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Publication Title | Low Voltage Battery Disconnect

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Homebrew

Low

Voltage

Battery

Disconnect

Automatic Battery Shutoff

For Medium Power DC Loads

G. Forrest Cook ©1997 G. Forrest Cook

Lead acid and NiCd batteries will last a long time if they are operated within the proper charge and

discharge voltages. A charge controller circuit is a necessity for preventing battery over-charge. Conversely, a low voltage disconnect circuit (LVD) prevents excessive battery discharge. By using a combination of both circuits it is possible to keep the battery operating within the proper range.

This article describes a low to medium power LVD which operates like a common on-off toggle switch. The circuit is very efficient, consuming a mere 8 milliamperes while running and essentially no power when off. The LVD was designed to use commonly available parts. The prototype was built entirely from junk-box parts.

Theory

The heart of the LVD is the power MOSFET transistor, Q1. Transistor Q1 operates as a switch in the positive line of the external circuitry. Switching of the positive side of the circuit allows for a common negative ground between the battery and the load which aids in many applications, especially automotive ones. To achieve this “high side” switching, it is necessary to generate a gate drive voltage that is higher than the supply voltage. This is accomplished by a voltage tripler circuit. Op- Amp U1b generates a 5 Khz square wave. This is fed into the diode/capacitor ladder circuit which successively boosts the voltage to about 3 times the peak voltage of the square wave.

This signal is then used to gate on the power MOSFET. Resistor R5 is used to discharge the gate circuit when the LVD is shut off, allowing the MOSFET to turn off.

The voltage comparator circuit consists of U2, a standard 5 Volt regulator which is used as a voltage reference, U1a wired as a comparator, and VR1, a voltage divider to provide a set point for the low voltage shutoff. When the battery voltage is above the threshold U1a provides a positive output which is used to create a bias level via the R2/R3 voltage divider, that allows the U1b oscillator to run. When the battery voltage drops below the set point the output of U1a goes to zero and the U1b oscillator shuts off, causing the voltage tripler and MOSFET to shut down. Resistor R7 gives the comparator circuit some hysteresis to prevent comparator oscillation near the shutoff voltage. The circuit is analogous to a solid-state latching relay in that it shuts its own power off when the MOSFET turns off. This is achieved with diode D6 and the on-off switch. When the circuit is switched on capacitor C11 acts like a momentary short-circuit, pulling the op-amp Vcc line up to the battery voltage. The whole circuit fires up long enough to turn the MOSFET on, after which operating current flows through the MOSFET and diode D6.

When the switch is shut off the comparator is forced off by shorting pin 3 to ground. This turns off the MOSFET. Resistor R6 prevents switch S1 from shorting the VCC directly to ground when potentiometer VR1 is set to VCC. Capacitor C11 is discharged through the other half of the switch and current limiting resistor R8. Discharge of the capacitor is required for circuit start-up the next time the switch is turned on. Diode D7 is used to protect the MOSFET from negative spikes generated by motors or other inductive loads. Capacitors C7, C8, C9, and C10 provide filtering in various parts of the circuit.

Fuse F1 protects the circuit from overload and should be a fast blow fuse that is rated at about 80 percent of the maximum current that Q1 can handle. Numerous MOSFET transistors can be used for Q1. Parts selection is based on cost and maximum current. The IRFZ34 MOSFET is rated at 30 Amps continuous current and should be used for switching heavy loads. A lower power MOSFET such as an IRF520 may be used for up to 8 Amp loads. A heat sink and thermally conductive grease should be used on the MOSFET unless the load current is always kept under a few amps.

An alternate method of switching the circuit on and off is to replace the DPDT switch S1 with a pair of momentary push buttons for separate on-off controls. The “on” push button connects between the Q1 Drain and the D6 cathode. The “off” push button connects between U1A and ground. If push buttons are used, capacitor C11 and resistor R8 may be left out of the circuit.

Homebrew

38 Home Power #60 • August / September 1997

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