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I need to mount some relays that I have soldered wires to. If I place them on a non-conductive surface and use a generous amount of standard glue from a hot glue gun, will that be relatively safe? I'll be running mains level voltage through the relays.

According to this aricle hot glue melts somewhere between 121 °C and 193 °C which I hope is much hotter than the relays would ever get.

Is there a better way to mount these relays? Obviously using a PCB would be best, but the cost is a bit prohibitive for a custom designed PCB.

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    \$\begingroup\$ 'relatively safe' is not a good idea when working with mains voltages. It might mean that you die less soon... \$\endgroup\$ – Wouter van Ooijen Nov 8 '12 at 19:18
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    \$\begingroup\$ Relays get warm to hot under load as also during frequent switching. Hot glue melts. \$\endgroup\$ – Anindo Ghosh Nov 8 '12 at 19:19
  • \$\begingroup\$ @WoutervanOoijen I understand that working with mains is always a safety concern. I originally left of the "relatively" but realized I would get a response like this if I didn't include it. =) Thanks for your concern. Do you have an suggestion for an alternative? \$\endgroup\$ – Matt Ruwe Nov 8 '12 at 19:20
  • \$\begingroup\$ @AnindoGhosh Good point. Updating the question. \$\endgroup\$ – Matt Ruwe Nov 8 '12 at 19:23
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There are a variety of hot glue adhesives, and there's no blanket statement that could be made to cover all of them.

However, I has seen industry use (ie, on the assembly line) of hot glue around the capacitors of the high voltage section in a CRT for vibration purposes. The glue came into contact with many of the leads and PCB tracks in that area of the PCB, so I know that there are hot glues which are suitable for electrical use, and appropriate even for high voltage insulation, though insulation was not the primary goal in this case.

You can test your hot glue with a mega-ohmmeter if you have concerns about the particular formulation you are using.

Is there a better way to mount these relays?

There are relay mounts and sockets for many styles of relays. You can also get relay connectors and relays with mounting tabs. These are preferable to adhesives for many applications.

When I've needed to mount relays in a chassis I've used zip ties and hot glue for short term and light duty usage, and metal brackets for heavy duty or long term usage.

I don't typically use hot glue on the bare wires and terminals themselves, and if I do I use heat shrink tubing to insulate the wires and terminals first.

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  • \$\begingroup\$ Note that in your particular case, the relays may experience very high voltages when opened (on either side, depending on how they are driven and the load) so you'll need to make sure the hot glue you use is appropriate for those voltages. I have used generic cheap hobby hot glue extensively in small low voltage projects and my own simple measurements suggest that this usage is OK. You'll need to perform your own measurements for your own peace of mind, though. \$\endgroup\$ – Adam Davis Nov 8 '12 at 19:23
  • \$\begingroup\$ I like the suggestion of "relay connectors" and "relays with mounting tabs". Can you point me at an example of what you're thinking about? \$\endgroup\$ – Matt Ruwe Nov 8 '12 at 19:32
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    \$\begingroup\$ The most common and cheap are those used with automotive relays, such as this socket: digikey.com/product-detail/en/VCF4-1001/PB316-ND/254533 which would work with this relay with a mounting tab: digikey.com/product-detail/en/1432772-1/PB685-ND/807762 using these terminals: digikey.com/product-detail/en/42281-1/A27926CT-ND/456890 of course you could skip the socket and just shrink wrap the terminals. Go to your supplier, search for relays, and then select Chassis Mount under Mounting Type. Then select your drive and load properties. \$\endgroup\$ – Adam Davis Nov 8 '12 at 19:47
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Even if hot glue is a good insulator, you have to look for the components found in it: they may be corrosive or harmful. My experience with hot glue in the past was bad. I have used it on the bottom of a low power (5 V) PCB to insulate it from the metallic enclosure - this has led to mysterious failures of boards after 3-6 months.

The material used in PCBs to hold large components, such as transformers and capacitors, in position is usually silicone RTV, which is specially made for electronics and contains no acids that could harm copper and tin solder joints.

An example can be found on digikey: https://www.digikey.com/catalog/en/partgroup/rtv-silicones/32319

An alternative would be to use relays or contactors that contain screw terminals for wire connections and a DIN rail for mounting.

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I have used glue sticks in high (22kV and above) and low voltage electrical circuits for many years and have never encountered any problems. In terms of resistance, the glue will be open circuit on an insulation tester. You would need considerable heat to melt the glue sticks, so there should be no problems in terms of heat causing the joints to melt. All in all, in my experience, it's a go!

