Author Topic: how to determine amps?  (Read 642 times)

Offline tschmidlin

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how to determine amps?
« on: September 07, 2010, 01:35:58 PM »
Does anyone know of a way to figure out how many amps a battery operated product draws?

I'm trying to size an ac adapter for my kids cd player - I know it needs a 9V adapter with positive outside, negative inside.  But I don't know the amps I need, and don't want to undersize it.  I've got a DMM and can measure it I guess, if I could figure out where to place the probes.

I know some of you guys are more electrically inclined, so what do I do?

I contacted the manufacturer, they said it is discontinued and they can't provide me any information.
Tom Schmidlin

Offline denny

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Re: how to determine amps?
« Reply #1 on: September 07, 2010, 01:37:19 PM »
Does it also run on batteries?  If so, it doesn't need much current.
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Offline tschmidlin

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Re: how to determine amps?
« Reply #2 on: September 07, 2010, 01:53:38 PM »
Yes, it runs on 6 c-batteries but I'm tired of buying them and replacing them every week.  I have a 9V one lying around I could use, but I don't want to use a 500 mA one if it needs 800 mA.  I figure it doesn't draw much, but isn't there some risk if you use an undersized adapter? 
Tom Schmidlin

Offline dhacker

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Re: how to determine amps?
« Reply #3 on: September 07, 2010, 01:53:51 PM »
Is there anything on the outside that lists wattage? If so, you can figure amps by taking the wattage and dividing it by 9 volts. Like Denny said, probably not a big current draw.

What is powering it now, if anything?
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Offline dhacker

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Re: how to determine amps?
« Reply #4 on: September 07, 2010, 01:56:05 PM »
To measure the current, the DMM needs to be in series with the power circuit. In other words, run the power THROUGH the meter to the CD player. that will give you what you need to know
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Offline dhacker

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Re: how to determine amps?
« Reply #5 on: September 07, 2010, 02:01:09 PM »
Typical current draw for a C battery is 50 to 100ma. I'd say your 500ma adapter will be fine . . If it feels like it's getting excessively warm you may need to upsize it a little. Just be sure to get the polarity on the plug correct!.
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Offline Slowbrew

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Re: how to determine amps?
« Reply #6 on: September 07, 2010, 02:07:50 PM »
Are the batteries connected in Series, parallel or some combination?  How the batteries are connected will affect how much voltage is being supplied the system.  Most use batteries in series which would be 6*1.25V or 7.5V. 

A 9 volt might work but could cause overheating issues in the player.

Paul
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Offline denny

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Re: how to determine amps?
« Reply #7 on: September 07, 2010, 02:10:47 PM »
Typical current draw for a C battery is 50 to 100ma. I'd say your 500ma adapter will be fine . . If it feels like it's getting excessively warm you may need to upsize it a little. Just be sure to get the polarity on the plug correct!.

I agree, 500 mA should be fine.  And there's no harm in having more current available than the device needs.  If it doesn't need the extra current, it just won't use it.
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Offline rabid_dingo

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Re: how to determine amps?
« Reply #8 on: September 07, 2010, 02:22:13 PM »
Yes, it runs on 6 c-batteries but I'm tired of buying them and replacing them every week.

Is it like a mini stereo system? It just seems like a lot of batteries for a CD player. Invest in rechargeables.
They are getting more and more efficient. I did when my youngest boy was born. I have about 40# of
batteries waiting for HAZ MAT day at the county fairgrounds. I just can't bring myself to throw them out.
Ruben * Colorado :)

Offline tschmidlin

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Re: how to determine amps?
« Reply #9 on: September 07, 2010, 02:26:27 PM »
Are the batteries connected in Series, parallel or some combination?  How the batteries are connected will affect how much voltage is being supplied the system.  Most use batteries in series which would be 6*1.25V or 7.5V. 

A 9 volt might work but could cause overheating issues in the player.

Paul
C-batteries are 1.5V, so it's 9V.  And 9V is explicitly written on it as the size of the recharger, so there's that.

I'll try the 500mA one, assuming the polarity is right :)  I'm not worried about oversizing it, just don't want anything to melt.

It's a little poratable CD player with a microphone for karaoke.  The sound is terrible, but the kids love it.

Tom Schmidlin

Offline Slowbrew

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Re: how to determine amps?
« Reply #10 on: September 07, 2010, 02:49:16 PM »
I was thinking they were 1.25v so ya, you should be fine.  As long as the charger is supplying the current you need (and it sounds like it is) you should be fine.

I'll hide behind the fact the last time I did any real work in electronics was in 1985 and then only in school.   ::)
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Offline dhacker

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Re: how to determine amps?
« Reply #11 on: September 07, 2010, 04:35:38 PM »
I will point out that MOST (not all) typical wall wart DC adapters have a positive center with negative outer sleeve. Sounds like your machine is the the other way around, so again Double Check the polarity!! (sorry to sound like a broken record)

 ;)
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Offline tschmidlin

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Re: how to determine amps?
« Reply #12 on: September 07, 2010, 04:41:53 PM »
To measure the current, the DMM needs to be in series with the power circuit. In other words, run the power THROUGH the meter to the CD player. that will give you what you need to know
Thanks - I snipped one of the wires coming from the battery case and clipped on a spade terminal and a female disconnect.  I used the DMM to measure the mA when I turned it on.  With everything running at full volume it peaked over 300 mA but never hit 400 mA.  So the 500 mA one work no problem.  Thanks for the help.

The adapter has a polarity switch on it, so I'm fine.
Tom Schmidlin