How do I install a remote Enable/Disable Switching Circuit?


Some temperature controller and laser diode driver components have remote Enable and Disable capability designed in with the current limit functionality. The user usually adds a simple on-off switch. This can be implemented with a simple switch across the limit inputs such that
open = output current enabled and closed (shorted) = output current disabled.

To add a simple enable/disable switch, connect it to the LIM + and LIM- pins as shown in Figure 1.

Switch connected to LIM- and LIM+ pins

By shorting the LIM+ and LIM- pins, the output current is disabled. Leaving the pins open, the output current is enabled up to the maximum capacity of the controller.

A more sophisticated switching method may be needed when the output needs to be controlled by a computer or other remote circuit. This requires either a relay or analog switch such as Figure 2.

Block Diagram for an analog switch

When connecting the analog switch to the LIM pins, use the following table to determine the output status:

Logic Control Input Output Status
Connect LIM+ to Drain Terminal
Connect LIM- to Switch 1
0 Enabled
1 Disabled
Connect LIM+ to Drain Terminal
Connect LIM- to Switch 2
0 Disabled
1 Enabled

The benefits of using an analog switch, such as the ADG436, rather than a relay for more complex situations are that it is more energy efficient (uses µA rather than mA), is smaller, and is less expensive.

Using Resistors to Set a Safety Current Limit
If a resistor is used in parallel with the switch or relay, a safety current limit can be set. Figure 3 below shows the different resistor and switch configurations that are possible for Wavelength’s products.

Optional Limit Resistor, Adjustable Trimpot, or Switch Connections

If quicker switching is required, a MOSFET circuit may be used. Contact your Wavelength Sales Engineer for more information.