CA3500 Load Sense Disabled, end spurious turn ons

CA3500 Load Instability

I’ve been battling with Intermatic CA3500 load stability issues for a while. The problem relates to spurious turn on of the switched portion of the outlet. I thought I’d share my learnings so others could avoid a lot of wasted time. With the push to CFL and LED lighting this may become a more common issue.

The problem is due to the load sense circuitry in the CA3500. Its purpose is to allow a table lamp or the like to be switched locally at the lamp and have a switched off CA3500 turn on without Z Wave intervention. I suppose this was a clever idea so people not familiar with the home automation wouldn’t try to replace otherwise good light bulbs thinking they were dead. Imagine seeing a lamp plugged in the wall, turning the lamp switch on and off, but the lamp never turns on, of course you’d think it was a dead bulb.

Unfortunately I have come to learn this well intentioned circuitry has problems with load devices that aren’t purely resistive (i.e. anything other than an incandescent lamp bulb). So CFL bulbs, downstream GFCI devices, and electronic items like TVs and audio gear all play havoc with the load sense circuit and can cause unintended spurious turn on of the CA3500.

I now understand why this is so and have a work around to disable the load sense capability. Unfortunately the CA3500 does NOT have a configurable parameter (Parameter 29) to disable load sense. It comes new with load sense enabled and it can’t be disabled. It would seem a trivial software update for the CA3500 but the designers chose not to make this configurable while other manufacturers did. More here on using parameter 29: (,4891.msg29888.html#msg29888) .

So how to disable load sense on a CA3500? It’s actually pretty easy. Load sense works by detecting when the load goes open circuit. I don’t know the exact scheme but the CA3500 somehow sends a current down the hot line when it is in the off position. I presume for safety reasons this is a very high impedance source current, I estimate around 370,000 ohms. So compared to 370k an incandescent light bulbs is essentially zero ohms. So when a turned off CA3500 sees a lamp load the test current will be gobbled up by the bulb load. Then the bulb is switched off (at the lamp) and the lamp load goes to essentially infinity and the CA3500 sees this absence of a load and switches on the CA3500. This is key. The turning on of the CA3500 is during the loaded to unloaded state change, not vice versa.

So now that we understand the detection mechanism a few things snap into focus.

[ol][li]Load sense is a very high impedance circuit. That makes it vulnerable to noise triggering.[/li]
[li]Any type of electronic load is more typically a switching power supply that has electronics on the input side to turn the AC to DC. This type of load is not purely resistive and can appear to the source as a changing impedance similar to the turning off of a light bulb. The result, false triggering. Since the load sense turns on the CA3500, it stays on until ZWave shuts it off[/li][/ol]
So how do you fix it? A 2cent component does the trick. Simply place a 56,000 ohm 1/2W resistor across the hot and neutral connections of the switched side of the CA3500. Make up a power cord with the resistor shunted across hot and neutral and then put your device after this and you won’t see spurious triggers any more. This will stop the load sense circuit from ever seeing what it thinks is an open circuit and thus the load sense circuit will never engage. Now the outlet will only respond to Zwave commands and depressing of the CA3500 local button.

Don’t want to use a resistor, use a 4W child’s incandescent night light as a load and put it somewhere where it’s not a distraction or a fire hazard. Then plug the complex electronic load after this.

Not sure if this has been a nuisance to you but perhaps this helps understand what is going on and a couple of options to work around it.

Use at your own risk. Author makes no assurances of UL and other safety agency certification from such concepts.

Good job figuring this out. You’re very persistent.

Nice write up, as well.

Linking to the original thread on this subject. You should probably post a link to this solution post, over there.

Excellent Work. I have several CA3500 outlets and I just got the same problem you had. Thanks for all the hard work.