What is Dimmer Dot

Dimmer Dot

The "Dimmer Dot" is a Variable Transformer and provide continuously adjustable voltage A.C. supplies. In its "Auto Transformer" from (type-D, as manufactured by Green Dot Electric Limited), it is an efficient, reliable method of controlling A.C. voltage from zero to line voltage by amplitude modulation of the A.C. waveform. Since the Current, Power, Heat, Light and Motor speed are further functions of the voltage, the "Dimmer Dot" Variable Transformer finds its varied use. A few of the applications are

  • Control of A.C. voltage in testing and development work.
  • Over voltage and undervoltage tests.
  • Voltage control of aging racks for lamps, vacuum tubes, dry disk rectifiers, etc.
  • Voltage control in power supplies -as for Servo-type, Automatic Voltage Stabilizers, constant current supplies etc.
  • Voltage control for meter calibration, phase-angle control in the calibration of wattmeters and power-factor meters.
  • Motor speed control.
  • Control of electric heaters and ovens in laboratory, pilot plant, and production line.
  • Lighting control in theater, auditoriums, photographic studio and dark rooms.


The versatility of control are further high lighted in a separate publication "Tech Note" on "Dimmer Dot" Variable Transformers. The circuitories there in are only a few of the applications.


The "Dimmer Dot" Variable Transformer, type-D consists of single layer winding on toroidal silicon-steel core. As the control knob is rotated, a carbon traverses the winding, tapping off a portion of the total voltage across the winding. The brush is in continuous contact with the winding, and the voltage between turns is always less than I volt, even in the largest model; in the smallest model it is only about 0.3 volt. The discrete voltage increments obtained as the knob is rotated are always less than the voltage between turns. Since the brush spans more than one turn, the change in voltage is practically continuous. The brush is so designed that excessive heating can not occur in the turns that it spans.


Dimmer Dot have many operating advantages over any other manually operated voltage control device for alternating-current use. Amongst the more important features are: 

GOOD REGULATION - Output voltage is substantially independent of load.

 SMOOTH CONTROL - The "Dimmer Dot" can be set very closely set to any output within its range.

 HIGH EFFICIENCY - Very low losses under all load conditions.

 LONG LIFE - With the exclusive "Dura Dot" contact surface, the life of the "Dimmer Dot" Variable Transformer is essentially the same as that of any standard fixedratio power trans.

LINEAR OUTPUT VOLTAGE - Output voltage is continuously adjustable from 0 to 112% of line by approximately 340 degrees and in certain models it is more in certain models. The dial reads directly in no load output voltage at rated line voltage or some dials are calibrated fo rpercentage of the full range control.

 MODERATE TEMPERATURE RISE - Less than 50 degrees for continuous operation.

 ADVANCED MECHANICAL DESIGN - Rugged construction - no delicate parts-two or more units can be ganged on same shaft-shaft is reversible for simple conversion from


Constructional features as different from other Variable Transformers lie in respect of INSULATION - that the insulation between windings and core is provided by mouldings so as to prevent any possibility of slackening of the turns during use and also to proved for a uniform hard bed under surface for the winding so as to make a strong base and under surface for long life of the track.


WINDINGS-are of high conductivity copper insulated with synthetic enamel of thepolyvinyl acetal-phenol formaldehyde resin type providing good space factor, extremely high abrasion resistance and having a good ability to resist the effects of prolonged heating. The windings are done by toroidal winding machines specially designed and developed by Green Dot during the past number of years to produce Constant winding tension and even pith. The winding are prevented from any possible movement during use by providing serrations on the winding rings and the further fixing the under pressure & use of high temperature bearing epoxies, through a special technique developed over years of experience.


BRUSH MECHANISM-"DIMMER DOT" Transformers are known for their trouble free brush gear. They have produced a self aligning brush mechanism which fulfils the following requirements.

      - Minimum heating and wear

      - Maximum contact area

      - Constant contact resistance and losses 

The temperature rise of brush mechanism and winding in any Variable Transformer, is dependent upon the contact pressure and the area of contact, which in turn decides the current in the short circuit, which in turn decides the current in the short circuited turn and brush face. "DIMMER DOT" brush mechanism have been developed to provide a constant brush contact area. The brush mounting holds the brush vertically in the direction of travel, so as to ensure that the same contact are is presented to the winding at all times with constant pressure. This enables the full contact are to be maintained, with resultant minimum contact temperature. They are provided with extra grip holder, greatly increasing their physical strength and offering great resistance to damage from shock and rough usage. The track cannot be shorted by metal brush holder fittings even in the worst conditions.


