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I have a relay that is switching 240V supply to a plug pack. The plug pack in turn supplies 24V to a solenoid coil.

The relay has fused in the closed position and I'm trying to figure out why and the best way to protect it when I get a new one.

Am I correct in assuming the flyback from the solenoid is isolated by the plug pack?

And if so, will an RC snubber circuit be the correct way to prevent arcing when cutting supply to the plug pack?

The relays specs can be seen here: link

It's rated for 6A 250VAC and is switching a 0.9A plug pack.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Am I correct in assuming the flyback from the solenoid is isolated by the plug pack? That sentence makes no sense to me. Draw a schematic ! The relay contacts are fused by the current being too high. Many (poorly designed) power adapters have a large inrush current when they're connected to mains. This can fuse the relay's contacts. An RC snubber will not help you there. The inrush current needs to be limited. \$\endgroup\$ Nov 24 '16 at 8:16
  • \$\begingroup\$ Sorry, I hope the drawing makes sense. \$\endgroup\$
    – sqgun
    Nov 24 '16 at 11:20
  • \$\begingroup\$ Your schematic shows a simple transformer which produces AC output. You show the output as being 24 VDC. Which is it? \$\endgroup\$ Nov 24 '16 at 18:53
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Yes the inductive load is isolated from the mains side by the plugpack DC-DC convertor.

The surge you are experiencing is in all probability the plugpack input capacitors charging when the relay contacts close. The relay contacts could close anywhere on the input AC cycle, and if it closes anywhere near the peak of a half cycle the input surge may be 10's of Amps for a short time. This can weld or seriously pit the relay contacts.

Instead of a relay you could use a zero crossing Solid State Relay (SSR). Since they turn on at the zero crossing of the mains, the capacitor charge current follows the input sine wave to the peak voltage. You still get a surge but it's much better controlled.

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  • \$\begingroup\$ Thank you. Is there any other way to control the input surge? My current setup will require 12 zero crossing SSRs and they seem to be $70-$130 each. \$\endgroup\$
    – sqgun
    Nov 24 '16 at 11:06
  • \$\begingroup\$ Here are some suggestions: e2e.ti.com/blogs_/b/powerhouse/archive/2015/03/31/… In your case perhaps Figure 4 is suitable, it uses an NTC resistor (RT1). Proper power supplies have these built-in, yours probably doesn't but that does not prevent you from adding one in series with the relay contacts. Do not that the NTC will get a bit warm during operation, this makes sense as it to be at a higher temperature so that it has a low resistance (when operating). \$\endgroup\$ Nov 24 '16 at 12:24
  • \$\begingroup\$ Note that an NTC needs to cool down after power off (it needs to be cold at the next power-on). So if you switch on/off in quick succession you cannot use an NTC. \$\endgroup\$ Nov 24 '16 at 12:25
  • \$\begingroup\$ @sqgun There are a bunch of multi channel options designed for Arduino such as these: amazon.com/Channel-OMRON-G3MB-202P-Solid-Module/dp/B00NIOLAR2 \$\endgroup\$ Nov 24 '16 at 15:48
  • \$\begingroup\$ @sqgun There are a bunch of multi channel options designed for Arduino such as these: amazon.com/Channel-OMRON-G3MB-202P-Solid-Module/dp/B00NIOLAR2 This particular SSR is not zero crossing, but does have sufficient pulse capability to handle your load. A 4 Channel version can be had for less than $9 on Ebay: ebay.com/itm/… this may be ideal for you. \$\endgroup\$ Nov 24 '16 at 15:55

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