Voltage spike from SLR320 when first powered

Issue

When an SLR320 light level sensor has power applied a transient voltage (approx. 17VDC) is seen on the output.

Product Line

Field Devices

Environment

SLR320

Cause

In order to provide dual output capability (voltage and current) with both AC and DC supply, it is necessary to use a capacitor to filter the AC supply. This capacitor is also active in current mode because it is placed behind the rectifier at the G clamp. An additional resistance is added inside to minimize this load current.

This load current of the capacitor (which is the cause of transient voltage) cannot be reduced to zero, otherwise the voltage output cannot be used at AC supply within its specification (max. output voltage @ min. load resistance @ min. supply voltage, Y clamp).

Resolution

This behavior is expected and by design. The transient voltage generated by the load current is settled within approximately 40ms and occurs only at power up.

The transmitter is continuously powered so this would not normally be an issue and would have stabilized before a controller that was powered up at the same time would be booted and scanning it's inputs.