no-load voltage on ac-dc adapter

Question

I recently acquired a toy that uses this transformer:

but since I live in a 220V area, I cannot use it directly.

I have this generic AC-DC adapter:

which in theory is all I need because it has a 4.5V output and enough amperage.

However, I'm a bit concerned since when measuring the voltage of the ac-dc adapter when it's set to 4.5V output, I get ~10.5V. I know that the voltage measured in a no-load scenario is supposed to be higher than when it actually has load, but it seems a little bit too much.

When set to 9V, it outputs ~16.6V

Is it safe to use this ac-dc adapter with this toy? Or it might be broken and supply more volts than it indicates?

Answer 1

This is one of the hazards of trying to use a greatly overrated (3:1) unregulated supply. That's actually the first time I've seen a no-load voltage specified on the label, very handy. Yes, it could potentially damage the toy.

You could try the adapter on the 3V setting and see if the product works, but it might be a bit on the low side.

Another possibility is to add a dummy load to the universal adapter (something like 10-15 ohms 5-10W), but it might get very hot. Or you could poke around and find a ~300mA 4.5V adapter that can accept your mains voltage.

Answer 2

Test them both with resistive loads.

Test load points:

• Test #1 4.5V @ 0.3A is a 15 Ohm, 1.5 W resistor
• Test #2 4.5V @ 0.15A is 30 Ohm 0.75 W resistor
• Test #3 4.5V @ 0.075A is 60 Ohm 0.5 W resistor

Measure the voltage across each load for both power supples using the three test loads above. If they regulate to about the same voltage levels then it should be fairly safe to plug in. Most unloaded supplies will float pretty high and most electronic devices pull them down very fast when plugged in due to the inrush current to the capacitors.

Answer 3

The kind of universal adapter you have there is most commonly created with a 317 voltage regulator. Open it up and have a look on how it works. If it is one of these things (I have one too that looks the same, same specs, just different sticker) then the following applies:

The 317 needs at least ~10mA current to start properly regulating. At 4.5V that is 450Ω. That means you waste almost 50mW, I hope your power bill won't mind.

Apply that resistor and measure the voltage. If it is roughly at 4.5V then go on, if it is still at 10.5V something is really wrong. Normally the circuit of such devices has a voltage divider based on 240Ω from \$V_{OUT}\$ to the \$ADJ\$ terminal, putting in various resistors as you slide the switch (possibly also selecting different windings of the transformer).

Lets stay a moment here. Often single transformer versions of that thing have ~16V-18V output from the transformer. Assuming something is broken in the path from \$V_{OUT}\$ to \$ADJ\$ and just the ~587Ω resistor for the 4.5V select is active, the 317 would regulate to 10V-12V. Check if this is the case.

Back on track, when you verified that you get 4.5V out at 5mA load, you can be quite sure to not damage the toy by too much voltage (assuming you always leave the resistor in place). Now if you have a lab bench power supply that does proper current limiting, set it to 300mA 4.5V and connect the toy and measure current flow. Is there any time that the current flow touches the 300mA (and wants to go beyond it)? If that is the case it might be that the designers of the toy made an unfortunate decision: rely on the implicit power limiting "functionality" of the adaptor to limit things like inrush current. I myself accidentally blew up a toy CD player this way. This is especially likely since the original supply is unregulated.

So if you could determine (either by inspection of the toys circuit, or the lab bench power supply and measurement method) that it doesn't do this, I would say it is quite safe to use that universal power supply.

If any of those things above did not go as expected (or if you couldn't measure the current thing), then all I can say is: good luck, I hope you can find the toy elsewhere to buy again...