Switch/Relay for high voltages

Hi All,

Long time lurker here, I've learned a lot from this forum, but only now have I needed to ask a question myself!

I'm building a device which needs to be able to switch a voltage supply of 120VDC and ~0.1A, and it will have this current set on for about an hour, then once it detects sufficient temperature change, it will switch it off.

I've been looking at different switches and relays, for example this fairly standard one: http://www.elecrow.com/1channel-relay-module10a-p-1048.html

Which is rated for 10A@250VAC or 10A@30VDC.

Or this high-current one: http://www.elecrow.com/large-current-relay-module-30a-p-1161.html

Rated for 30A@240VAC or 30A@30VDC.

Now, I understand that VAC is rated much higher than VDC since VAC drops to zero at 120 Hz or whatever, so power dissipated would be substantially less.

That said, would it be possible for these switches to operate my device?

I exceed their maximum DC voltages by a factor of four, but as you can see I'm using a much lower current, so surely the power dissipated in the device will be substantially less than their rated maximum and it will be OK?

Thank you for any assistance, sorry if this is quite obvious, I think that surely it would be OK since I'm running a low current, but I just wanted to confirm...

If you are only going to draw 100 mA these should be able to work.

IMO, the contacts should not weld at this low of current.

Oh great, so contact “Welding” is an internal thing where the mechanical components inside the device become stuck together because of heightened temperatures?

My initial guess was that power = VI, so for that high-current one it is rated at 3030 = 900 Watts, but all I’m doing is 1200.1 = 12 Watts, so surely it won’t cause any temperatures close to the point at which it might weld…

Thanks for replying so quickly!

Contact welding can occur with high inrush currents. If you have a big capacitor then its DC power consumption is (ideally) zero but it sucks a lot of amps when the contacts initially close. If you have a big inductor, then it makes sparks when the contact opens.

The AC voltage rating of a relay is usually much higher than the DC voltage because the voltage drops to zero during the time that the contacts are moving to the open position. This extinguishes any arc across the slowly-opening contacts, which can be significant even at 'low' DC voltages like 30V.

You need to choose a relay switch which is rated for AC application at a level higher than the maximum current your expect you will be switching at the AC voltage you will be using. Any DC ratings are of no use in this case.

The higher the rated current (compared with the max you are switching) of the switch, the longer it will live. In addition on a professional design you should also be taking into consideration the power factor (p.f) of the particular load you will be switching. Switch relays are normally rated for p.f =1 which corresponds to a purely resistive load. i.e a filament lamp or a heating element. If your load is a motor (inductive load) or a florescent lamp (capacitive load) then the p.f is either greater than or less than 1 and the switch is having a hard time to disconnect the load. In that case you should go for an even higher rated switch.

Switches are also rated for mechanical and electrical endurance which is an indication of how many operations it can make before failure.

Thanks for the replies

Re: MorganS Ok, I understand. So, the problem is really in the closing/opening motion, not in the actual continuous conductance. Regarding the loading, it'll be all resistive and with constant current draw, so I hope that since I'm only pulling a low current, the spark will not cause minor damage, despite the fact that I'm at a large voltage (so the spark can jump quite a long way).

Re: Watcher I was probably not clear enough in my original post, I'm using only DC power through this switch, so I think the DC rating should be important? It will never have an AC load. The loading is entirely resistive, so p.f. should be OK i hope.

Thank you for your responses!

Yes, sorry I didnt realize you will be using only DC and yes in your case the DC ratings are of importance.

I would choose the highest voltage rated switch I could afford that also fits my application.
This would ensure a bigger clearance between the moving contacts of the switch and hence longer life.
On the downside it might need more energizing current to operate and will be more bulky.
Everything is a matter of tradeoffs in engineering. You need to balance how much reliability you really need and lifetime versus cost and complexity.
Your current though is small, so you ll be ok for most applications with small and cheap components.

Ok, thanks for that feedback Watcher. I figure I'll just search out the biggest switch I can find regardless of its data sheet VDC description and test with that.

Do you happen to know how the solid-state switches would perform with increased VDC? I have one rated up to 250VAC and 2Amps, would that be able to handle a similar DC load?

Is this rectified and unfiltered AC? If so you can just use an SCR. Very cheap, very small and very reliable.

Your solid state switch should also work.

Hi Pegwatcher, no it is all DC. I'll take a look at the SCR, seems to be a useful device...

