Determining defective resistors in inkjet printers

1. A method of determining defective heating resistors R i in each of a plurality of inkjets in an inkjet printer, wherein each heating resistor R i is connected in parallel with a common capacitor, the method for each heating resistor R i comprising:

a) providing a variable power supply effective in a first condition to produce a first operating DC voltage and, in a second condition, to produce a second known test DC voltage V t ;

b) inserting a known reference resistor R r in series with the heating resistor R i and capacitor and open circuit the capacitor in response to sensing that the power supply has changed from the first condition to the second condition;

c) digitizing the voltage V i at the electrical junction between the heating resistor R i and the reference resistor R r ; and

d) using the digitized voltage to determine if the heating resistor R i is defective.

2. The method of claim 1 further including reducing the charge on the capacitor before digitizing the voltage V i .

3. The method of claim 1 wherein the resistance value of the heating resistor R i is calculated by the relationship

R i =R r V i /( V t −V i ).

4. The method of claim 2 wherein the resistance value of the heating resistor R i is calculated by the relationship

R i =R r V i /( V t −V i ).

5. Apparatus for use in determining defective heating resistors R i in each of a plurality of inkjets in an inkjet printer, wherein each heating resistor R i is connected in parallel with a common capacitor, comprising:

a) a variable power supply effective in a first condition to produce a first operating DC voltage and in a second condition to produce a second known test DC voltage V t ;

b) a known reference resistor R r ;

c) first circuit means for inserting the known reference resistor R r in series with the heating resistor R i and capacitor in response to the power supply changing from the first condition to the second condition;

d) second circuit means for open circuiting the capacitor in response to the power supply changing from the first condition to the second condition,

e) an analog to digital circuit electrically connected to the junction of the heating resistor R i and the known reference resistor R r for digitizing the junction voltage; and

f) means responsive to the digitized junction voltage V i for determining if the heating resistor R i is defective.

6. The apparatus of claim 5 wherein the second circuit means includes a transistor.

7. The apparatus of claim 5 wherein the second circuit means includes an N-channel FET connected between the negative side of the capacitor and ground.

8. The apparatus of claim 5 wherein the first circuit means includes a switch in parallel with the known reference resistance R r , said switch having an on-resistance which is less than R r /3.

9. Apparatus for use in monitoring the status of individual circuit elements within an array of circuit elements of a same first type, wherein each circuit element of the first type is connected to a common circuit element of a second type, comprising:

a) a variable power supply effective in a first condition to produce a first operating voltage and in a second condition to produce a second known test voltage;

b) a known reference circuit element having a electrical characteristic which may be compared to an electrical characteristic of the circuit elements of the first type;

c) a first circuit for inserting the known reference circuit element in series with the circuit element of the first type in response to the power supply changing from the first condition to the second condition;

d) a second circuit for isolating the circuit element of the second type from the circuit elements of the first type in response to the power supply changing from the first condition to the second condition;

e) an analog to digital converter electrically connected to the circuit element of the first type and the known reference circuit element for digitizing the voltage; and

f) means responsive to the digitized voltage for determining if a circuit element of the first type is defective.

10. The apparatus of claim 9 , wherein the first operating voltage is a DC voltage.

11. The apparatus of claim 9 , wherein the first operating voltage is an AC voltage.