Electronic apparatus for biological research Electronic apparatus for biological research electronicappara00dona Year: 1958 OTHER DESIGNS OF RELAY Polarized relays are particularly suitable for use with 'chopper' amplifier circuits (Chapter 39). For this application special types are available with platinum contacts for low noise, and with screening between the coil and the contacts to minimize spurious pick-up. Chopper relays should be of the each-side stable type, giving good contact pressure with high speed of operation; chopping frequencies of up to 300 c/s can be used. A chopper relay s
Electronic apparatus for biological research Electronic apparatus for biological research electronicappara00dona Year: 1958 OTHER DESIGNS OF RELAY Polarized relays are particularly suitable for use with 'chopper' amplifier circuits (Chapter 39). For this application special types are available with platinum contacts for low noise, and with screening between the coil and the contacts to minimize spurious pick-up. Chopper relays should be of the each-side stable type, giving good contact pressure with high speed of operation; chopping frequencies of up to 300 c/s can be used. A chopper relay should be adjusted when running in its final circuit by observing the contact waveform on an oscillograph. A particularly sensitive form of polarized relay is based on a moving-coil meter movement (Chapters 32 and 33) with contacts instead of a pointer; such a relay may operate on less than 1 ^aW. If an appreciable load is to be switched, an intermediate relay is essential as the contacts of moving-coil relays have a very low power rating. Moving- coil relays are both delicate and expensive, and in most applications can be replaced by a transistor and an ordinary relay (see Chapter 29). There are, however, a few purposes for w hich the use of a moving-coil relay is essential. Ordinary non-polarized relays can be made to operate only on current of one polarity with the aid of a rectifier {Figure ). Naturally none of the f ' , j f f f ' I I ^' I I I I ^^ I (a) (b) (0 (d) Figure Rectifier-polarized relay: (a) and (6) constant-current feed; (c) and {d) constant-voltage feed other advantages of the polarized relay is thereby obtained. Conversely, a polarized relay can be operated by a current of either polarity if it is con- nected in the circuit of Figure Figure Operation of polarized relay on current of either polarity OTHER DESIGNS OF RELAY Many different designs of relay exist besides those already discussed. In all cases the fundamentals described for
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