Can I use a Step-Down Transformer to lower the output voltage of an AC adapter?

Question

Here is the deal...

I have an IBM LCD monitor that I got on the cheap from Ebay, the issue was that it didn't have a base or a power supply.

Big deal, I thought because the plan was to wall-mount it and I had 6-7 old ac adapters lying around that would have roughly the same voltage and amperage.

So, now that I got it in the mail, I find that it requires a 12v 3.0 amp adapter. Unfortunately, all the adapters I have that provide enough amperage are all 19V or higher.

Back to the question, couldn't I just add a transformer inline with the output cable to step down the voltage to a level that will not fry the monitor?

Where could I find a step-down transformer that small?

The adapters I am looking to use are all old notebook adapters. I imagine these are too well regulated to work considering what you are all telling me.

The main point of this was to figure out a way to do it and avoid having to buy the adapter outright. There is a good level of satisfaction knowing that you were able to avoid spending money by doing things yourself.

I found a 15v 4A adapter, but again it is a notebook adapter. Looks like I will have to suck it up and buy a new one, unless anyone has any other ideas...

Answer 1

It depends.
If the adapters you have generate a regulated DC output, then trying to reduce the voltage by running the AC side off a transformer isn't going to be successful.

However, if the adapter produces an unregulated DC output, or an AC output, then if you can find the correct transformer it should work. For example, using a 24V adapter with a transformer sold to run 120V appliances off a 240V mains would give you the 2:1 ratio that you need. You might have to do some wiring. If the output of the adapter is AC, the step-down transformer can go on either the mains side or the output side of the adapter. If the output of the adapter is DC, you would have to put the step-down on the mains side.

I would be a little careful here, though. It is likely that the monitor needs a regulated DC supply, in which case you have to buy something that supplies the required voltage and current. Usually, monitors that I have seen come with a brick power supply similar to what is used for laptops, and those generally provide a high-current, regulated, DC output.

Answer 2

First try putting a load (like a 12v automotive interior or brake lamp)on that 19V 3A adapter and measure the voltage. Most cheap adapters are just a transformer and a bridge rectifier, so the output voltage goes to the peak value of the AC out, instead of RMS, when it's unloaded. Any supply that is marked for 3 amps or more and 12-20 volts DC out ought to do the job for you though because all it's doing is acting as input to the internal, well-regulated, power supply. Make sure you get the polarity right.

Answer 3

Don't try to step down the output voltage of an existing adapter. An AC adapter consists of a step-down transformer, which converts the high-voltage AC (house wiring) to low-voltage AC, and a rectifier, which converts the low-voltage AC to DC (the kind of current used in electronic equipment).

Since a transformer only works with AC, it cannot step down the output which is already DC. There are DC-to-DC voltage converters out there, but they are outrageously expensive.

I would recommend finding another adapter. But if you must use an existing one, there are 12V voltage regulator chips out there that will limit the voltage from the adapter. It is probably harder to find one of those than the correct adapter, though, and you'd have to wire up a circuit.

Answer 4

The problem would be that the output of the power supply would be DC, not AC. Once you have DC transformers (being AC devices) would not work.

You can buy AC wall-mount transformers that will output 12V at 3amps from places online like Jameco or Mouser.

Answer 5

Just buy the 12V 3.0 adapter made for your monitor. You'll never find a 36VA 19VAC to 12VAC step-down transformer.

Answer 6

You can probably find the adapter you need at Radio Shack. I did that for an electronic keyboard a couple years ago. Or you can order it from IBM.

Answer 7

Power inverters convert direct current (DC), the power that comes from a car battery, into alternating current (AC), the kind of power supplied to your home and the power larger electronics need to function.