Questions about a relays

[atferrari]

Questions about a relay

My questions about this particular DPDT 5V relay:

a) Specs for AC and DC do not look very consistent. Why?

b) What if I use it to switch a load on 30V DC with 850 mA (maximum)?

c) I come from reading Wikipedia but I am not sure about this:

NC (normally closed) is the contact that stays closed when the coils is NOT energized?
NO (normall open) is the contact that stays open when the coil is NOT energized?

Thanks for any help and the time you spent here.

 

[MikeMl]

All relays/switches have a similar disparity between AC and DC ratings. It has to do with the contacts arcing/welding when breaking a circuit with a current flowing.

The specs should be adhered to if you want to get the rated cycles (life). By violating the ratings a little bit, you will likely shorten the life a little bit.

You have the NO/NC correct.

 

[JimB]

Just to expand a bit on what Mike is saying.

An AC current goes to zero twice every cycle, so that as the contacts pull apart, an arc is drawn but is soon extinguishes as the current drops to zero during the cycle.

A DC current on the other hand is flowing all the time and so as the contacts pull apart and draw an arc, there is nothing to extinguish that arc until the gap between the contacts is wide enough.

JimB

 

[atferrari]

Thanks Mike and Jim.

Clear enough!

 

[KeepItSimpleStupid]

There is another difference too and that is sliding contacts are not usually seen for AC. The typical ceiling fan uses one, BUT it should only be operated when the fan isn't on.

Note that a typical AC wall switch clunks. Connections are made quickly and abrubtly. AC typically has some sort of arc supression or something that might blow out the flame.

With DC, you would like to be able to PULL the contacts apart.

High voltage and RF relays you cannot switch with the voltage on. What might happen when you have a 40 kV voltage inside a glass, hopefully evaculated jar and you suddenly change the contacts? Bad things.

 

[atferrari]

What might happen when you have a 40 kV voltage inside a glass, hopefully evaculated jar and you suddenly change the contacts? Bad things.

As I said I have modest 30V to switch. I do not plan to switch not even a single kV in the near future.

 

[KeepItSimpleStupid]

I did at one time (15 kVDC at 1.5 A) and 1 KW into 50 ohms @ 13.56 MHz and used something like this: https://www.electro-tech-online.com/custompdfs/2012/09/h19_17.pdf

Note these contacts have different ratings depending on frequency.

 

[alec_t]

High voltage and RF relays you cannot switch with the voltage on

I must be missing something here. I thought the sole purpose of a relay was for switching signals/power on/off.

 

[davenn]

I must be missing something here. I thought the sole purpose of a relay was for switching signals/power on/off.

I dont know about HV, but with RF it depends on the design as to if it can handle hotswitching or not.
In the majority of RF applications I have delt with over the years, DC power was cut to the final amplifier
before the antenna relay was switched between TX and RX. The problem with RF is that with relays not designed for hot switching
The generated arc will ionise the air surrounding the contacts and create an arc-sustaining plasma. That arc will be sustained for
as long as RF current flows

With hot switching RF relays, the contacts will most likely be in a vacuum or inert gas to avoid the ionisation and plasma
like the ones KISS linked to above

cheers
Dave

 

[alec_t]

Thanks for the info, Dave.