A circuit device for step-by-step control of an electrical voltage

1,124,660. Digital to analogue converters. INGENJORSFIRMAN EL - AUTOMATIK A.B. 23 March, 1966, No. 12936/66. Heading G4H. A digital-to-analogue converter comprises a reference resistor Ra, Fig. 2, across which the output voltage is produced and a group of resistors R 1 -R 12 connected at one end to a voltage source, there being for each resistor a two-way switch O 1 -O 12 arranged to connect the corresponding resistor to the reference resistor Ra or to a further resistor Rb of equal value. The load on the supply is therefore always the same irrespective of how many switches are operated. Two groups of switched resistors may be provided producing signals over resistors Ra and Re connected in series. The supply is taken from the first secondary winding S 1 of a transformer T. A second secondary winding S 2 supplies a measuring bridge B which produces a signal representing, for example, a weight. This is applied to one end of a transformer winding P', while the output from the digital-to-analogue converter is applied to the other. The secondary winding S1 gives a signal proportional to the difference. A series of resistors R 25 -R 34 are provided, controlled by switches, to introduce a bias to compensate for the time taken for the mechanism to respond to the error signal. In the case of an automatic concrete mixer, the digital-to-analogue converter switches may be controlled by a record and the bridge B by the weight of tho ingredients. The hopper flap does not close instantly a balance is achieved, and the switches O 30 -O 34 are adjusted so that it begins to close before a true balance has been reached.