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I have used hot glue before. It is a good insulator, and it is also strong for phone charging cables, etc. However what I learned with boards, such as inside an adapter for my computer, is that the hot glue, when applied can melt parts of the board and cause damage. So keep that in mind when using it. Other than that, it is a good option in my opinion.

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Bad experiences with hot glue in computers where applied to the 5 and 12 volt DC. Eg to ensure a cable does not fall out in transit a dab if hot glue applied. Within two years there is an acrid smell, huge amount of smoke, large orange glow within the computer case, severe charring, damage to the cabling and associated area. Obviously there are many types of hot glue so there's no standard that you can apply and state it's ok. Experience we've seen would be NO, unless qualified by the manufacturer for use in the area intended.

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Other answers already mention the variety of hot glue's chemical composition, possible corrosiveness, etc.; but another harmful possibility is electrical breakdown happening during prolonged application of voltage. In other words: maybe the glue's resistance could decrease (enough to short the circuit) over time under some voltage, as its composition breaks down. See https://en.wikipedia.org/wiki/Electrical_breakdown

Thus I think, as others already said, that one must not use just any hot glue for electrical insulation.

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  • \$\begingroup\$ My impression is that electrical breakdown happened as a result of very high voltages. And according to the article you linked, the breakdown (into a short circuit) is a result of carbon tracking, which I have only heard of in the context of kilovolts. I would be interested to know whether (solid) breakdown and carbon tracking can occur at mains voltage, but it's hard to imagine. Mains voltage can't even break down air (cannot arc at any distance). \$\endgroup\$ – piojo Sep 24 at 7:47
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It depends on what you use it for.

Hot glue tends to absorbs moisture and becomes conductive after a while. It also softens with heat! If you use it where heat is generated, better use something else instead of hot glue If you use it where lots of moisture is present, better use something else. If both moisture & heat -> hot glue is definitely not your choice!

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  • \$\begingroup\$ I tested hardware store hot melt by soaking in water for months. The conductance did not change and I used it as a board coating. Performance was better than coatings specifically made for the job. \$\endgroup\$ – Robert Endl Feb 11 '19 at 18:04
  • \$\begingroup\$ and how you test that "performance" ?? \$\endgroup\$ – OWADVL Feb 11 '19 at 19:57
  • \$\begingroup\$ Advantages: low cost, no funky chemicals when curing, no 24 hr cure time, easy storage, amazingly low leakage current when used for chemical sensor preamps (my interest). I tested several brands, some with colors, some with glitter. All worked the same. The higher viscosity even when hot will put many people off. It'd probably not for everyone. \$\endgroup\$ – Robert Endl Feb 12 '19 at 16:06
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You can find both hot glue sticks and hot glue guns made specifically for these sorts of applications. They will have higher melting points and not contain corrosive chemicals. The below site has some products that may work.

https://www.hotmelt.com/collections/hot-melt-glue-sticks/application-electronics

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    \$\begingroup\$ A brand new user responding to an 8 year old post with a link to a commercial site comes dangerously close to spam. Are you affiliated with the link somehow? \$\endgroup\$ – Lundin Sep 24 at 7:48
  • \$\begingroup\$ @Trent Are you associated with the company in the link. If so please say so. Such references can be acceptable but MUST be declared. \$\endgroup\$ – Russell McMahon Sep 24 at 10:58
  • \$\begingroup\$ @Lundin Your concern is noted and justified. The cited site does seem to have a wide range of hotmelts for specialist purposes and is of potential value to others. If he is associated and declares his relationship it seems acceptable. \$\endgroup\$ – Russell McMahon Sep 24 at 11:01
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    \$\begingroup\$ @RussellMcMahon Yeah it didn't seem like obvious spam so I didn't want to flag. They might simply be a new user acting in good faith. However, the reason that I asked if they are affiliated is that sneaking in advertising to your own products by posting on-topic "answers" isn't really nice either. Companies who wish to advertise their products should contact SO the company instead. \$\endgroup\$ – Lundin Sep 24 at 11:07
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    \$\begingroup\$ @Lundin - I agree with your concern. I have seen the the technique of writing a marginally interesting answer (so can't be flagged as "not an answer") and then adding a link into the answer that the writer is promoting, happen more often over the last few years. One big clue here is that the link is not needed as part of the answer - there is nothing on that linked page which refers to insulation or mounting relays, which were the actual question. If the link really answered the question, that would be different... \$\endgroup\$ – SamGibson Sep 24 at 11:34

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