Blowing ULN2803A Relay Drivers

[ajw22]

@ogdento I saw this in 4.1.1 in Jandy Troubleshooting Manual for AquaLink® RSAll Button and OneTouch™ Control Systems

The relay coil provides resistance (approximately 300 Ohms); if there is no resistance, the driver chip circuit will be damaged.

I have seen many systems where installers do not connect unused relays to the PCB sockets.

Jandy Aqualink, Pentair EasyTouch, and IntelliTouch all have buttons on the OCP that can turn on unused AUX circuits. That makes it easy to accidentally turn on an AUX without a relay plugged in and blow the relay driver.

On the IntelliCenter, you have to define an AUX in the software to get a button for it, so it is more difficult to accidentally turn on an unused AUX circuit.

I wonder if this is why some relay drivers are blowing and if we should recommend plugging all unused relays into the PCB.

 

[ogdento]

I think what they meant by "if there is no resistance" is "if the relay coil is a short" (or very low resistance), then the driver could be damaged... A short or low resistance would draw too much current (over 500mA) through the transistors for that channel and smoke them. An unconnected relay would appear like an open/infinite resistance rather than a low/short

 

[ajw22]

An unconnected relay would appear like an open/infinite resistance rather than a low/short

Thanks. That makes it clearer.