9v Power Supply

This article is a continuation of our 3v to 5v POWER SUPPLY article where we introduced 3 power supplies that produced a 5v regulated output from 3v.
Two of the power supplies can be purchased on eBay as modules for less than the cost of the individual components and we take advantage of these bargains.

MODULE 1:
The first module is a Phone Charger and can be purchased for about $2.00 incl postage:
Pocket Portable Cellphone Emergency Charger

This article shows how to “Jack-up” the output by providing a voltage-divider on the output to produce 9v. The voltage-divider actually comes in the form of a 5v1 zener on top of a 5v detector within the chip. An actual resistor-voltage-divider would consume current when idle and the resistance values would need to be quite small as the chip needs a relatively high current when operating.
These 9v power supplies will take the place of a 9v battery and when using two AA cells, they will last 3 times longer and cost much less.
The results of our tests were better than expected. The voltage hardly dropped when up to 200% current was delivered and the only point to remember is the high current taken from the battery due to the step-up-voltage situation, when even 30mA is delivered.
The only problem with this type of circuit is the noise it produces. It operates at about 200kHz and if the output is connected to an AM radio, it produces a lot of hash in the background. Only loud stations can be received. However this is not the case with FM reception and these supplies can be connected to FM receivers.
Jacking up the output is not an easy thing to do.
The aim is to create a divider that consumes no current when the load is removed. And we have achieved this with the modified PHONE CHARGER circuit in a very clever way.
Here is the original Phone Charger:

The original circuit-board and socket:

The socket is removed by touching the soldering iron on each leg FROM THE TOP and pulling the leg from the board.

The original top-side of the PC board

The 3 components are then added to the PC board:

The SS14 Schottky diode is turned 90° to allow the 3 extra
components to be fitted to the board

The wiring of the extra components

The modified Phone Charger to produce an output of 9v

These circuits are classified as LOW IMPEDANCE circuits and this means the input needs to have a capacitor so the circuit can “push against the capacitor” via the positive rail and produce the required output. If a capacitor is not present, the circuit relies on the low-impedance of the battery. (The output needs to have a capacitor to store the pulses and deliver a DC output.)
This means you cannot put a milliammeter (this is the correct wording for a milli-amp meter) in the positive (input) line and expect the circuit to work correctly. The high-resistance of the meter will cause the output to decrease considerably - because the circuit simply will not work. It needs a low-impedance supply because it requires a very high current due to the fact that the output voltage is being multiplied about 3 times - and the current is further multiplied by two because the inductor is being "charged" for 50% of the time and delivering for 50% of the time. To maintain a low-impedance supply while taking current-readings, a 1 ohm resistor (brown-black-gold-gold) is placed in the positive line and a multimeter (200mV or 2v range) is connected across the resistor. This means each mV reading will represent 1mA current.
For the PHONE CHARGER circuit, the idle current was less than 0.1mA and could not be read on the meter. The output voltage dropped to 5v (in idle mode) and kept the 1u ceramic capacitor charged.
To turn-on the PWM controller chip requires a reasonably–high current for a very short period of time. This is achieved by the 10u tantalum capacitor and when the chip draws current, a voltage of 5.1v is dropped across the zener. The 10k resistor is needed to quickly discharge the 10u if the power is removed and re-applied. If the 10k is removed, it takes more than 10 seconds to discharge the 10u so it can re-apply the spike of current.
We placed a 47R across the output to check the regulation. It got HOT (200mA) but the output voltage remained at 10v. This is classified as “100% regulation” and is an amazing achievement for microscopic components.
The photo shows the surface-mount components added to the top of the PC board. The SS14 Schottky diode is turned 90 degrees so the additional components can be fitted. This is shown in detail in the second photo above.
If you are connecting a 9v battery snap to the output, remember it is being used “in reverse” and the negative lead must be connected to the positive output on the board so the snap can be connected to another snap on a radio or other device needing 9v.

MODULE 2:
The second module is a 3v to 5v PWM boost circuit with an output current of 500mA (or more under ideal conditions).
It can be purchased on eBay for less than $3.00 posted:

DC-DC Power Supply Converter Step Up Boost Module 1A 3V to 5V

It can be "jacked up" to 12v by changing the value of R1 or R2.

There are a number of modules that look exactly like the photos above but the voltage-divider will have different values.
Our module had 20k (203) and (180) where the "180" surface mount resistor was a special value and measured about 150k.
By increasing the value to 270k the output voltage was 8.6v and 330k created an output voltage of 10.5v. You can adjust this value by using a higher value and placing a 1M or 2M2 on top or altering R2. Don't omit R1or the output voltage will rise above 30v.
FB is the "Feedback" pin or Vreference pin and it needs to see about 1.2v. Rather than going into the formula to determine the output voltage, it is much easier to change the values of R1 and R2 and measure the output.
Increasing R1 will increase the output voltage. And placing a 220k across the 20k resistor will increase the output from 8.6v to 9.4v. It is much easier to put a resistor across R2 than replace R1.

The 1206 surface mount resistor was the only size available and a small length of fine tinned copper wire was needed to join one end to the pad. A 0805 surface mount component would fit perfectly.

The module fails to regulate if the input voltage falls below 2.5v and this will limit the output current when 2 cells are used as the output voltage will very easily dip to 2.5v if a high current is drawn. The input voltage must be 2v below the output voltage, so a 6v supply could be used for 9v output.

A point to note: This module drives an AM radio without any background hash but the FM reception is impossible to receive.

MODULE 3:
The output voltage of this module could not be "jacked-up."
Any attempt at increasing the output voltage resulted in the chip getting very hot.

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17-6-2013