The SCR can only turn off when current drops to almost zero. It will just latch on with pure DC and it will not turn off when you remove the gate current unless the Anode voltage is removed or interrupted. They work great with AC and AC-rectified-but-unfiltered where the voltage drops to zero twice each cycle.

Your solid state switch might work but some of these (at least older ones) work only with AC, not DC.

Your solid state relay product has to be mosfet or igbt based. They are used to switch DC. If you use a triac or thyristor based product, they expect to see a zero mark on a sine wave to turn OFF. DC has no zero mark, so they do not turn OFF.


For you and others in the future, what you are referring to is not commonly known as "high voltage", but rather "mains voltage" (or "household voltage").

Basically anything from 110 VAC to 240 VAC falls into that range - but for the term "mains voltage", I would consider up to 440 VAC (common in commercial/industrial) to fall under that.

Anything over that could be considered "high voltage" - but more specifically, the term "high voltage" only really comes into play when you are discussion kilovolt levels and higher (ie - anything from 1 kV on up - AC or DC).

It's useful to be aware of these distinctions, because for me, your title comes across as misleading (I know that it wasn't intentional on your part - so don't worry there). When I see something stating "switch/relay for high voltages" - I think of completely different circuits/devices than I do when I see the statement "switch/relay for mains voltages". At that point, I only realize what is going on once I start reading the thread (and even then that may not be a good indicator). I'm sure others here would agree.

Just a small FYI to learn from and move forward with. Good luck with your project.

Neft, you repeatedly call the relay a switch.

the first rule of all intelligence is to understand the meaning of the words.

if you are coming to a new field of study, learn the terminology. here is a fundamental rule of education. teachers do not understand the words in the fields in which they teach.

My brother teaches agronomy, I challenged him to teach one semester with ONLY using a dictionary and the words. no other discussion, just make sure the students understood the words.

end of the semester, all the students got 100% on the test.

please call a switch a switch, find out what a switch is and how it works call a relay a relay, find out what it does and how it works.

cr0sh: For you and others in the future, what you are referring to is not commonly known as "high voltage", but rather "mains voltage" (or "household voltage").

That is quite amusing as in the (power transmission) industry, it is actually called "Low Voltage (LV)". :grinning:

Neft: I exceed their maximum DC voltages by a factor of four, but as you can see I'm using a much lower current, so surely the power dissipated in the device will be substantially less than their rated maximum and it will be OK?

High voltage DC is hard to switch off... There is no zero-crossing in a DC load so you have to rely on the spark self-quenching, which is a very non-linear behaviour. I suspect 0.1A is low enough, but I wouldn't be at all surprized if it wasn't. It might be borderline and sometimes work, sometimes not (thus melting your relay contacts and starting a fire perhaps).

Go for a relay rated at 120V dc for a boring life I suggest.

One data point I can offer is 1kV at 0.5A - 3cm spark can be held continuously (well a 500W fireball to be exact). With AC you'd only need 1mm separation to break the spark at that voltage (with luck).


at 1:20 is that a switch or a relay ?

after watching that video, it is no wonder people on here warn about the dangers of high voltage. !

if you notice in the video, there is a 'spark gap' when contacts are opened. you have to look closely to see it. at 1:20 if you watch carefully, you might catch a glimpse of the electricity jumping the contacts

when a relay is changing state, the voltage will jump the gap. if it is doing so on the close section, the surface can heat and melt, then once joined, the melted section is may not allow the coil to open the contacts.

the relays have a multitude of specs. AC voltage and current DC voltage and current Motor rating. resistive

the engineers who designed these things and created the ratings were not just jotting down numbers. the ratings are there for allow you to pick one that will work for your application.

you can get a bicycle to operate at 100 mph, but the slightest problem will result in total failure.

order a relay that is rated for your use, that way, you will not have problems.

Neft, your application will not have this problem, because your relay is only closed for an hour, or so. In real life, a relay carrying DC for a month or more will likely have the contacts permanently connected, not from welding, but from metal migration caused by the DC. Back in the days of leased phone lines and modems, I had this problem in quite a few cases. The modem could not be taken off-line because the relay contacts were stuck together. These were sealed relays. AC does not have this problem.

Another relay thought: the arcing you see when contacts close is actually desirable, if limited to a very short time. The contact metal will oxidize a tiny bit over time, and will eventually insulate the contacts. The arc will clean the oxide off. There may be some metal that doesn't have this problem.


Paul_KD7HB: There may be some metal that doesn't have this problem.

Platinum if I recall correctly, or palladium.

In "the old days", critical switching relays (such as the 3000 series) had such contacts.