active power buffer with temperature
Frequently,a directvoltage isrequired ina circuitthat mustbe free ofhum and noise,but not regulated.For instance,the supplyto an audiooutput stage mustbe able to vary with themains voltage and theload. Another exampleis when in the workshopa variable-ratiotransformer (variac) isto produce a well-filtereddirect voltage withgood loading capacityfor general purposes.
The hum emanating from an unregulatedpower supply is normally causedby too high a current drain or toosmall a reservoir capacitor. Enlargingthe capacitor is often the simplest, butnot always the most effective, way ofdealing with the problem. Hum is themanifestation of a ripple on the outputvoltage and it is best to suppress thisin an active manner.looks like that of a conventional seriesregulator, but has no regulating amplifierwith control comparator.
Therefore,the output voltage automaticallyadapts itself in accordance with theinput alternating voltage and the currentthrough the load. Consequently,the entire hum voltage is applied to thecollector-emitter junction of darlingtontransistor T3. The advantage of such anarrangement lies in a drastic reductionof the maximum dissipated power (atthe highest mains voltage).The lower part of the figure is atemperature monitor in which T4 is thesensor. If this transistor detects an overtemperature,the monitor circuit pullsthe base of driver T2, and thus that ofT3, to ground. This effectively cuts offthe output current, so that no morepower is dissipated.