This supply gives both positive and negative outputs. Appropriate Fuses should be used to protect from fire hazard and overload of transformer.

Voltage Regulators LM7812 and LM317

You could use LM7824 or LM7815 or LM7812 for 24V, 15V and 12V respectively. You could use LM7924 or LM7915 or LM7912 for -24V, -15V and -12V respectively.

Dual Polarity Power Supply

The Filter capacitor C1 4700uF has an impedance of Xc = 1 / (2 * 3.14 * f * C) which comes to 0.6 ohms at 50 Hz.

Power Electronic Circuits

The impedance of the load at 2A for 24V is R = V / I that is 12 Ohms which is more than 20 times the impedance of the capacitor at 50 Hz. That means less than 1 / 20 of ripple current will flow thru the load. The Regulator also reduces the ripple a little.

Voltage multipliers are used to obtain low current high voltage, from an existing AC Source. Step up transformers can be used, but the insulation problems and care that has to be taken for HV above 1kV, make it difficult. After SMPS technologies and Ferrites were developed, HV for CRTs was generated by blocking oscillators and step up Transformers at High Frequency AC.

Mains Voltage and Power Circuits

Voltage Doublers and Multipliers

Caution Instruction –

Use this with Low voltages like 24V AC for Learning. Do not Use it with High Voltage AC. If you are learning, first work with other circuits using batteries or Low Voltage Mains Adapters.
This circuit uses Diodes and Capacitors, just like the text book circuits.

More Reading –

Voltage Multipliers – The Creative Science Centre

The Circuit below is a paper design and not tested. It can be used for education and information, this can help you make your own design. Please do not just wire it up and expect it to work.

Regulated High Voltage Power Supply

Now let me see if i can explain the circuit, This is a regulated AC power supply. This circuit uses the Mosfet to turn off when voltage goes beyond a reference point. That means it just chops the Sine wave above a point, that also implies that the output may not be pure sine and may have harmonics. The Transformer if well designed may smoothen the chops. Even a Series Inductor or Resonant Circuits may reduce harmonics.

The opto coupler 4N50 Provides isolation and good Current Transfer Ratio. That may mean you may not get a shock and that even a small current signal in Opto-LED will give a saturated or Low Impedance in Opto-Transistor. The Mosfet is used like a Impedance Control switch turned On-Off by Opto. The Optocoupler diode is controlled by the Opamps which work Closed loop. The transformer output is compared with reference to drive opto-led.

Regulated High Voltage Power Supply – del20032

This Circuit is based on Teledyne Solid State data book application note. They may not be making these parts anymore but they are available from others.

This LED Indicator uses a LM339, a quad comparator. LM339 can work on single or dual supplies, it has a open collector output that can drive 15mA, low power consumption. The circuit is an untested design but it should work.

Mains Voltage and Power Circuits – Similar circuits for Mains Voltage Monitoring.

There are many better circuits in the various circuit archives i have linked on the front page, you just have to look around. When you measure the open circuit voltage of a battery with a high impedance DMM (10M), the value may be a bit misleading. Apply a dummy load to bleed the battery a bit so that proper readings can be taken on Load. The load below is a 100 ohms wire-wound fusible ceramic resistor which will heat a bit when you test 12V batteries.
Battery Level LED Indicator

Theory of Operation.

R16 a 5W ceramic wire wound bleeder or dummy load. R15 is a part of an attenuator for obtaining ranges. D2 is a protection clamp diode. R10-D1 forms the 5V reference for comparators. Then an attenuator obtains 1.2, 1.4, 1.6, 1.8 V steps for each comparator. This circuit is similar to Audio Level meter or VU meter circuit.

Comparators in Interface Circuits

The comparator compares the battery sample voltage to the fixed reference step. If ‘+’ pin is more positive than ‘-‘, or is ‘+’ is more dominant, then output goes floating ‘open collector’, so No LED light . But if ‘-‘ is more dominant the output transistor of comparator goes low impedance or saturates or turns ‘ON’. But only spec current can be switched, do not compare with electrical switch ‘ON’. Also on a dual supply 0V is more dominant or positive compared with -12V, even though it appears -12V is a big number. The direction of current is what decides, all measurements are relative.