Fault detection circuit for printers with multiple print heads

1. A fault detector apparatus for a printer having a first print head and a second print head with a plurality of print head elements in each print head, the apparatus comprising:

a test circuit in signal communication with all the print heads to test the plurality of print head elements, the test circuit comprising:

a test power supply for generating a test voltage;

a first resistor in series connection between the test power supply and the plurality of elements in the first print head;

a second resistor, the first resistor and the second resistor in series connection with the test power supply and the plurality of elements in the second print head; and

an analog to digital converter in signal communication with the second resistor;

wherein in response to a flow of current from the test power supply through the first resistor and one of the plurality of elements in the first print head, the analog to digital converter detects a first voltage; and

wherein in response to a flow of current from the test power supply through the first and second resistors and one of the plurality of elements in the second print head, the analog to digital converter detects a second voltage;

wherein in response to a flow of current through the one element in the first print head operating in accordance with an acceptable operational characteristic, the first voltage is described by:

res

⁢

⁢

116

res

⁢

⁢

116

+

element

⁢

⁢

1

⁢

(

v

⁢

⁢

66

)

;

and

wherein in response to a flow of current through the one element in the second print head operating in accordance with an acceptable operational characteristic, the second voltage is described by:

res

⁢

⁢

116

+

res

⁢

⁢

132

res

⁢

⁢

116

+

res

⁢

⁢

132

+

element

⁢

⁢

2

⁢

(

v

⁢

⁢

66

)

where:

v 66 represents the test voltage;

res 116 represents a resistance of the first resistor;

res 132 represents a resistance of the second resistor;

element 1 represents a resistance of the properly operating element in the first print head; and

element 2 represents a resistance of the properly operating element in the second print head.

2. The apparatus of claim 1 , wherein:

the printer comprises one print head.

3. The apparatus of claim 1 , wherein:

the first print head is an impact print head comprising a plurality of electromagnetic coils; and

the second print head is a thermal print head comprising a plurality of heating elements.

4. The apparatus of claim 1 , wherein:

the first resistor has a resistance about equal to the resistance of the one element in the first print head operating in accordance with an acceptable operational characteristic;

the first resistor and the second resistor have a combined resistance about equal to the resistance of the one element in the second print head operating in accordance with an acceptable operational characteristic;

in response to the flow of current from the test power supply through the first resistor and the one element in the first print head operating in accordance with an acceptable operational characteristic, the analog to digital converter detects the first voltage equal to about half of the test voltage; and

in response to the flow of current from the test power supply through the first and second resistors and the one element in the second print head operating in accordance with an acceptable operational characteristic, the analog to digital converter detects the second voltage equal to about half of the test voltage.

5. The apparatus of claim 1 , further comprising:

a first power supply in power connection with the plurality of elements in the first print head;

a second power supply in power connection with the plurality of elements in the second print head;

a first power switch in series connection between the first power supply and the plurality of elements in the first print head; and

a second power switch in series connection between the second power supply and the plurality of elements in the second print head;

wherein the first power switch is opened prior to the flow of current from the test power supply through the first resistor and the element in the first print head; and

wherein the second power switch is opened prior to the flow of current from the test power supply through the first resistor, the second resistor, and the element in the second print head.

6. The apparatus of claim 5 , further comprising:

a first diode in series connection between the first power supply and the first resistor to isolate the test power supply from the first power supply; and

a second diode in series connection between the second power supply and the second resistor to isolate the test power supply from the second power supply.

7. The apparatus of claim 1 , further comprising:

a processor in signal communication with the analog to digital converter;

wherein the processor is receptive of a signal representative of the first voltage detected by the analog to digital converter, and responsive to generate a notification signal if the detected first voltage deviates beyond a threshold from the first voltage described by:

res

⁢

⁢

116

res

⁢

⁢

116

+

element

⁢

⁢

1

⁢

(

v

⁢

⁢

66

)

;

and

wherein the processor is receptive of a signal representative of the second voltage detected by the analog to digital converter, and responsive to generate a notification signal if the detected second voltage deviates beyond a threshold from the second voltage described by:

res

⁢

⁢

116

+

res

⁢

⁢

132

res

⁢

⁢

116

+

res

⁢

⁢

132

+

element

⁢

⁢

2

⁢

(

v

⁢

⁢

66

)

.

