Marko Mäkelä’s electronics projects: 5 Volts from a Dynamo Hub with a Low-Dropout Rectifier and Linear Regulator

Reducing the Voltage Dropout

In a conventional full-wave diode rectifier bridge (Graetz circuit), the current always flows through two diodes. There will be a voltage loss of around 0.7 V across each diode. It is possible to replace the four diodes of the rectifier bridge with MOSFETs, whose on-resistance is mere milliohms.

Similarly, the voltage loss across the LM7805 can be 2 V, four times that of special low-dropout regulators, such as the LM2940T-5.0. The voltage loss in the simple rectifier and LM7805 circuit should be 3.4 V, while the circuit presented here should only lose about 1 V.

Schematic Diagram

The schematic diagram and the board layouts on this page have been produced with CadSoft EAGLE 4.16.

If this circuit were mass-produced, it would probably make most sense to integrate it within a LED headlight. In the simplest form, the power LED and its power switch would be connected between +UB (the anode of D3) and ground.

Principle of Operation

Synchronous Rectifier

Jürgen Heidbreder, also known as JuergenH on the German MTB-News.de forum Elektronik rund ums Bike, built an ingenious LED power supply circuit for bicycle dynamos. The original circuit consisted of a MOSFET rectifier bridge and a power LED that can sink all the current that the dynamo is able to source. Later, two zener diodes were added for protecting the MOSFETs from voltage spikes. The left part of our schematic diagram up to the anode of D3 is a copy of that circuit. In the headlight, the LED would be connected between +UB and GND.

In the presented synchronous rectifier bridge, if the rectified voltage (at +UB) exceeded the absolute value of the input voltage, the current would flow backwards, from the circuit to the coil of the dynamo. This means that we cannot connect a capacitor directly between +UB and ground. The Schottky diode D3 prevents the unwanted current flow, but it also causes a loss of approximately 0.5 V.

It is possible to eliminate D3 by building an active synchronous rectifier that would only switch on the MOSFETs when the input voltage exceeds the output. Such a circuit was presented by Wolfgang Schubert in Elektor 6-7/2006: Power-MOSFET-Gleichrichter (Power MOSFET Bridge Rectifier). The circuit adds a quad op-amp and 14 resistors, some of which require very small tolerance. This would make the circuit much bigger and more expensive.

Low-Dropout Regulator

The rectified voltage is fed to the 1 mF capacitor C1. Thanks to the zener diodes D1 and D2 that clamp the voltage to 7.5 V and the Schottky diode D3 that drops 0.5 V, the voltage across C1 should never exceed 7.0 V. The regulator (IC1) would tolerate up to 26 V, and the maximum gate-source voltage of the IRF7319 (T1 and T2) is 20 V. However, zener diodes with a lower clamping voltage were selected, so that C1 can be made smaller due to a lower voltage rating (10 V) and the diodes will have more time to dissipate the excess peaks of the alternating input voltage. The first prototype used 18 V zener diodes (1N5355B) and a 25 V capacitor.

The LM2940T-5.0 requires fairly stable input voltage and a low-ESR output capacitor. Because the maximum voltage is derated in high temperatures, the output capacitor C2 was chosen to be rated for 10 V, although its maximum voltage should be 5 V. The circuit should work from -40°C to 85°C, but this has not been tested.

Bill of Materials

D1, D2

1N5343B zener diode, 5 W, 7.5 V

D3

10MQ040N or B140 Schottky diode, 1 A, 40 V reverse voltage, SMA

T1, T2

IRF7319 dual MOSFET, SO-8

C1

1000 µF 10 V miniature electrolytic capacitor

C2

47 µF 10 V low-ESR tantalum chip capacitor

IC1

low-dropout voltage regulator LM2940T-5.0 (TO-220) or LM2940IMP-5.0 (SOT-223)

other

some connectors, wire and heat-shrink tubing for connecting and protecting the circuit

Circuit Board Layouts

There are two circuit board layouts for the TO-220 case of the LM2940T-5.0 voltage regulator: with and without a thermal connection. The regulator IC1 and its input capacitor C1 are on the top side, and all other components are on the bottom side.

