High Current Transformerless Power Supply Circuit

The simple configuration of a transformerless power supply circuit presented below is able to provide high current at any assigned fixed voltage level. The idea seems to have solved the problem of deriving high current from capacitive power supplies which earlier seemed a difficult proposition.

Introduction

I have discussed a few transformerless power supply circuits in this blog which are good only with low power applications, and tend to become less effective or useless with high current loads.

The above concept utilizes high voltage PP capacitors for dropping the mains voltage to the required level, however it is unable to raise current levels as per any desired particular application.

Although, since the current is directly proportional to the reactance of the capacitors, means the current can be lifted just by incorporating more capacitors in parallel.

But this puts a risk of high initial surge currents which might destroy the involved electronic circuit instantly.

Adding Capacitors to Increase Current

Therefore adding capacitors might help to increase the current specs of such power supplies but the surge factor must be first taken care of for making the circuit feasible for practical usage.

The circuit of a high current transformerless power supply explained here hopefully, effectively handles the surge developing from power transients such that the output becomes free from the dangers, and provides the required current supply at the rated voltage levels.

Everything in the circuit is kept just as its old counterpart, barring the inclusion of the triac and zener network which actually is a crowbar network, used for grounding anything that goes above the rated voltage.

In this circuit the output would hopefully provide a stable voltage of around 12+ volts at around 500 mA of current without the dangers of any accidental voltage or current influx.

CAUTION: THE CIRCUIT IS NOT ISOLATED FROM MAINS AND THEREFORE INVOLVES HIGH RISK OF ELECTROCUTION, APPROPRIATE PRECAUTION NEEDS TO BE EXERCISED.

UPDATE: A better and a more advanced design can be learned in this zero crossing controlled surge free transformerless power supply circuit

Parts List

• R1 = 1M, 1/4W
• R2,R3 = 1K, 1/4 WATT
• C1----C5 = 2uF/400V PPC, EACH
• C6 = 100uF/25V
• All DIODES = 1N4007
• Z1 = 15V, 1 watt
• TRIAC = BT136

A neatly drawn PCB for the above high current transformerless power supply may be seen below, it was designed by Mr. Patrick Bruyn, one of the avid followers of this blog.

Update

A deeper analysis of the circuit showed that the triac was dumping a significant amount of current while restricting the surge and controlling the current.

The approach taken in the above circuit for controlling voltage and the surge is negative in terms of efficiency.

In order to obtain the intended results as proposed in the above design and without shunting precious amps, a circuit with exactly opposite response needs to be implemented, as shown above

Interestingly, here the triac is not configured to dump power rather it's wired in a such a way that it switches OFF power as soon as the output reaches the specified safe voltage limit, which is detected by the BJT stage.

New Update:

In the above modified design the triac may not conduct properly due to its rather awkward positioning.

The following diagram suggests a correctly configured version of the above, which can be expected to operate as per the expectations.

In this design we have incorporated an SCR instead of a triac since the positioning of the device is after the bridge rectifier and therefore the input is in the form of a DC ripples and not AC.

Improving the above design:

In the above SCR based transformerless power supply circuit, the output is surge protected through the SCR, but the BC546 is not protected.

In order to ensure a complete protection for the entire circuit along with the BC546 driver stage, a separate low power triggering stage needs to be added to the B546 stage. The amended design can be seen below:

The above design can be further improved by modifying the position of the SCR as shown below:

So far we studied a few transformerless power supply designs with high current specs, and also have learned regarding their different modes of configurations.

Below we would go a little farther and learn how to make a variable version circuit using an SCR. 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:

Circuit Operation

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.

The Crowbar Network

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.

Circuit Diagram

Another SCR based Circuit

This SCR based high current Transformerless Power Supply consists of rather a few, easily accessible electronic parts.

The output voltage level (and also the level of feasible current as provided in the Table I) is adjustable by rotating the rotary switch S1.

Table#1

Zero Crossing Controlled High Current Transformerless Power Supply Circuit

The following circuit shows how a zero crossing concept can be implemented to create an effective transformerless high current power supply circuit, which is highly customizable.

Advantages of this Circuit are as follows:

100% surge free ensures that the load, zener diode and the capacitor are completely safe all the time regardless of the input switching conditions of the power supply.

No heat dissipation ensures that the circuit's efficiency is maximum.

How the Circuit Works

We know that the main issue with transformerless power supply circuit is the switch ON surge current, which happens due to the sudden peak AC entering the electronic circuit connected with the power supply.

This sudden in-rush voltage and current leads to the burning of the vulnerable electronic components attached with the power supply.

This means that, if the load is allowed to be switched ON whenever the AC waveform nears the zero crossing then such mishaps can be avoided.

The above circuit does exactly this.

The NPN TIP122 conducts only when the AC waveform is below the zener value.

When the TIP122 conducts the AC waveform is already within the safe range of the load and this safe voltage gets stored in the 1000uF capacitor for powering the load.

The process continues for each cycle and only when the AC peak has dropped down safely to the zener value, which keeps the load powered consistently, with an optimized voltage and current inputs.