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Breaktru Forum  |  eCigarette Forum  |  Modding  |  Topic: Amp limit at ***VAC
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Author Topic: Amp limit at ***VAC  (Read 4707 times)

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Offline digiw0rx

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Amp limit at ***VAC
« on: October 24, 2013, 03:48:00 PM »
I see this on lots of switches, like 4A at 250VAC, 8A at 125VAC, am i right in thinking that 8VDC would provide a much higher amp limit for these switches?

Offline CraigHB

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Re: Amp limit at ***VAC
« Reply #1 on: October 24, 2013, 08:04:47 PM »
Switches are typically rated with a voltage/amperage or they're rated in Volt-Amps or VA.  Switches intended for use with DC supplies tend to be rated with voltage/amperage.  Switches intended for use with AC supplies, especially utility power, are typically rated in VA.

For purely DC, Volt-Amps are the same as Watts.  For inductive loads like motors and transformers powered by AC sources, Volt-Amps differ by Watts in terms of a power factor.  Usually a 40% power factor is a safe assumption so a switch rated for 100 Volt-Amps is good for 60 Watts. 

If a switch is rated with a voltage and a current, it's a partially safe assumption to multiply them for a power rating.  So a 3A, 24V switch is good for 72 Watts.  At 10 Volts, it should be able to handle 7 Amps.  Though that only works to a point.  It also depends on the heat dissipation of the contacts themselves which is not rated.  It's doubtful a 3A/24V switch could handle 24 Amps at 3 Volts.  The main thing to take away is that within reason, you can use a higher current with a lower voltage without significant loss in switch longevity.

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Re: Amp limit at ***VAC
« Reply #2 on: October 25, 2013, 01:38:38 PM »
I have done several searches in past years for DC vs AC switch rating and have not concluded 100% that what I read was absolutely true. Probably because the people commenting are not experts but I do highly regard Craig's analogy.
« Last Edit: September 18, 2014, 03:48:02 PM by Breaktru »

Offline digiw0rx

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Re: Amp limit at ***VAC
« Reply #3 on: October 25, 2013, 05:14:22 PM »
Thanks for those responses guys! informative as always, love this forum!

I was just a little concerned about putting together mods with 10 amp limits if the switch would of been the weakest point.

EDIT

I read a couple of articles online about the rating of the switch being to do with the arc'ing which occurs when closing/opening a circuit, at 8.4 volts i'd imagine only a tiny arc would be present?

Online Breaktru

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Re: Amp limit at ***VAC
« Reply #4 on: October 25, 2013, 05:49:41 PM »
If you are building a step down converter mod such as the OKR or PTR, the 10A is in the output portion of the circuit which I doubt you would ever come near anyway. The P.B. would be in the input of the circuit and the current would be lower. The datasheet doesn't state what it is but I'm sure there is a formula for it. Or...... you can put an amp meter on it to determine what it is.

DC-DC Converters/Regulators
Step down = Higher Output current - Lower Input current
Step Up = Lower Output current - Higher Input current

Offline CraigHB

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Re: Amp limit at ***VAC
« Reply #5 on: October 25, 2013, 06:23:06 PM »
DC-DC converters conserve power so power in is equal to power out plus converter losses.  A safe assumption is 10% so for example, if you're outputting 5V and 3A, that would 15 Watts.  Adding 10% for losses would be an input power of 15 plus 1.5 for 16.5 Watts.  With series cells nominal voltage is 7.4V so input current would be 16.5 Watts divided by 7.4 Volts for 2.2 Amps.

The same is true for a boost converter.  In that case, nominal input voltge would be 3.7V so for the example above, input current would be 4.5 Amps.

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Re: Amp limit at ***VAC
« Reply #6 on: October 25, 2013, 06:30:33 PM »
I think you confused me Craig
Are you saying that what I had said is not correct?

Okay sorry, I just read it again. You are giving the formula. Got it.
Step down = 3A out, 2.2A in
Step up / boost = 3A out, 4.5A in
« Last Edit: October 25, 2013, 06:36:53 PM by Breaktru »

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Re: Amp limit at ***VAC
« Reply #7 on: October 25, 2013, 06:38:49 PM »
Craig, what do you think about the DC vs AC switch rating in Reply #2
Any truth to it?

Offline CraigHB

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Re: Amp limit at ***VAC
« Reply #8 on: October 25, 2013, 06:49:41 PM »
Yes, it's a pretty simple calculation really.  Just add 10% to output power and divide by input voltage to find supply demand.

Of course, converter efficiency determines that "10%" number.  In same cases it may be as low as 5%, in others it may be as high as 15%.  Just depends on converter efficiency which varies for any converter depending on load.  Generally, the heavier the load, the lower the efficiency.

Craig, what do you think about the DC vs AC switch rating in Reply #2
Any truth to it?

Yes, it's true that voltage is always ramping up and ramping down with the sine wave of an AC power source and that may have an effect on contact wear.  However, inductive AC loads like motors and transformers induce nasty voltage spikes on switches which can also have an impact on contact wear.  To be honest, I'd have to do some research on the issue to say for sure, but I do find it surprising to see a huge difference in power ratings between AC and DC like that.

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