I highly doubt the "
SCR can't turn on till 20 volts theory. Is it the
choke limiting
current draw?
This is a problem created by the nature of the firing
circuit which derives its
power from the series connection with the load. Inside a
SSR is a photocuppler. Basically, a photosensitive transistor or photosensitive
triac is wired through a full wave rectifier,
current limiting
resistor and
capacitor to the gates and MT2s of a pair of back to back SCRs inside a potted module. Enough
voltage must be present between MT1 and MT2 for this
circuit to operate. Because this
circuit exists between the
line input and the load output, in cannot operate with 0 volts across it as the waveform crosses between halves of the AC waveform. If the firing signal came across the photocuppler at the 0
voltage cross
point, the
SSR would not
switch into conduction. So, the firing signal is delayed until there is some
voltage across MT1 to MT2. Because of this, the front portion of the output waveform is missing. This squaring off on the front of each half can cause problems with some electronic and magnetic ballasts, as well as electronic
power supplies.
In addition, some dimmers regulate the maximum
voltage output to a
preset level such as 120 volts. If the input
voltage is higher (lets say 127 volts) the
dimmer control
circuit would automatically dim the output to produce the equivalent of 120 volts even when set at 100%, further distorting the waveform.