amps vs watts at voltage question

[James S]

Hi folks,

I have a part, a crydom solid state relay actually, and it is rated for 2 amps at 240 volts. That means it can switch up to 480 watts at that voltage correct?

However, I plan to use it at my line voltage of 120 volts. Can I switch the same wattage at this voltage level? That would make it 4 amps to get the same wattage. Or am I limited to only 240 watts at that voltage?

 

[NightStorm]

James,

I think you'll find that the relay will handle upward of 240 volts before arcing between the contacts and that the contacts will handle 2 amperes of current before overheating. Its a different game when dealing with relays and switches. Sorry to disappoint you.

Dan

 

[batterystation]

PercaDan is correct. It is more like Amps in a fuse rating. The fuse will blow at 2 amps regardless of 12volts or 120volts. Wattage is a strange thing. In theory if you were correct, you could switch that relay at 100 amps if the voltage were low enough but we know this is not the case.

 

[snakebite]

in his case the triac inside would get stuck on.as in shorted.it is a ss relay.

 

[batterystation]

Oh I missed that.

 

[asdalton]

Whenever dealing with voltages and currents in a circuit, you always have to ask yourself: Where are the voltage drops? Where is the current flowing? The P=IV formula implies that current I is actually flowing across a voltage drop V, causing power P to be dissipated in that region of the circuit.

Like other people have said, those specifications for maximum voltage and maximum current do not describe simultaneous conditions. The maximum voltage across the relay occurs when the switch is open. The maximum current occurs when the switch is closed, and under those conditions the voltage drop across the switch is much smaller than 240 volts. The relay would melt down if it were actually dissipating 480 watts internally.

 

[INRETECH]

Get the proper part for the application, get a SSR rated for 4 or more amps

I worked in the lighting field, and we used to use 120A SSR's

 

[MrAl]

Hello there James,

The current ratings of most devices are unrelated to the voltage
ratings. This is because they usually depend on independent physical
dimensions of the part. The current depends on the cross
sectional area of some section of the part, while the voltage rating
depends on some minimum distance between adjacent current
carrying oppositely polarized conductors. This means we end up
rating the device for current based on area and rating the voltage
based on distance. These are physically very different,
and there is seldom any interaction between the two.

Also to take into consideration is the fact that a manufacturer
might rate the part differently for different applications,
such as AC or DC. This is mainly because of arc-over and
might make it possible to use a given device at a higher
current when using AC then when using DC.

The latter means the only sure way is to consult the manufacturers
data sheet on the specific part number. If they rate it at
two different voltages then you can compare and decide. Im pretty
sure in your specific case however there is either a triac or
two back-to-back SCR's internally, or perhaps a single SCR and
a bridge rectifier. In either of these cases you would go by
the current rating as a max at any voltage that is lower then
the max voltage, just as you go by the voltage as a max at any
current that is lower then the max (2 amps).

The manufacturer has the last word, of course, so look up the part
and see what comes up :-)

I have to agree with INRETECHs advice too.
Take care for now,
Al