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Trying to revive an old electronic keyboard. It is labelled to take 9-12V and I have an appropriate connector, but don't have an idea of how much current to allow for.

It can also be powered by 6 D cells, and presuming that they should last for some non-trivial length of time, that should give some kind of reasonable approximation, but I don't know the calculation to do.

So, if I want to use a wall-wart or line-lump transformer of big enough but not overkill from my junk box, how to reasonably estimate from the 6-D batteries to how much current to allow for?

sdg
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3 Answers3

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how much current to allow for?

You have to supply more current than the keyboard needs. If you supply the right voltage then the keyboard will draw itself the needed current.

What is important here is to supply it with the right voltage. So 9-12V and also check if DC or AC.

  • If you supply it with more current then nothing bad will happen.

  • If you supply it with less current then it will not work, nothing bad happens.

  • If you supply it with more voltage then it needs you will destroy it.

If you know, how long did the keyboard last on that batteries, then you can calculate the power consumption based on the D batteries capacity.

I am assuming the capacity of 8000mAh http://en.wikipedia.org/wiki/D_battery

If the keyboard was able to play for 4 hours, then:

8Ah / 4h = 2A

So the average current draw was 2 Amperes. Bare in mind that this is only the average. So you should supply more.

drzymala
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Just as a guess, figure they wouldn't design the thing to take more than 1 A from the batteries. Fortunately small power supplies, like wall warts, are readily available in the 1-2 A and 9-12 V range. I'd go for a 12 V 1 A supply and very likely that will be fine. That's 12 W after all, which sounds like quite a lot for a keyboard. It would probably work with 12 V and 500 mA, but you're not going to save much money compared to a 12 V 1 A supply.

Olin Lathrop
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  • Not just a guess, a perfect reasoning! Voting up. Watch out for using a switching mode or regulated wall wart - cheap old ones have no regulator and can easily output more than 12V even you expect 12V exactly. – Gee Bee Mar 16 '16 at 17:02
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If you have an appropriate (but possibly too physically big) power supply in your junk box, measure the current taken when connected (you might have to make an adapter to break-in to the power cable) to the keyboard. Some keys should be played with volume up full to get a decent estimate.

Do you have a multimeter that measures DC current? If you only have a voltage measuring instrument, put a 1ohm resistor in series where you would have fitted the ammeter and measure the volts across it.

Current is what you need to know but if it's power it's PSU output voltage x measured (or inferred) current

Andy aka
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  • @martini - please feel free to justify what you think is incorrect about what i've said so I have the benefit of your wisdom. – Andy aka Mar 31 '13 at 19:55
  • Measuring the voltage on the 1Ω resistor will not give you the correct answer.Ok when the current will be small it might work, but it is a bad advice. Second, If you play some higher notes on a keyboard the current will not be so dramatic as when you play the lower notes.Well, this also is not so important.What I want to say is that OP is probably not so good in electrical engineering and advising him to messing with an ammeter is a very bad idea.And don't tell him to connect a possibly too big power supply. He asked for what to connect to the keyboard and not how to measure the current! – drzymala Mar 31 '13 at 20:08
  • @martini D type cells are likely not to be used in applications requiring a continous DC current draw of more than 200mA (http://www.techlib.com/reference/batteries.html) and assuming this is true the volt-drop across 1 ohm will be no-greater than 200mV. This will create an error that is unimportant. Maybe you know something I don't? If he's got a multimeter then he probably knows how to use it. If he's got a 1 ohm resistor then even more so. – Andy aka Mar 31 '13 at 20:26
  • I rethought my complaint about your answer. I think he does know what he is doing. I apologize. I think I did not got his intention. But I got a question for you. Have you had some experience with battery powered keyboards? How long can they last on batteries? – drzymala Mar 31 '13 at 20:38
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    @Martini - Apology accepted. I have a yamaha DGX-205 portable grand and a yamaha DD65 drum kit but never run them from batteries so I don't know. – Andy aka Mar 31 '13 at 20:47
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    Advising to use a "too big" power supply without qualification is irresponsible. If he connects a 24 V supply, for example, that would quite likely damage his equipment. We do engineering here, so we talk about Volt and Amp ratings of supplies, not their "bigness". – Olin Lathrop Apr 02 '13 at 12:17
  • @OlinLathrop - I have added the word "physically" to my answer. – Andy aka Apr 02 '13 at 12:22
  • Completely the wrong response. I give up. -1 – Olin Lathrop Apr 02 '13 at 13:10
  • @Olin I did use the word "appropriate" in my answer and as the OP knows what the voltage rating for the instrument is (he says it in his question) I think my answer is perfectly OK. – Andy aka Apr 02 '13 at 13:14
  • "Appropriate" is circular and therefore useless. It works fine for someone that already knows the answer, but is of no use to someone that doesn't since they don't know what appropriate is. Physical size is also irrelevant, so your last edit actually made things worse. The power supply has to have the right voltage. The current capability can be much bigger, but you didn't actually say any of that and someone that doesn't already know could easily be misled. Again, we do engineering here, so talk in real units and values, not "big". – Olin Lathrop Apr 02 '13 at 14:58