AC Volts and Amp comparing to DC Volts and Amps
Discussion
BingoBob said:
It's still 12am peak and 5 amp continuous, regardless of the voltage.
I agree with Mr Bingobob here.At these sort of voltages the difference between AC and DC currents really amounts to nought.
So 12 amp peak and 5 amp continuous is the answer
As an example have a look at these pages on the Farnell site. (linky below)
You can see there is a drastic derating (about 10 fold) of contact voltage from AC to DC. Ratings for current are not distinguished for different voltages.
http://uk.farnell.com/jsp/search/browse.jsp?N=2031...
Hope that helps
I have say the safe way would be to stay with 5A continuous is 5A continuous but something tells me that the voltage makes a difference but what I can't remember is if the AC and DC make a differance. Though again a could be wrong.
I did a Watt conversion and at 240 V at 5A = 1200W which is sort of hair drier / hoover range and 12 V at 5A = 60W which is a medium light bulb.
http://www.supercircuits.com/resources/tools/Volts...
Then again one is AC and the other is DC.
I did a Watt conversion and at 240 V at 5A = 1200W which is sort of hair drier / hoover range and 12 V at 5A = 60W which is a medium light bulb.
http://www.supercircuits.com/resources/tools/Volts...
Then again one is AC and the other is DC.
Swiss_Toni said:
I have say the safe way would be to stay with 5A continuous is 5A continuous but something tells me that the voltage makes a difference but what I can't remember is if the AC and DC make a differance. Though again a could be wrong.
I did a Watt conversion and at 240 V at 5A = 1200W which is sort of hair drier / hoover range and 12 V at 5A = 60W which is a medium light bulb.
http://www.supercircuits.com/resources/tools/Volts...
Then again one is AC and the other is DC.
Not sure I understand your logic. I did a Watt conversion and at 240 V at 5A = 1200W which is sort of hair drier / hoover range and 12 V at 5A = 60W which is a medium light bulb.
http://www.supercircuits.com/resources/tools/Volts...
Then again one is AC and the other is DC.
What do you want to do with your relay? It's basically a magnetically operated switch. At low voltages (<500) it doesn't matter whether it's AC or DC.
The current is the important thing. You are looking at your watt conversion in the wrong way. You start with the power of your device and then the current will depend on the voltage. So if you are powering a 100W bulb, for example, then if you use 240V it will draw ~0.4A. If you were to try to power the bulb with a 12V supply then it would draw 8.3A.
If a switch is rated for 5A at mains voltage, its corresponding DC rating will often be less than that. The heating effect is proportional to the square of the current (P=I^2 x R : notice no volts component here), so it might appear that they should be the same, but that isn't the case.
When switching DC, there is a tendency for the current to want to continue flowing. This means that after repeated operation damage can occur to the switch contacts because of arcing, and in extreme cases lead to the contacts welding together. Obviously not good if in a safety critical application. AC doesn't have that tendency as it is always reversing from one direction to the other and this helps to break the current flow as each time it reverses it passes through a short period of 0 current. The physical design of the switch is a factor here.
There isn't an easy way to tell the DC rating from an AC rating, since it varies according to design. Some switches may be OK with similar currents, but others won't. The safe thing to do is get a switch or relay with a manufacturers rating for the conditions you want to use it for. If you can't get that, then assume that the rating is less and leave healthy safety margin.
When switching DC, there is a tendency for the current to want to continue flowing. This means that after repeated operation damage can occur to the switch contacts because of arcing, and in extreme cases lead to the contacts welding together. Obviously not good if in a safety critical application. AC doesn't have that tendency as it is always reversing from one direction to the other and this helps to break the current flow as each time it reverses it passes through a short period of 0 current. The physical design of the switch is a factor here.
There isn't an easy way to tell the DC rating from an AC rating, since it varies according to design. Some switches may be OK with similar currents, but others won't. The safe thing to do is get a switch or relay with a manufacturers rating for the conditions you want to use it for. If you can't get that, then assume that the rating is less and leave healthy safety margin.
Edited by tank slapper on Friday 9th March 22:19
tank slapper said:
If a switch is rated for 5A at mains voltage, its corresponding DC rating will often be less than that. The heating effect is proportional to the square of the current (P=I^2 x R : notice no volts component here), so it might appear that they should be the same, but that isn't the case.
When switching DC, there is a tendency for the current to want to continue flowing. This means that after repeated operation damage can occur to the switch contacts because of arcing, and in extreme cases lead to the contacts welding together. Obviously not good if in a safety critical application. AC doesn't have that tendency as it is always reversing from one direction to the other and this helps to break the current flow as each time it reverses it passes through a short period of 0 current. The physical design of the switch is a factor here.
There isn't an easy way to tell the DC rating from an AC rating, since it varies according to design. Some switches may be OK with similar currents, but others won't. The safe thing to do is get a switch or relay with a manufacturers rating for the conditions you want to use it for. If you can't get that, then assume that the rating is less and leave healthy safety margin.
Good advice and all true. When switching DC, there is a tendency for the current to want to continue flowing. This means that after repeated operation damage can occur to the switch contacts because of arcing, and in extreme cases lead to the contacts welding together. Obviously not good if in a safety critical application. AC doesn't have that tendency as it is always reversing from one direction to the other and this helps to break the current flow as each time it reverses it passes through a short period of 0 current. The physical design of the switch is a factor here.
