Hi everyone,
I have a relay with contact capacity of 28V -DC , 7A.
Can I use it to power a device with supply voltage of 300V DC and current of 30 m?? I think It is just about contact power . 7*28=196W is much larger than 300*0.025=7.5w . It means that this replay could be used in this case ??

 

NO. The separation of the contacts will probably not be enough for high voltage.

Les.

 

It will probably work for a while, but the contacts will arc and destroy themselves eventually.

No, it is not about “contact power”. It is about contact voltage and current, which is why they are rated that way. The power of the load has nothing to do with it. When the contacts are open the current is zero, so no power. When the contacts are closed, the voltage is zero, so no power. The power stays in the load where it belongs.

 

Posts #2 and #3 are both correct.
Relay contacts have several different requirements: First, they must resist the controlled voltage while they are open. That is a gap space function. Then, when the relay operates, the contacts must close and conduct the current without excessive heating. BUT while they are closing, usually relay contacts bounce a bit, meaning that they rapidly open and close. This can lead to an arc that produces heat, and in worst cases, melt a bit of the contacts allowing them to weld closed. Then, when the contacts open, at the first instant the voltage may jump across the tiny gap during the first instant and establish an arc. If the contacts open far enough the arc will extinguish. So both the speed of opening and the distance control what voltage the relay contacts can interrupt. So there you have the explanation of the mechanisms that are involved. The contact POWER relates to a small part of that. but mostly that is not the issue.

 

Posts #2 and #3 are both correct.
Relay contacts have several different requirements: First, they must resist the controlled voltage while they are open. That is a gap space function. Then, when the relay operates, the contacts must close and conduct the current without excessive heating. BUT while they are closing, usually relay contacts bounce a bit, meaning that they rapidly open and close. This can lead to an arc that produces heat, and in worst cases, melt a bit of the contacts allowing them to weld closed. Then, when the contacts open, at the first instant the voltage may jump across the tiny gap during the first instant and establish an arc. If the contacts open far enough the arc will extinguish. So both the speed of opening and the distance control what voltage the relay contacts can interrupt. So there you have the explanation of the mechanisms that are involved. The contact POWER relates to a small part of that. but mostly that is not the issue.

The specification of this relay allow it to operate under 280V AC, that's OK to let it work with 300V DC

 

No. Although the peak voltage of 280 volts RMS is 395 volts 300 volts DC is more likely to sustain an arc when the contacts open. This is because the instantaneous voltage of AC drops to zero twice every cycle of the waveform.
Les.

 

No. Although the peak voltage of 280 volts RMS is 395 volts 300 volts DC is more likely to sustain an arc when the contacts open. This is because the instantaneous voltage of AC drops to zero twice every cycle of the waveform.
Les.

But the current is just only 28mA, I think the current is too small to create an arc

 

But the current is just only 28mA, I think the current is too small to create an arc

Welcome to AAC.

You are making some unwarranted assumptions about the nature of electric arcing, as well as how and why relay contacts are rated concerning voltage and current.

An electric arc forms when a high potential exists between two conductors. That is, when a pair of conductors have a difference in the state of charge—negative vs. positive—some of the charge from the negative conductor find a path through space to the positive one. (it is negatively charged because it has an excess of electrons compared to the relatively positively charged half of the pair)

The effect of this movement is the ionization of the air between the two, creating a low resistance plasma path that requires a much lower voltage to maintain. The voltage will drop and the current needed to maintain that plasma, and as a result, the arc, will depend on the distance between the two conductors.

For AC current, the ionized path only exists for 20ms (50Hz, only 16.7ms for 60Hz). The zero crossing of the AC waveform shuts down the current which is maintaining the plasma, making the distance required to extinguish the arc much smaller than for DC.

When designing a relay, the engineer will take into account the distance between contacts, and the rating of the relay is based on that distance. Once an arc is struck by DC, the voltage required to maintain it is much lower than for AC. If a relay could handle different DC voltage mechanisms based on current, there would be curves in the datasheet, not plain ratings. The two numbers can’t be plugged into the formula for power because they are independent maximums not a connected pair.

The voltage relates to arcing and the current rating is ampacity, just like for wire.

 

If the 300 volts DC rating is an actual spec-sheet rating, then MAYBE the TS can get away with it. Of course, if it is not a sealed relay, it may also be possible to adjust the contact spacing to increase the gap a bit. Certainly there is a bit of risk involved, but that trick has worked for a lot of people over the years.