Abstract:
The disclosed power supply is compatible with two voltage ranges, the midpoint or mean of the higher voltage range being approximately double the midpoint or mean of the lower voltage range. Two equivalent loads and two current sources are re-configured using a relay in response to a detected input voltage exceeding a pre-determined threshold voltage. The disclosed circuit re-configures the load being driven to match the input voltage, rather than re-configuring the voltage to match the load.

Description:
BACKGROUND 
     Commercial, emergency, military and passenger vehicles commonly employ direct current (DC) electrical systems operating at voltages of 12 volts or 24 volts DC. It is common for manufacturers to produce electrically driven subassemblies for each voltage range, e.g., a 12 volt product and a 24 volt product. A power supply for use with a known voltage source is typically simple and inexpensive. Alternatively, products designed for use with a wide range of voltages are provided with “switching” power supplies that transform the available voltage using known “buck”, “boost”, “sepic or “buck boost” topology circuit configurations. Switching power supplies are more complex and expensive than a simple DC power supply. Switching power supplies use high speed switching of a transistor and typically generate undesirable RF noise which then must be filtered or suppressed by shielding. 
     There is a need for a simple and inexpensive power supply circuit for motor vehicle electronic sub-assemblies that will allow the electronic sub-assembly to be used with both common motor vehicle voltage ranges. 
     There is a need for a simple and inexpensive power supply circuit for motor vehicle electronic subassemblies that does not generate RF noise. 
     SUMMARY 
     The disclosed circuit is compatible with the 12 volt and 24 volt electrical systems commonly found in motor vehicles. Motor vehicle electrical systems experience significant variation in available voltage depending upon a number of factors, including the state of the vehicle battery, whether the vehicle is running or not, and the electrical load applied to the electrical system. For a 12 volt DC motor vehicle electrical system, the voltage may vary between 11 and 16 volts. For a 24 volt electrical system, the voltage may vary between 22 volts and 32 volts. The disclosed circuit is configured to be compatible with both of these voltage ranges. This is accomplished by employing a relay driven by a threshold voltage detector to reconfigure the load when the applied voltage exceeds a threshold voltage, indicating the circuit is connected to a vehicle employing a 24 volt electrical system. 
     In the disclosed circuit, the load being driven includes two series strings of light emitting diodes (LEDs). LEDs are current-driven devices, so the disclosed circuit includes a pair of substantially equivalent current sources configured to provide regulated current through the LEDs. When the circuit is connected to a 12 vDC electrical system, the relay remains in its de-energized state and the relay contacts connect each string of LEDs between the input voltage and a current regulator. When the circuit is connected to a 24 VDC electrical system, the input voltage exceeds the threshold voltage and the circuit energizes the relay, re-configuring the circuit so that the two strings of LEDs are in series with each other between the input voltage and one of the current sources. In each configuration, the LEDs drop most of the input voltage, with the remainder dropped across a field effect transistor (FET) or bipolar transistor, which regulates current through the LEDs. The disclosed circuit re-configures the load being driven to match the input voltage, rather than re-configuring the voltage to match the load. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURE 
         FIG. 1  is a schematic diagram of an embodiment of the disclosed dual range power supply circuit. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a power supply circuit  10  which allows the same electronic subassembly to be compatible with 12 volt or 24 volt motor vehicle electrical systems. The disclosed power supply circuit is illustrated in the context of an LED light, but may be compatible with other electronic subassemblies. 
     A low voltage loss bridge rectifier  12  at the input  14  provides reverse polarity protection and bi-directional input voltage capability, similar to a standard incandescent bulb. An input voltage threshold detector  16  includes a Zener diode D 6  connected in series with a resistor voltage divider R 6 , R 8 . The base of a transistor Q 4  is connected to the voltage divider R 6 , R 8  so that when the voltage applied to the input exceeds the Zener diode D 6  breakdown voltage (referred to as the threshold voltage), the transistor Q 4  is turned on. Transistor Q 4  conducts, applying voltage to the coil K 1 C of relay K 1 . Relay K 1  has two pairs of contacts K 1 A and K 1 B, which remain in the de-energized or first position shown in  FIG. 1  when the voltage at input  14  (the input voltage) is below the threshold voltage set by Zener diode D 6 , Q 4  is turned off and voltage is not applied to the coil K 1 C of relay K 1 . When the input voltage exceeds the threshold voltage set by Zener diode D 6 , Q 4  is turned on and voltage is applied to the coil K 1 C, which switches the relay contacts from the de-energized state (first position) shown in  FIG. 1  to the energized state (second position). 
