(From the First Southern California Astrophotography Seminar, March 15, 1975)

A SIMPLE POWER CONVERTER
FOR TELESCOPES

R. CHANDOS

Many circuits have been published for converting l2V D.C. sources (such as automobile batteries) to 115V 60 Hz or variable frequency AC for powering synchronous motors of telescope clock drives. Most of these circuits are relatively complex, and require the use of large electrolytic capacitors, integrated circuits or other components not readily available to the amateur astronomer. The included circuit overcomes some of these problems and also provides excellent frequency stability and tolerance of supply voltage and impedance variations. Parts layout and construction are non-critical, and substitution of recommended parts by available (junk box) types is possible.

The circuit operates as a R/C multivibrator, with transistors Ql and Q2 forming a differential comparator. A positive feedback signal is provided to the base of transistor Q2 via resistor R3, and a negative feedback signal is provided to the base of transistor Ql via R4. The delay caused by the R4/C1 combination forces the circuit to operate in a bi-stable fashion, with oscillation frequency dependent upon the value of R4 and C1. Comparator switching points are set by the resistor voltage divider R1 and R2 and positive feedback resistor R3. For this reason, resistors R1, R2, R3, and R4 should be 1% types (preferably metal film which has excellent temperature characteristics) and C1 should be of the paper or polycarbonate type. Transistors Q3 and Q4 are required to increase the output current of the comparator circuit to a level sufficient to drive the output transistors Q5 and Q6 into saturation, generating a 60 Hz square wave at the primary of the output transformer.

Transistors Q1 and Q2 should be high beta NPN silicon type such as 2N2484 or 2N930, Q3 and Q4 should be of the medium power PNP silicon type such as 2N2905, and transistors Q5 and Q6 should be power devices such as 2N3055. Motorola hobby type substitutes for the above types are available. The output transistors should be mounted on a small heat sink or the chassis box even though they will operate barely warm. Some 24V filament transformers commonly available are designed to operate very close to core saturation, and if used as the output transformer, may cause Q5 and Q6 to operate quite warm 150°F). This causes excess current to be drawn from the automobile battery, and if it is objectionable, the transformer can be re-wound increasing the number of turns by 1-1/2 times while retaining the same 10-1 primary to secondary turns ratio.

A synchronization input can also be incorporated to cause the oscillator frequency lock on to an external source such as a crystal oscillator (for extreme frequency stability) or a voltage controlled oscillator as required for automatic star tracking.

Periodic synchronization pulses of approximately 60 Hz can be inserted through a series resistor and coupling capacitor into the base of transistor Q2. Exact values for the resistor and capacitor can not be given here because they are a function of the amplitude and shape of the synchronization pulse.

The circuit will operate clock drives requiring less than 15 watts and with component value changes and an increased size output transformer can be made to drive motors requiring 50 watts or more.

NOTE: When I built this circuit, R4 had to be 22K Ohms to get close to 60 HZ. You need to experiment with your circuit after it is built to find the resistor value that gives you the exact speed you want. The double/normal speed circuit was not in the original 1975 schematic, I added it along with an on/off switch to get 2X guiding.