# What is difference between 2 transformers rated to give output the same voltage but different current (ampere)?

What is difference between 2 transformers rated to give output the same voltage but different current (ampere)?

Such as, if a stepdown transformer is rated as 12V 600mA, and another is rated as 12V 1000mA (or 1A); what is difference in their function? in spite of same voltage, would they produce different current on load on secondary?

• That means such current-ratings are to indicate upper -limit of current that the transformer could provide/ tolerate. Not the actual current through the transformer. Is that? Jul 13, 2016 at 12:50
• replied as "Yes" by user @Trashman in their answer. Jul 14, 2016 at 10:16

The real difference is in the power handling. The actual power (and therefore current) provided by the transformer is determined by the load on the transformer.

The 1000 mA transformer can handle a load that consumes more power and also more current.

For the sake of simplicity, I'm going to assume a resistive load with no reactance, and therefore a power factor of 1. You will probably likely want to look up those terms if you're interested in learning more about transformers and AC power in general*.

For example,

If you have a 5W load (device you want to power) connected, it will draw 5W / 12 V = 417 mA (approximately). If you connected this load to either tranformer, it would draw 417 mA.

If you have, say a 10 W load, it would draw (or attempt to draw) 10 W / 12 V = 833 mA (approximately) - it would draw this if connected to the 1000 mA transformer. If you tried to connect this load to the 600 mA transformer then one of the following would occur:

1. The load would not function (because the voltage droops or sags too low to power the device)

2. The load would draw more current than the transformer can handle and possibly damage the transformer (depending on the transformer and load, could be immediate, or could slowly heat up and deteriorate over time)

3. The transformer fuses would "Blow" if installed and properly size for the transformer (to prevent the damage in #2)

This is a simplified explanation. Most transformers can actually handle a little more than what their rating states - so they won't always get damaged, but you should always stay within the manufacturer's specifications to ensure maximum life of the equipment.

*If the loads are reactive and have a pf other than 1, you cannot simply divide power by voltage to get current.

• I think it might be worth clarifying that #1 is because the output voltage of a transformer will "sag" under load, with the sag generally becoming worse when the load exceeds the rating of the transformer. A typical transformer would have two meaningful numbers: (1) how much current can be drawn while the transformer outputs a useful voltage; (2) how much current can be drawn without damage. Some small transformers could drive a dead short all day without damage because the output would sag to nothing while the current was low enough to be endured. Some other transformers... Jul 12, 2016 at 17:37
• ...might overheat under sustained conditions beyond the rated maximum even if the voltage wouldn't sag much until then. Jul 12, 2016 at 17:39
• Both good points. I tried to hint at these, but left them out for sake of trying to keep the answer as simple as possible. Jul 12, 2016 at 17:56
• I also suspect that the original asker isn't actually asking about a transformer per se, but probably a "wall wart" which is a plug-in transformer and rectifier that comes with a lot of electronics. Don't see too many 12 V AC Transformers Jul 12, 2016 at 17:59
• @Always Confused, I can't reply to your comment directly so I'll reply here. Essentially, you have it right in your comment. It's the limit, not the actual current. Jul 13, 2016 at 22:05

On simple load, needed current can be calculated using Ohm low: $$Current = Voltage / Resistance$$ You can read more here https://en.wikipedia.org/wiki/Ohm%27s_law