Electronics 2000

Hi there

I've got a drum machine that asks for a 9VDC 50mA PSU. Is it possible with simple, passive components (IE I've got a couple of boxes of resistors and some bits of breadboad) to build a current divider circuit that somehow limits the current into the drum machine to 50mA, when driven from a universal adapter rated at 300mA, 9VDC? I've tried it before with a 150mA PSU and the audio outs are carrying a hum in the background that isn't there with batteries.

I suppose my question is, can this be worked out easily, or do I need to know things like the resistance caused by the drum machine's circuit?

Thanks!

The rating of 50 mA is a maximum rating. It's the maximum current that the drum machine will need, so it is actually limited by design not to use more than 50 mA. The 300 mA rating on the adapter is also a maximum, but in terms of how much current the adapter is able to deliver. If you plug the adapter in the mains but don't connect anything, the output current is 0 mA, not 300 mA. The output current depends on what you connect to the output. Hard load, high current. Soft load, low current. No load, no current. The hum is AC voltage, which is present on top of a bad quality DC voltage. The more you pay for an adapter, the less AC you should expect. But the AC voltage is always enlarged as the output current goes up.
If you have a multimeter you can measure this AC voltage on the adapter output on different brands of adapters. The AC voltage on batteries is 0 V, yes. Pure DC voltage.

That's a great reply Thomas, very clear and useful. I'll go looking for a better quality PSU, thanks for your time!

You're welcome - happy hunting

Just to add that what you are looking for is a PSU with the word "regulated" in its title. If you get one where the ripple is specified (and it's a suitably low ripple voltage) then it's more likely to do what you want.