Protection circuit for detachable operating unit used in audio device

A detachable operating unit for connection with a main unit comprises at least two power supply units for applying operating voltages to the detachable operating unit, the power supply units generating two different voltages, respectively; at least two power supply terminals for engaging the detachable operating unit and the main unit to receive two different voltages applied to the power supply units, respectively; a ground terminal for engaging the detachable operating unit and the main unit; a protective circuit for applying a reverse voltage in repsonse to a differential voltage between voltages of the power supply units, the protective circuit being disposed between one of power supply units which generates the operating voltage lower than that of the other power supply unit, and the ground terminal.

BACKGROUND OF THE INVENTION 
The present invention relates to a protection circuit for a detachable 
operating unit used in an audio device. More specifically, it relates to a 
protection circuit for a detachable terminal such that when the detachable 
operating unit is connected to a main audio device, the main audio device 
applies two different voltages to the detachable operating unit by two 
power supplies through two contact points, respectively. 
In an automotive audio device for an automobile such as a car stereo, a car 
compact disk player and a car radio, a detachable operating unit 3 having 
an input key unit 1 and a liquid crystal display unit 2 is detachable from 
a main audio device 4 as shown in FIG. 2. When the car is stopped or 
parked, the detachable operating unit 3 may be separated from the main 
audio device 4 to prevent the audio device from being stolen. 
The detachable operating unit 3 comprises an IC circuit 5 controlling the 
input key unit 1, the liquid crystal display panel unit 3, and an 
illumination lamp 6 for illuminating a surface of the liquid crystal 
display unit 2. 
An operating voltage VDD of the IC circuit is, for example, +5 V at a 
relatively low level. On the other hand, an operating voltage ILL of the 
illumination lamp 6 is, for example, +14 V at a relatively high level. 
Therefore, both operating voltages could not be supplied from a single 
power supply unit. As a result, the operating voltages are supplied by two 
power supply units through power supply terminals 7a and 7c, respectively. 
A contact terminal 7b is used for the ground (GND). 
However, in the case where the detachable operating unit 3 having such a 
structure for the operating voltages VDD and ILL separately applied to the 
detachable operating unit 3 is engaged with or disengaged from the main 
audio device, if the timing of connection of the ground terminal GND to 
the main audio device is delayed from that of the power supply terminals 
7a and 7c, a reverse voltage in response to a differential voltage between 
the operating voltages ILL and VDD (for example, 5 V-14 V= -9 V) is 
inadvertently applied to the IC circuit 5 thereby damaging the IC circuit. 
Therefore, to overcome the above-described problems inherent in the 
conventional apparatus having the operating detachable portion, a 
detection switch 8 has been included on the engaging surface of the main 
audio device. This detecting switch 8 detects whether the operating 
detachable unit 3 is engaged or disengaged. As a result, once the switch 8 
determines that a operating detachable unit has been engaged with the main 
audio device without a failure, the operating voltages VDD and ILL are 
thereafter supplied to the power supply terminals 7a and 7c, respectively. 
However, to prevent the IC circuit from being broken by controlling the 
supply timing of the operating voltages to the power supply terminals, the 
audio device is required to include a detecting switch, an ON/OFF 
switching unit of the power supply, a control circuit for controlling the 
ON/OFF switching unit of the power supply, and software for controlling 
the ON/OFF switching unit and control circuit, so as to increase the 
number of mechanical and electronic parts and additional processing. The 
mounting space of audio device is also increased, resulting in an 
undesired enlargement of the device. 
SUMMARY OF THE INVENTION 
In view of the above problems inherent in the prior art, it is an object of 
the present invention to provide a simple protection circuit for a 
detachable unit in which when the detachable unit is engaged with or 
disengaged from a main device, if contact timing of a ground terminal to a 
main device is delayed from that of the power supply terminals thereof, an 
internal circuit such as an IC circuit may be protected from an accidental 
reverse voltage. 
To accomplish the above and other objects, according to the protection 
circuit for detachable operating unit of the present invention in which 
the detachable operating unit could be engaged with or disengaged from the 
main device, when the detachable unit is engaged with the main device, the 
different operating voltages are supplied to the main device through 
terminals, respectively, the detachable unit is characterized by 
comprising a protecting diode connected between the power input terminal 
of one of operating power supply units which has a voltage lower than that 
of the other operating power supply unit and a ground terminal with 
reverse polarity. 
According to the present invention, when the timing of connecting the 
ground terminal to the main device is delayed from that of the power 
supply terminals, the reverse voltage in response to the voltage 
difference between the two operating power supply unit is applied to the 
protective diode. As a result, the reverse voltage generated between the 
lower operation power supply unit and the ground terminal is regarded as 
the voltage drop caused by the voltage through the protective diode in the 
forward direction so as to prevent internal circuit such as the IC circuit 
connected to the operating power supply generating voltage lower than that 
of the other operating power supply unit from being broken by the reverse 
voltage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will now be described in more detail with reference 
to the accompanying drawings. 
One example of a detachable unit according to the present invention will 
now be described. 
In a detachable operating unit 3, a protective diode 9 is connected between 
a power supply terminal 7a and a ground terminal 7b with a reverse 
polarity. 
As shown in FIG. 1, the detachable operating unit 3 according to the 
present invention as shown in FIG. 1 is composed of substantially the same 
components as those shown in FIGS. 3 and 4. Therefore, the duplicated 
description will be omitted. 
In FIG. 1, assuming that the timing of connection of the ground terminal 7b 
is delayed from that of the power supply terminals 7a and 7c thereof as 
shown in FIG. 4, a reverse voltage in response to a difference voltage 
between operating voltages ILL and VDD, namely, the reverse voltage (for 
example 5 V-14 V= -9 V) is applied to an IC circuit 5. However, the 
reverse voltage is applied to the protective diode 9 with the reverse 
polarity. The reverse voltage is generated between a power input terminal 
of the IC circuit and the ground terminal in which the reverse voltage is 
regarded as a voltage drop (about 0.6 V) caused by applying the voltage 
through the protective diode 9 with forward polarity. Therefore, the 
reverse voltage in the same manner as the conventional apparatus is not 
applied to the IC circuit 5 so as to avoid the breakage of the IC circuit 
5. 
In this embodiment of the present invention, the protective diode is 
disposed around the IC circuit 5. The protective diode could be formed on 
a tip of IC circuit 5 in a manufacturing process. 
According to the present invention, the protective diode is connected with 
the reverse polarity between the ground terminal and the power supply 
terminal which generates a voltage lower than that of the other power 
supply terminal thereof so as to absorb a reverse voltage. This avoids 
breakage of the internal circuit connected to the power supply unit which 
generates a voltage lower than that of the other power supply thereof. It 
is therefore unnecessary to employ an engaging/disengaging detecting 
device, the manufacturing cost is increased, and it is possible to 
decrease the size of the audio device.