VMIVME-2120 Caution Regarding External Voltage Selection

Outputs of VMIVME-012120-175 board always turned on and no data written to the boards registers. The load is an LED (with current limiting) connected to an external 28VDC supply.

As indicated in the Product Manual, Figure 2-3 page 35, jumper JC selects the reference voltage for the output driver clamp diodes. The factory setting is +5V. This means that if 28V source is connected in series with the LED to the driver collector, the driver internal clamp diode will be forward biased, and current will flow from 28V source, through the LED, through the clamp diode, and into the 5V source. Thus, the LED will turn on regardless of the output driver state. The LED only allowed a small current to flow through the clamp diodes and no damage was done to the board. If a high current load had been connected, the output driver could have been destroyed. In addition, the 5 volt supply might be forced out of regulation, putting other boards in the system in jeopardy.

A conceptual schematic of the driver output and clamp diode circuit is given in Figure 2-2 on page 34. The purpose of the clamp diode is to protect the driver output stage from large transient voltages that typically result from switching inductive loads. These diodes are always reversed biased and do not conduct any current during normal operation. To ensure proper operation of the clamp diode circuits, the clamp diode reference voltage must be selected such that it is equal to or greater than the source voltage connected to the load. This means that for the factory default JC jumper setting, the maximum load source voltage must be no more than 5 volts. To accommodate a 28V source, the jumper JC should be moved to EXT and 28V connected to P2 A1. The external voltage must be less than 56 volts. A 56 volt 5 watt zener diode prevents the clamp diode voltage from exceeding this value. Warnings regarding inductive loads and user ground connections given on page 34 of the Product Manual should be followed closely.