How to know the actual amperes?

Discussion in 'General Electronics Chat' started by paul.1911, Jun 29, 2012.

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Mar 25, 2012
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Hi
I'm noob in electronics things as i'm learning commerce from 7 years
I just want to ask for my knowledge

I've tested 12v 60W Halogen bulb with 12V 1amp transformer
as per my knowledge 60W bulb will require 5 amps at 12v to glow on its full efficiency.
but when i check the ampere it was drawing 3 amps...

How it is possible? If the source amp is 1???
Does a transformer also needs to be 5 amp to make the bulb glow on its full efficiency??

2. #12 Expert

Nov 30, 2010
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Yes. The transformer must be capable of delivering the whole 5 amps that the lamp is rated for. What you have done is invite the transformer to melt by asking it to deliver 5 amps when it is only designed for 1 amp.

Mar 25, 2012
33
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Then why it was showing 3 amps?
How can i check the actual ampere when i don't know the source ampere?
Take the example of my transformer.

4. MrChips Moderator

Oct 2, 2009
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Using an under-rated transformer is asking for trouble. The transformer will overheat, melt the insulation, short circuit and start a fire.
Don't skimp and use anything less than 5A.

5. WBahn Moderator

Mar 31, 2012
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It was drawing 3A because that is the most it could deliver into that load. The transformer was rated at 1A, meaning that you should not attempt to draw anything more than that otherwise you are overloading the transformer and risking damage to it and possibly a fire. It does not mean that it is able to magically limit the current to the 1A. It will provide whatever it can as long as it can. In this case, the combined resistance of the bulb and the transformer windings limited the current draw to 3A. A significant fraction of the total power consumed is probably going into heat in the transformer.

I don't understand the question about how you can check the actual amperage. You've been taking some kind of measurement to find out that it's drawing 3A, right?

paul.1911 likes this.

Mar 25, 2012
33
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I mean.. i know that my transformer is rated as 1 amp
but how can i check this amp if i don't know or it was not rated??

7. WBahn Moderator

Mar 31, 2012
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Oh, I see what you are getting at.

I would first look for any markings on the transformer and see if you can look them up on line. In some cases, this could involve some detective work in first tracking down the manufacturer, perhaps through some logo or trademark.

Failing that, my guess is that you would need to power it up and increase the load until some maximum voltage was being dropped across the transformer. Now, if you don't know anything about the transformer, including what the rated input voltage is, then I don't have any firm suggestions for you (other than to find a different transformer). It will be interesting to see if folks here that use transformers a lot have some good practical suggestions.

Mar 25, 2012
33
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I've some more questions....
Suppose i've a transformer with more than 5A take as 7 or 8
Will that bulb get blew? I mean get fuse if i deliver more than 5A? It will turn black right?
if this is the same will be with 12v 7.2A battery? I think not.
what will be?
pls i want to clear my doubts.

9. #12 Expert

Nov 30, 2010
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Go to the top of the "Chat" page and read, "Ohm's Law for noobies".

10. MrChips Moderator

Oct 2, 2009
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3,365
As WBahn suggests, connect the transformer to AC mains with no load attached. Measure the output voltage. Apply a known load or measure the current on a variable load. Measure the voltage output as the current increases. At the same time put your hand on the transformer and monitor the temperature rise. You will know when you are overloading the transformer.

paul.1911 likes this.

Mar 25, 2012
33
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question erased!

Last edited: Jun 29, 2012
12. WBahn Moderator

Mar 31, 2012
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Now you are getting into the realm of automotive modifications, which is specifically prohibited on this forum, both for legal and out of safety concerns. Look at the stickies at the top of the forum lists and you will find one on this subject. It contains links to other forums and sites that might suit your needs.

Mar 25, 2012
33
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okay... thanks anyway!

14. Wendy Moderator

Mar 24, 2008
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As has been stated, the owners of All About Circuits has elected not to host discussions of automotive electrical system modifications/enhancements due to safety concerns, the potential of legal ramifications and the possible circumvention of vehicle regulations at the state and federal level.

This thread is against the AAC forum rules, Chapter 6, as seen here: