Can I use a higher current AC/AC adaptor?

[[email protected]]

Hello

I have to replace a broken power adaptor that was rated at AC/AC 12v
500ma. I have found that I already have a spare power adaptor rated
AC/AC 12v 1000ma.

I know little of electronics, and was wondering if it is safe to use
the 1000ma supply? Does the equipment only draw as much current as it
needs or will it somehow overload it?

Many thanks

Paul

 

[John Fields]

Hello

I have to replace a broken power adaptor that was rated at AC/AC 12v
500ma. I have found that I already have a spare power adaptor rated
AC/AC 12v 1000ma.

I know little of electronics, and was wondering if it is safe to use
the 1000ma supply? Does the equipment only draw as much current as it
needs or will it somehow overload it?

---
The problem with small supplies is that they have poor regulation.

That is, with a 1000 mA load on it the output will likely be at 12V,
but with a 500 mA load it surely won't. It'll be higher, and if it
gets high enough could damage what it's driving.

One solution would be to put an extra load on the supply so that it
supplies 500mA to your device and 500mA to the dummy load. For a
12V source and a 500mA load you could use a:

E 12V
R = --- = ------ = 24 ohm resistor.
I 0.5A


It would be dissipating:


P = IE = 0.5A * 12V = 6 watts


So I'd go for something like a 24 ohm 12 watt resistor, and that
resistor's still going to get pretty hot.

Another way out would be to connect a 12V, 6 watt incandescent lamp
across the supply.


But...

All of that'll be unimportant if the device you have now can take
the higher voltage.

Do you have any specs for the device and can you measure the
supply's voltager>

 

[[email protected]]

Thanks for the very informative reply.

I think I'll be safe and buy a new 500ma adaptor. I know that the
eqipment wont like a higher voltage.

Paul

 

[Alan Nishioka]

The problem with small supplies is that they have poor regulation.

But isn't a 12V AC power adaptor simply a transformer in a box?
It seems to me his 1000ma adaptor should work fine.

Alan Nishioka

 

[John Fields]

But isn't a 12V AC power adaptor simply a transformer in a box?

---
It might, but it might not.

Assuming +/- 30% regulation from no load to full load means that a
transformer rated to have a 12VRMS output with a 1000mA load will
have about 13.8V across it when it's loaded with 500mA load. Add
another 1.38V for 10% high line and the output will rise to about
15.2V


With the properly rated transformer in there, loaded with 500mA, the
output will be 12VRMS nominal and, with a 10% high line, 13.2VRMS.

So, the question becomes one of whether the appliance take the extra
2 volts. Maybe, maybe not, but I'd prefer to err on the safe side.

 

[Alan Nishioka]

I'm still confused. I must be missing something. If it is just a
transformer, shouldn't the output voltage be determined by the turns
ratio alone?

I always thought that an AC wall wart rated 1000ma simply meant that
the wires were thick enough to carry that much current.

Alan Nishioka

 

[John Popelish]

I'm still confused. I must be missing something. If it is just a
transformer, shouldn't the output voltage be determined by the turns
ratio alone?

I always thought that an AC wall wart rated 1000ma simply meant that
the wires were thick enough to carry that much current.

That determines the unloaded voltage. When load current passes
through the secondary (and its transformed equivalent through the
primary), those windings drop voltage proportional to their
resistances and those currents.
]
Small transformers are often made with intentionally high resistance
windings, so that their current is limited in the event of a short
circuit load, so that they do not reach dangerous temperature (catch
fire). This causes them to have a lot of voltage droop as their load
current reaches rated load. Their rated voltage applies at rated load
current, so their output voltage will be higher at lighter loads.

 

[John Fields]

I'm still confused. I must be missing something. If it is just a
transformer, shouldn't the output voltage be determined by the turns
ratio alone?