FRAMES - the wound core are held strongly and insulated from mounting fixture so as to proved minim high voltage standing of 2.5KV. The economised models are also available for OEM uses. The facilities can be provided for fitting stops and micro switches.


BEARINGS - the units are fitted with bearings, of bush type of nylon or glass filled nylon, for higher sizes, heat and light stabilized unaffected by atmospheric conditions, or sintered modified bronze or sintered bronze, self lubricating type as not to need lubrication through the life time of the product or the ball bearings as optional or the motorised unit. While ganging up the units of two or more numbers, the intermediate bearing guides are removed to provide rotary smooth operation. The Torque figures for various models can be had on request.


GENERAL - the "DIMMER DOTS”-are manufactured in certain fixed sizes determined by the voltage and current ratings. Type-D are manufactured in 0.7, 2, 4, 8, 15, 28, 40, 50, 60, 75, 100, 150 and 200Amps., and voltages of Input 270 Volts for 1Phase with taps at 0-30-240-270 voltages so that for 3 Phase operations, 3 single Phase unit can be connected in Star so as to provide for 470 Volts phase to phase connection facility. 

Invariably all 3 Phase units are ganged assemblies of 3 single Phase units mounted on a common shaft. The facility exists for changing the shaft to make front shaft unit to back shaft unit suitable for back of panel mounting for 4, 8, 15 and 28 Amps units of single phase type. The 3 Phase units of back of panel and mounting have to be placed on supported structure at the back of the panel and the frond shaft can be brought through panel hole, obviously such arrangements can be for the air natural cooled unit. For the oil cooled units the containers have to be placed at the back of the panel and mounted or suitably held by keeping the unit with shaft in vertical position. 3 phase units are not available in 0.7 Amps. All the units are either fitted with screen printed insulating plates with schematic wiring diagram shown thereon or with a connection marking plate fixed there upon. The standard connection diagram and markings on the terminal plates are as follows


BACK OF PANEL - "DIMMER DOT" Units type D, of 4, 8, 15 & 28 Amps. ratings, single phase may be used either surface mounted or back of panel mounted. For ease of packing and transportation, at times, standard models are supplied for surface mounting, but the user can easily convert them to back of panel mounting, simply by removing the shaft locks or clamping rings by simply loosening he screws thereupon. The spindle or shaft is then adjusted after inserting from the other end, spacers/washers are suitably placed so that proper free movement of the moving arm without getting disturbed from the tail end and stopper is obtained with proper usable length of the Carbon from the Carbon Brush Holder available on track. Sufficient pressure from the spring holding the Carbon Brush must be ensured as available on the winding track. 


METRIC CONVERSION - Small screws used in the models at present are B.A. sizes, 2, 4 & 6 B.A. having been used at small paces and conversions are being carried out to metric size. The changes are being carried out in a phase manner & is being intimated by sales department separately. 


TRACK PREPARATION - "DURA DOT" track-" Green Dot " has developed over the years sophisticated track preparation method which provide fine tracks, designed to providing close tolerance surfaces. The optically ground tracks are plated with a hard silver alloy, ensuring constant resistance. After a quarter of million operating cycles, the Brush wear is only approximately 0.001 inch. This will allow for approximately 30 millions operating cycles before Brush wear demands a replacement Brush. "DURA DOT" contact surface is one of the most exclusive achievement made by "Dimmer Dot" for greater reliability, less maintenance and longer life of the "DIMMER DOT" variable Transformers. The contact surface eliminates all oxidation problems involved in the use of a carbon brush and carecopper contact and have the further following features to claim 


LONGER LIFE - essentially the same as that of any fixed ratio transformers. Failures in any "DURA DOT" Dimmer are almost unknown. They are caused only by some factorswhich affect any standard fixed ratio transformer: such as due to failure of insulation because of prolonged over extended periods at currents in excess of rated. HIGH INITIAL SURGE CURRENTS- all Dimmer with "DURA DOT" contact surface will withstand initial surges ten times the rated current; they do not have to derated even for incandescent lamp loads (with may draw up to nine times rated current when first turned on) 

OVER LOADS - the time current curve given in these pages is the safe allowable overload for "DURA DOT" Coated Dimmer-these overloads are considerably higher than the older types of Dimmers. 

LESS MAINTENANCE - under normal conditions, maintenance of "DURA DOTS" is negligible-an occasional wiping of the contact surface with an alcohol-moistened rag being all that is required-the "DURA DOT" contact surface is subject to no deterioration when "DIMMER DOT" is operated with their rated loads.