Replacement power supply specs question

Question

I have an old Philips clock radio/Bluetooth/android phone charger with a missing power supply. The device calls for 7v 2.5a with positive center tap. In my junk drawer I have a 7.5v 2.1a wall wart. I've read the posts about being as close as possible to original specs, but I'm wondering if being a half volt high and a tad under current rating is acceptable? Thanks!

Answer 1

In general, it's not a good idea to replace one power supply with another that has different specifications, especially in your situation where the voltage of the replacement is higher and the current is lower. The voltage should be be the same, and the current capacity of the replacement supply should be greater or equal, not lower.

That said, there is usually some tolerance built into circuits. It's likely the circuit may be designed to handle an input voltage of 7v ±10%. In your case, the nominal voltage of the replacement supply is 7% high. I would measure the voltage of the replacement supply, and if it is not over 7.5v, you might be okay.

It's likely there are using some sort of regulator anyway inside the radio to drop the voltage down to 5v or something like that, in which the 1/2 volt probably doesn't matter. But if the no-load voltage of the replacement supply is over 7.5v, I still wouldn't try to use it.

The current rating is a little dicier, since you are 16% low. That may be outside the tolerance designed into the circuit. In this case, about the only thing you can do (after verifying the voltage isn't too high) is to go ahead and plug it in and see if the radio works.

Answer 2

It is really difficult to know the answer to your question. In general, when replacing a power supply, you need the exact same voltage and the rated amperage must be the same or greater than the old power supply.

However, most power supplies have a tolerance where they can supply a certain percentage of amperage greater than rated. This will not be good for the power suppply, but at least it will not catch fire if within the tolerance. Voltage tolerance has to do more with your appliance's ability to tolerate the increased voltage which is almost always - NO.

Please check out this Choosing power supply, how to get the voltage and current ratings? it explains things much better than I ever could. Then you can decide if that power supply is suitable for your clock radio.