There isn't an easy way to tell the DC rating from an AC rating, since it varies according to design. Some switches may be OK with similar currents, but others won't. The safe thing to do is get a switch or relay with a manufacturers rating for the conditions you want to use it for. If you can't get that, then assume that the rating is less and leave healthy safety margin.
Edited by tank slapper on Friday 9th March 22:19
Most 5amp relays I've looked at do not derate for 12 volts, but as tank says use parts designed for the task in hand. Ensure there is protection (fuse) upstream rated no higher than the components (including wires) drawing off it.
Interesting stuff, I'm glad I've asked.
The interesting thing is that I want to use the following remote switch for the old F22 work around (F15 in my case as it's a 2012).
In thread http://www.pistonheads.com/gassing/topic.asp?h=0&a... people look to be using the following remote switch http://www.maplin.co.uk/single-channel-wireless-re... .
The key is that this remote switch is rated "Relay output rating: 12A peak / 5A continuous @ AC 230 V"
Now with the above discussion I should be fine as with my car F15 is 5A and job done. However PRE-2011 it's F22 which is a 20A fuse which therefore exceeds the rating.
That being the case if you have F22 you should then you should use a separate relay rate for 20A continuous though no one has mentioned such a thing.
Now to confuse the situation further I found another similar remote switch http://www.focalprice.com/HA613W/FSLHWXJF102_4CH_R... and the rating of the relay in this one is “Relay: 10A / 120V AC, 20A /14V DC” which is contrary to the above info as the DC Ampere are actually higher then the AC one!
Have I missed something
The interesting thing is that I want to use the following remote switch for the old F22 work around (F15 in my case as it's a 2012).
In thread http://www.pistonheads.com/gassing/topic.asp?h=0&a... people look to be using the following remote switch http://www.maplin.co.uk/single-channel-wireless-re... .
The key is that this remote switch is rated "Relay output rating: 12A peak / 5A continuous @ AC 230 V"
Now with the above discussion I should be fine as with my car F15 is 5A and job done. However PRE-2011 it's F22 which is a 20A fuse which therefore exceeds the rating.
That being the case if you have F22 you should then you should use a separate relay rate for 20A continuous though no one has mentioned such a thing.
Now to confuse the situation further I found another similar remote switch http://www.focalprice.com/HA613W/FSLHWXJF102_4CH_R... and the rating of the relay in this one is “Relay: 10A / 120V AC, 20A /14V DC” which is contrary to the above info as the DC Ampere are actually higher then the AC one!
Have I missed something
You need to know what the current draw of the actuator for the exhaust valves is. Or rather, what the current through fuse 22 is. 20amp is a large value. Of course, 20A is the current at which the fuse will blow. The actual current through it will nominally be much less. It does suggest that quite a big current is going through it though - I thought the valves were vacuum activated. Are they in fact electrically operated?
There's nothing to stop your 5 amp relay driving a bigger relay, of course.
This is done frequently in industrial applications. If your remote switch can only switch loads of say, 2amps and you need to switch 10A then use use the output of the switch relay to drive the coil of the bigger relay that can switch the 10A.
If that makes sense.
There's nothing to stop your 5 amp relay driving a bigger relay, of course.
This is done frequently in industrial applications. If your remote switch can only switch loads of say, 2amps and you need to switch 10A then use use the output of the switch relay to drive the coil of the bigger relay that can switch the 10A. If that makes sense.
Swiss_Toni said:
“Relay: 10A / 120V AC, 20A /14V DC” which is contrary to the above info as the DC Ampere are actually higher then the AC one!
Have I missed something
Yeah, that's odd. Maybe because at higher voltages there is more likelyhood of a spark forming and burning the contacts at higher currents. Have I missed something
Either way, that one is hideous
I would use this one which can be powered from the car and which you can also use to turn the headlight washwipe off with:
http://www.maplin.co.uk/2-channel-rf-remote-contro...
That does look like a better option.
I like the idea of headlight washer on/off but that fuse (F26) is in the engine bay where as F22 / F15 is in the boot.
I've just pulled F26 and be done with as I don't drive the car weather that nasty (sleet/snow) so I don't need it.
Interesting on the headlight washer, I was having a chat with the dealer on this and he says on older Vantages if used at high speed they can bend/break taking out the headlight unit. They had one guy who was doing 250 km/h through Germany and used the washer and the headlight pods and everything had to be replaced (not cheap!). The guy couldn't explain why he thought that anything would get cleaned at that speed! The newer cars are automaticly speed limited to use under 120 km/h.
I like the idea of headlight washer on/off but that fuse (F26) is in the engine bay where as F22 / F15 is in the boot.
I've just pulled F26 and be done with as I don't drive the car weather that nasty (sleet/snow) so I don't need it.
Interesting on the headlight washer, I was having a chat with the dealer on this and he says on older Vantages if used at high speed they can bend/break taking out the headlight unit. They had one guy who was doing 250 km/h through Germany and used the washer and the headlight pods and everything had to be replaced (not cheap!). The guy couldn't explain why he thought that anything would get cleaned at that speed! The newer cars are automaticly speed limited to use under 120 km/h.
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