Am I correctly concluding that these two AC/DC adapters are interchangeable

Question

I'm in the US with 120V mains. I have a "heavy" style AC/DC adapter (weighs a pound or so) and a lighter one (weighs a couple of ounces) which came off a 1TB Seagate NAS. Can I replace the heavy one with the light one?

Here are the specs for the heavy one:

AC100V  50/60Hz 19VA
DC 12V 1000mA

and here are the specs for the light one:

AC INPUT: 100-240V~ 50-60Hz, 0.65A max, 51-80 VA
DC OUTPUT: 12V  2A

If I understand correctly, the DC output of the lighter one is 12V and is capable of handling a draw (e.g. from a DC motor) up to 2000mA, so in that regard it is just as capable as (more capable than) the heavy one.

But I know pretty much diddly about electric, so I don't know how to interpret what 0.65A max, 51-80VA means on the AC input side.

P.S. The writing is so small, I can't tell if that is a tilde ~ or a minus-sign after the 240V.

Answer 1

You already comprehended that the current delivering capabilities of the "lighter" one are basically double the heavy one.

It is doubtful that you will get the same type of outputs from both of these in terms of voltage over various amounts of load current. The "heavy" one is a simple transformer wall wart with a diode bridge rectifier and maybe a filter capacitor. At small to no load this heavy one will most likely be outputting a voltage that is nearly half again as much as 12V. As load is increased up to the rated 1A the average voltage will come closer to 12V but the output will also be pulsing with ripple at 60Hz.

The "light" unit will tend to have a much smoother output voltage and less ripple effect. Such ripple will, if any, be at a much higher frequency matching the switching frequency of the energy converter inside. There will also be an output much closer to the rated voltage at no load and staying fairly close to that level as the load is increased up to the maximum 2A level.

Compatibility between these two will depend on:

1. Actual current load
2. Tolerance of the load to voltage level
3. How the load deals with noise and ripple
4. If the connector ends are same size and polarity

Answer 2

Probably

You are correct that the light one provides the same voltage and more current than the heavy one, so is more capable. In terms of voltage, current, power etc, it can do the job.

The 0.65A max, 51-80VA marking tells you how much current and VA the power supply draws. There is a range, because the power supply also has a wide input voltage range, and at some voltage ranges it will be more efficient than at others. The heavy one draws 19VA from mains, and puts out 12VA on the DC side. The light one draws 51-80VA and puts out 24VA. So it's looks like it's rather less efficient. It will be slightly more expensive to run, and you might notice it getting warm.

There are a few other aspects of the supply, which might be relevant if you're thinking of replacing one with the other.

• Isolation. You should not swap isolated and non-isolated supplies. The vast majority of power supplies are isolated these days, but it never hurts to double check.
• Ripple. The heavy supply might produce cleaner power than the light one. In other words, both supplies produce 12V on average, but the output varies a little around that average. The heavy supply probably varies at 60Hz, the light one at a much higher frequency. Some equipment doesn't like being powered by noisy supplies, but most things are just fine. You don't say what you want to power.
• Safety. There are some shockingly bad quality power supplies being imported by dodgy ebay sellers. If the light supply is one of these, don't use it at all. Though if I understand your question correctly, it was sold by seagate, so it'll be fine.
• Voltage droop. As Michael Karas points out in his answer, the heavy supply may or may not be regulated. If it's regulated, it makes no difference. If it isn't then it only provides 12V when it's providing 1A. At lower currents it provides a higher voltage, and vice versa. I've written the rest of this answer on the assumption that it is regulated.