I have bought this electronic drumkit by Roland about a year ago. It was second hand. It came with a 230v ac to 9 v dc 1000ma adapter. On the drum module, a few types of compatible adapters are written. It's official. The adapter I have is one of them and it's also made by Roland. Now the problem. I wanted to take a look at how many amps the drum module takes from the maximum of 1A of the adapter by screwing the module open and using my multimeter on the power dc input. Weirdly, it shows that it's using about 4.3 amps, which is way more than the adapter should be able to handle. Then, I unplugged the module and left the adapter plugged into my outlet and checked the output voltage. It shows 13.4, not 9. This really surprised me. I checked my multimeter by checking other power supplies for their voltages, and they show about the same as they are claiming to output.

Does anyone know what this could be?

  • 2
    \$\begingroup\$ You know that in order to measure current you need to connect the Ammeter in series, don't you? The output voltage of such an adapters might be higher when unloaded. \$\endgroup\$
    – Eugene Sh.
    Jan 31 '18 at 18:10
  • \$\begingroup\$ Hmm. That might be difficult to do since it's all pcb. I'll try to DIY my way out of this. Thanks for the reply \$\endgroup\$
    – Hendriks3D
    Jan 31 '18 at 18:12
  • \$\begingroup\$ I just measured the voltage when plugged in. It does drop, but to 11.8v as opposed to 9v. \$\endgroup\$
    – Hendriks3D
    Jan 31 '18 at 18:15
  • \$\begingroup\$ What does it state on the adapter as an output? \$\endgroup\$
    – Eugene Sh.
    Jan 31 '18 at 18:16
  • 2
    \$\begingroup\$ 0.12mA? I find that a little hard to believe. In any case, how are you planning to charge your phone from it? \$\endgroup\$
    – Finbarr
    Jan 31 '18 at 20:32

DMM's do not measure RMS but will display RMS for a sinewave ( Unless otherwise labeled)

This is done by measuring some other method like dual-slope integrated average or full-wave rectified Peak, then converted to RMS by some ratio for a sine wave.

Judging by your measurements for a 9V supply, of 13.4V I suspect it is an unregulated supply, which uses a full wave bridge to Caps and is thus 1.414 x the RMS voltage with no load. There is also transformer conduction losses so that at full load the voltage may will drop to 9V with a resistive 1A load. This means it may rise another 5 ~ 10% with no load resulting in 1.5x9V=13.5V.

When the cap is charging just before the peak, to supply a load, the duty cycle may be say 10% and thus 10x the current needed to supply the DC out. THus if your DMM measured Peak voltage or current and converted to RMS using 0.707 for a sine wave, then you may be actually measuring peak current of 4.3A/0.707 = 6.1 A but at a duty cycle ~ 10%~15% so the actual average current may be 0.6 to 0.9A amp

  • \$\begingroup\$ Wow, that explains it really well. Thanks. Can you suggest me a way to limit the current that a 1A car charger is drawing from the module to let's say 800mA? I don't want the charging phone to take over the whole power adapter. And I also don't want to buy a compatible 2A or 1.5A power adapter. If there's no other option, then so be it. \$\endgroup\$
    – Hendriks3D
    Feb 4 '18 at 15:25
  • \$\begingroup\$ 5V charger in car? who knows what you have \$\endgroup\$ Feb 4 '18 at 15:38
  • \$\begingroup\$ A car charger is 12v to 5v converter. \$\endgroup\$
    – Hendriks3D
    Feb 4 '18 at 15:44
  • \$\begingroup\$ There are simple methods if you use Ohm's law and modify the internals. But is it worth the trouble if it fails? \$\endgroup\$ Feb 4 '18 at 15:53
  • \$\begingroup\$ I have a test phone to see if it charges healthy. The car charger is next to nothing so, yes. I also suppose that I can't damage the drummodule if I put the charger in parallel to the power input of it. \$\endgroup\$
    – Hendriks3D
    Feb 4 '18 at 15:59

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