APPLICATION NOTE

REPAIRING THE DSC/DRC CURRENT SOURCES

The DSC Sine and DRC Random vibration controller cards have current sources on the two inputs available for powering integrated accelerometers without requiring external power supplies. These current sources have an 18 VDC open circuit voltage and output 4 MADC into the input pins.

The 18 VDC is generated by a voltage-doubler circuit operating from the 12 VDC computer power; it is a TCS429CPA driver I/C ( U22 on the DSC and U20 on the DRC ) and associated diodes and capacitors. The circuit outputs about 20 VDC which is regulated down to 18 VDC by the 78L018 regulator I/C ( U23 on the DSC and U37 on the DRC ). Each 4 MA current source consists of a 2N3904 NPN transistor ( Q1 and Q3 ) connected to the computer data buss which turns on the 2N3906 PNP current source transistor

( Q2 and Q4 ).

The inputs are short-circuit protected, but excess voltage applied to the input pins could damage the transistors. They can be checked with a meter in the usual way. Refer to the schematic diagram below for circuit details:

Click here to see schematic diagram.

Revised 10/27/99