The voltage regulator IC1 is also available in a space-saving SOT-223 package, the LM2940IMP-5.0. On the top corners of this tiny board, there are two mounting holes for the zener diodes D1 and D2 for over-voltage protection. The cathodes of the diodes are to be connected together outside the circuit board. The anodes are mounted to the board corners. In the picture of the assembled circuit, the anode leads have not been cut, but simply bent downwards parallel to the board. The AC voltage input can be connected to these leads. The holes for the voltage output leads are on the bottom edge, interleaved with the mounting holes of C1, the large input capacitor of the voltage regulator.

It is advisable to bend the body of C1 parallel to the board. In the picture of the assembled board, C1 has been bent on the top side of the circuit board, above the two IRF7319 dual MOSFETs T1 and T2. In this way, the assembled circuit will fit inside a round tube whose inner diameter is about 16 mm and length is about 27 mm, not counting the uncut leads of the zener diodes D1 and D2. If C1 is bent the other way, next to the circuit board, the circuit would likely fit inside a tube whose diameter is 11 mm and length is 52 mm.

Most components are surface-mounted. The rectifier bridge comprising T1 and T2 is on the top side. The voltage regulator IC1 and its support components D3 and C2 are on the bottom side.

The tiny circuit board can also be used as a rectifier for a self-made LED headlight. In that case, all solder-through components and all surface-mounted components on the bottom side can be omitted. Because the dynamo is a constant current source and a power LED is a constant current sink whose maximum operating current exceeds that of the dynamo, the zener diodes for over-voltage protection can be smaller: 500 mW in an SOD-80 (Mini-MELF) surface-mounted package. The mounting pads for such diodes are at the top edge of the circuit board, on both sides. The current output +UB for the LED anode is right below the top left corner, and the LED cathode will be connected to ground, at the bottom edge of the board. It is also possible to connect a power LED and a switch to the full circuit.

If the size and the durability of the circuit are not of concern, one could connect a 5 to 6 volt rechargeable battery pack in parallel to C1, similar to some commercial products.

Performance

The charging current of Nokia mobile phones is proportional to the input voltage and the state of the battery. The minimum voltage is about 4 V, the same as with many USB devices that are equipped with a rechargeable Li-ion battery. The charging current ranges from below 100 mA to over 500 mA in the voltage range of 4 V to 5 V, depending on the device and the state of the battery.

In practice, the charging process of old Nokia DCT-3 series mobile phones will continue even at 5 km/h. However, the charging will stop when riding slow with the headlight switched on. With a test load that switches on when the output voltage exceeds 4.2 V, I got 370 mA at 15 km/h, 420 mA at 20 km/h and 440 mA at 30 km/h.

The maximum voltage defined in the USB-IF Battery Charging Specification, Revision 1.1, 15^th April, 2009 is 5.25 V, which is just within the tolerance 5.00±0.25 V of the LM2940-5.0.

The navigator Garmin GPSMap 60CSx is equipped with a battery compartment for two AA cells. It works both with rechargeable NiMH cells as well as single-use cells. The operating current at minimal display brightness is about 120 mA. When the device is connected to a USB host, the current on the battery drops to about 200 µA. In other words, the battery life will extend from less than a day to some years. There is no charging circuit inside the device, and it was not yet established whether this device requires some USB host identification before drawing power.

The navigator Garmin Edge 705 is equipped with an internal battery. The device draws up to 500 mA when charging the battery and less than 80 mA when the battery is fully charged, no matter how the display brightness has been set. I have not been able to measure the current on the battery so far. During the 3 years of usage, the battery has always been almost fully charged after a ride, even with the lights switched on.

The main advantages of this circuit are small size, low cost and low electromagnetic interference. The drawbacks are intermediate performance (a switching regulator can do better) and the waste heat production at low or no load. The MOSFETs do not tolerate much voltage. Therefore, the zener diodes for over-voltage protection must kick in at a small voltage, heating their surroundings when the headlights are turned off and no devices are charging.