cell surface determines the maximum output current, which is usually indicated as the short-circuit current (i.e., at an output voltage of 0 V). Available versions range from small amorphous cells with an output current capacity in the micro-amps range, right up to square-metre size modules from monocrystalline silicon with an output short-circuit current rating of more than 5 A. Several identical modules may be connected in parallel to obtain a higher output current. The output voltage then equals that of a single cell. Finally, it is also possible to resort to a combined parallel-serial configuration.

Strictly speaking, 20 cells connected in series should be sufficient to charge a 12 V battery. In practice, however, a solid margin should be designed into such a system. Unfortunately, the output voltage of a solar cell is not constant. In fact, it drops with increasing temperature, and decreasing brightness of sunlight. This effect is far more pronounced with polycrystalline cells than with monocrystalline types. Because of this, the voltage characteristics of the relevant cells or modules should be studied before a solar power system is planned and built. To achieve the highest possible output power, the cell or module should be operated at the so-called maximum power point, MPP, at which the electrical output power reaches its maximum. The MPP shifts with light intensity and cell temperature.

Inside a module, the individual cells are connected in such a way that the lower part of a solar cell is always connected to the upper part of another cell. Professional modules constitute a symmetrical glass assembly with a layer structure: melting adhesive foil, solar cells, melting adhesive foil, glass. High-end frames consist of stainless V4A steel.