We have already seen quite a few transdormerless power supply designs in this blog, and also have learned regarding their specific applications.
In this article we would go a little further and learn how to make a variable transformerless circuit. The explained design not only provides the option of getting a continuously variable output but is also surge protected, and therefore become much reliable with its intended functions.
The circuit can be understood from the following description:
The left side section of the circuit is quite familiar to us, the input capacitor along with the four diodes and the filter capacitor forms the parts of a common, unreliable fixed voltage transformerless power supply circuit.
The output from this section will be unstable, prone to surge currents, and relatively dangerous to operate sensitive electronic circuits.
The portion of the circuit on the right side of the fuse transforms it into a completely new, sophisticated design.
It's in fact a crowbar network, introduced for some interesting functions.
The zener diode along with R1 and P1 forms a kind of voltage clamp which decides at what voltage level the SCR should fire.
P1 effectively varies the zener voltage from zero to its maximum rating, so here it an be assumed to be zero to 24V.
Depending upon this adjustment, the firing voltage of the SCR gets set.
Supposing P1 sets a 12V range for the SCR gate, as soon as mains power is switched ON, the rectified DC voltage starts developing across D1 and P1.
The moment it reaches the 12V mark, the SCR gets sufficient triggering voltage and instantly conducts, short circuiting the output terminals.
The short circuiting of the output tends to drop the voltage toward zero, however the moment the voltage drop goes below the set 12V mark, the SCR is inhibited from the required gate voltage and it reverts to it non conducting state.... the situation yet again allows the voltage to rise, and the SCR repeats the process making sure the voltage never goes above the set threshold.
The inclusion of the crowbar design also ensures a surge free output since the SCR never allows any surge to pass through to the output under all circumstances, and also allows relatively higher current operations.