8. The apparatus of claim 7 , wherein:

the processor generates the notification signal based upon a trend of change of the signal representative of the second voltage.

9. The apparatus in claim 1 wherein one of the print heads has only one print head element.

10. A fault detector apparatus for a printer having an impact print head with a plurality of electromagnetic coils and a thermal print head with a plurality of heating elements, the apparatus comprising:

a test circuit in signal communication with the impact print head and the thermal print head to test the plurality of electromagnetic coils and the plurality of heating elements, the test circuit comprising:

a test power supply for generating a test voltage;

a first resistor in series connection between the test power supply and the plurality of electromagnetic coils, the first resistor having a resistance about equal to a resistance of one of the plurality of electromagnetic coils operating in accordance with an acceptable operational characteristic;

a second resistor, the first resistor and the second resistor in series connection with the test power supply and the plurality of heating elements, the first resistor and second resistor having a combined resistance about equal to a resistance of one of the plurality of heating elements operating in accordance with an acceptable operational characteristic;

an analog to digital converter in signal communication with the second resistor; and

a processor in signal communication with the analog to digital converter;

wherein in response to a flow of current from the test power supply through the first resistor and one of the plurality of electromagnetic coils, the analog to digital converter detects a first voltage;

wherein in response to a flow of current from the test power supply through the first and second resistors and one of the plurality of heating elements, the analog to digital converter detects a second voltage;

wherein the processor is receptive of a signal representative of the first voltage detected by the analog to digital converter, and responsive to generate a notification signal if the detected first voltage deviates beyond a threshold from about half of the test voltage; and

wherein the processor is receptive of a signal representative of the second voltage detected by the analog to digital converter, and responsive to generate a notification signal if the detected second voltage deviates beyond a threshold from about half of the test voltage.

11. The fault detector apparatus of claim 10 , wherein:

the test voltage is one of 3.3 and 5.0 volts.

12. A fault detector apparatus for a printer having an impact print head with a plurality of electromagnetic coils and a thermal print head with a plurality of heating elements, the apparatus comprising:

a test circuit in signal communication with the impact print head and the thermal print head to test the plurality of electromagnetic coils and the plurality of heating elements, the test circuit comprising:

a test power supply for generating a test voltage;

a first resistor in series connection between the test power supply and the plurality of electromagnetic coils;

a second resistor, the first resistor and the second resistor in series connection with the test power supply and the plurality of heating elements; and

an analog to digital converter in signal communication with the second resistor;

wherein in response to a flow of current from the test power supply through the first resistor and one of the plurality of electromagnetic coils, the analog to digital converter detects a first voltage; and

wherein in response to a flow of current from the test power supply through the first and second resistors and one of the plurality of heating elements, the analog to digital converter detects a second voltage;

wherein in response to a flow of current through the one electromagnetic coil operating in accordance with an acceptable operational characteristic, the first voltage is described by:

res

⁢

⁢

116

res

⁢

⁢

116

+

element

⁢

⁢

1

⁢

(

v

⁢

⁢

66

)

;

and

wherein in response to a flow of current through the one heating element operating in accordance with an acceptable operational characteristic, the second voltage is described by:

res

⁢

⁢

116

+

res

⁢

⁢

1322

res

⁢

⁢

116

+

res

⁢

⁢

132

+

element

⁢

⁢

2

⁢

(

v

⁢

⁢

66

)

where:

v 66 represents the test voltage;

res 116 represents a resistance of the first resistor;

res 132 represents a resistance of the second resistor;

element 1 represents a resistance of the properly operating electromagnetic coil; and

element 2 represents a resistance of the properly operating heating element.

13. The fault detector apparatus of claim 12 , wherein:

the test voltage is one of 3.3 and 5.0 volts.