     As shown in  FIG. 1 , the de-energized contacts of relay K 1  place the LED loads  18 ,  20  between the input voltage and their respective current sources  22 ,  24 . When relay K 1  is energized, the relay contacts K 1 A and K 1 B change state. In the energized state, relay contact K 1 A connects the first LED load  18  in series with the second LED load  20  which is in turn connected to the second current source  24 . In the energized state, relay contact K 1 B disconnects the input voltage from the second LED load  20  which is now instead connected in series with the first LED load  18  by relay contact K 1 A. In this manner, the disclosed power supply circuit  10  re-configures the load to match the input voltage. 
     In the disclosed circuit, three of the selected high power white LEDs drop most of an input voltage in the 11-16 volt range, with the remainder being taken up by the current source FET Q 6 , Q 1 . The disclosed circuit operates the FED in linear mode and since the voltage drop across the load  18 ,  20  is matched to the input voltage, the FED can operate in a relatively efficient near-saturation mode. If one of the disclosed LED loads  18  or  12  and a current source  22  or  24  were connected to an input voltage in the higher range (22 v-32 v), voltage not dropped over the LED load would be dropped across the current source FED Q 6 , Q 1 . This mode of FET operation would be very inefficient, causing excess power to be dissipated by the FET and likely causing overheating of the transistor Q 6 , Q 1 . Six of the selected LEDs are a better match for the higher voltage range (22 v-32 v), leaving a much smaller voltage to be dropped across the current source FET Q 6 , Q 1  so that a majority of power consumed by the assembly is used to generate light from the LEDs in loads  18 ,  20 . 
     The circuit of  FIG. 1  is configured to detect the applied input voltage and generate a signal when the input voltage exceeds a predetermined threshold voltage. In the disclosed circuit, the predetermined threshold voltage is selected to be greater than the highest voltage typically generated in a 12 volt motor vehicle electrical system, e.g., approximately 18 volts. The disclosed voltage detector includes a Zener diode D 6  in series with a resistance voltage divider R 6 , R 8 . When the applied voltage exceeds 18 volts, the Zener diode D 6  breaks down and begins to conduct, sending current through the resistors R 6  and R 8 , which generates a voltage at the junction of R 6  and R 8 . This threshold voltage signal at the junction of R 6  and R 8  turns on a transistor Q 4  which applies input voltage to the coil K 1 C of relay K 1 , causing the relay contacts K 1 A, K 1 B to change from their de-energized (first) state to their energized (second) state. Relay contact K 1 A changes state, disconnecting the first LED load  18  from its current source  22  and connecting the first LED load  18  in series with the second LED load  20 . Relay contact K 1 B changes state, disconnecting input power from the second LED load  20  and connecting input power to a latch circuit  26 . The latch circuit  26  includes a 17 volt Zener diode D 5  in series with a resistor R 5  and a capacitor C 2 . This latching circuit maintains transistor Q 4  turned on so long as the input voltage is above 17 volts. The disclosed voltage detector  16  and latch circuit  26  ensure that once the input voltage rises above 18 volts and relay K 1  is energized, transistor Q 4  will remain turned on and the relay K 1  will remain energized until the input voltage falls below 17 volts. This circuit configuration prevents bouncing or chatter of the relay during start up and shut down of the circuit, which can diminish the life span of the relay K 1 . 
     Each current source includes a FET Q 6 , Q 1  and a transistor Q 5 , Q 2  arranged to regulate current through the FET. It should be noted that the regulating transistors Q 2  and Q 5  and the current source FETs Q 6 , Q 1  are arranged on the same printed circuit (PC) board as the LED loads  18 ,  20  in the disclosed circuit configuration. The selected regulating transistors are temperature sensitive, so that increasing temperature causes a reduction in current through the FET and the LEDs. This arrangement exposes the current regulators  22 ,  24  to the same temperature as the LED loads  18 ,  20  and automatically reduces current through the LEDs when the assembly temperature approaches temperatures which could damage the LEDs. 
     The disclosed circuit  10  also includes a high voltage shutdown  30 , which employs a 36 volt Zener diode D 11  and voltage divider R 9 , R 4 . Input voltage in excess of 36 volts causes Zener diode D 11  to break down and conduct, resulting in voltage at the junction of R 9  and R 4 . This shutdown voltage at the junction of R 9  and R 4  turns on transistor Q 3 , which effectively grounds the gate of Q 1 , shutting off the second current source FET Q 1  when the input voltage exceeds 36 volts. This prevents the circuit from being damaged by high voltages. 
     The disclosed circuit  10  provides a protected and durable electronic assembly which can be installed in 12 or 24 volt vehicle electrical systems, eliminating the need to manufacture separate assemblies compatible with these voltages. 
     An embodiment of the disclosed power supply is described with reference to the drawing. Variations of the disclosed embodiment may become apparent to those skilled in the art upon reading the foregoing description. The appended claims are intended to encompass all modifications, variations and equivalents of the disclosed subject matter.