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Using Arduino => Motors, Mechanics, and Power => Topic started by: TECH GEEK on Feb 12, 2011, 08:51 pm

Title: How do i measure the maximum current of an AC variable power supply with resetib
Post by: TECH GEEK on Feb 12, 2011, 08:51 pm
How do i measure the maximum current of an AC variable power supply with an auto-reseting over-current relay???(~8 ampsish) <---this is the topic
yes i know this topic has a long title(i ran out of room too! :smiley-eek: lol )... and no i can not make it shorter even tho i want to :smiley-roll-blue: <---this is snot the topic
Title: Re: How do i measure the maximum current of an AC variable power supply with resetib
Post by: jackrae on Feb 12, 2011, 09:35 pm
Do you mean "how do you measure the current it is being loaded at"  If so the simplest way is to buy a cheap clamp-on test meter.

If you mean, what can you load it to without causing it to trip, then simply read the rating plate.

The resettable trip is probably a simple thermal device, in which case it will have a non-linear current x time relationship, so the maximum load current for say 1 second is probably twice that for 5 seconds

jack
Title: Re: How do i measure the maximum current of an AC variable power supply with resetib
Post by: TECH GEEK on Feb 12, 2011, 10:09 pm
it's a lionel train trainsformer
says:

my main question is this...
The 7.5Volt.Amps. is for what voltage? 8V? 15? Both?
and if it is for only one voltage level, what is the maximum Volt.Amps for the other & what will the maximum V.A. be at differant Voltage levels between 8 &15?

Title: Re: How do i measure the maximum current of an AC variable power supply with resetib
Post by: retrolefty on Feb 12, 2011, 10:46 pm
Quote
my main question is this...
The 7.5Volt.Amps. is for what voltage? 8V? 15? Both?


Volt/amps is equivelent to watts if driving a resistive load. And if it's like the old variable transformers then the maximum 7.5 watt rating should apply anywhere between it's variable voltage output. The actual load current (or wattage or V/A) will of course depend of the load resistance being powered by the variable transformer.

Title: Re: How do i measure the maximum current of an AC variable power supply with resetib
Post by: TECH GEEK on Feb 12, 2011, 10:53 pm

Quote
my main question is this...
The 7.5Volt.Amps. is for what voltage? 8V? 15? Both?


Volt/amps is equivelent to watts if driving a resistive load. And if it's like the old variable transformers then the maximum 7.5 watt rating should apply anywhere between it's variable voltage output. The actual load current (or wattage or V/A) will of course depend of the load resistance being powered by the variable transformer.




i am trying to find the maximum amount of Amps the unit can handle before bad things start happening
i need 5 Amps minumum
Title: Re: How do i measure the maximum current of an AC variable power supply with resetib
Post by: retrolefty on Feb 12, 2011, 11:05 pm
Quote
i am trying to find the maximum amount of Amps the unit can handle before bad things start happening
i need 5 Amps minumum


Well I'm afraid you are out of luck using that source for your 5 amps minimum requirment. 7.5 V/A = 7.5 watts

Amps = watts / volts, so at 8vac output you will get .937 amps (and at 15vac output you will have .5 amps) maximum current rating
Title: Re: How do i measure the maximum current of an AC variable power supply with resetib
Post by: jackrae on Feb 12, 2011, 11:11 pm
Puting it simply, there's no way you will get 5 amps out of this without also releasing the smoke genie

If you want 5 amps at 8 volts then you need 40 volt-amp

If you need 5 amps at 15 volts then you need 75 volt-amps

simples

The reason that ac power units are specified as volt-amps is because the supplier has no way of knowing what power factor your load will be.  At unity PF volt-amps is equal to watts.  However at reducing power factor the available watts is also reduced since watts is specified at the equivalent unity power factor.

Hence if the unit is specified as 40 volt amps with an 8 volt output then you will always get 5 amps out of it, even if you have a low power factor (inductive or capacitive load) only represent say 20 watts

jack

Title: Re: How do i measure the maximum current of an AC variable power supply with resetib
Post by: TECH GEEK on Feb 12, 2011, 11:13 pm
ok thank you for educating me further