Replacing a battery with AC/DC adaptor

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  #1  
Old 10-10-13, 03:42 PM
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Angry Replacing a battery with AC/DC adaptor

Hi all,

I have an electrically operated handheld weed trimmer that uses a battery of 12V and 7Ah capacity.

I am fed up with charging it over and over and so far I have had to replace the battery twice.

So I finally decided to replace the battery with an AC/DC adapter.

I went online and bought a transformer that gives me an output DC of 12V and 6A .

When I place this in place of the battery the motor does not run smoothly.
It kind of kicks in and out or does not even start.
I measured the current and found out that sometimes it will go above 7A (when I get it running) even though the transformer is rated 6A. Sometimes the transformer shuts down and does not even start. I need to give it a good shake or a few taps to get it going again (Very ODD). I do not think the transformer is faulty.

It seems to me that the motor will just draw as much current as it can. Thereby the potential of damaging the transformer. I think even if I get a higher amperage then it may damage the motor. So my question is how to limit the output of the transformer to say 6A, which is the maximum the transformer ought to supply ?

I am actually surprised that there are no protections on the transformers to limit the output current.

Any suggestions would be appreciated .

Thanks

Raminee
 
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  #2  
Old 10-10-13, 04:12 PM
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Measure the voltage at the motor with the adapter hooked up and it running. It's probably a lot lower than 12 volts. The adapter probably can't provide enough power for the motor. Just because the battery is rated at 7 ah doesn't mean that's what the motor draws.

Can you temporarily put an ammeter in series with the load and use say your car battery to measure the current?
 
  #3  
Old 10-10-13, 04:21 PM
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I Will try but I did measure the voltage and the current.
Even though I can not get it to run consistently I managed to see a peak of near 7A on the ammeter. I measured the voltage across the output of the AC/DC adapter and it was 12.2 V .
So I assumed that it would be the same across the motor.

Q: can I put a 14 V car battery instead of 12V ?
Will it harm the motor ?
Also, what if the Motor draws too much current ? I am worried it will burn the motor out.

R.
 
  #4  
Old 10-10-13, 04:44 PM
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The motor will draw only the current it needs. 14 volts should not harm the motor but it will be closer to 12-13 with the battery not in a running car.
 
  #5  
Old 10-10-13, 07:00 PM
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Your weed trimmer came with a 7ah battery but that doesn't mean that 7amps is what the trimmer draws or it's maximum. I'm sure it can go upwards to 8-9 amps under heavy load. That heavy load is what is pushing the limits of the battery.

You can certainly use a car battery or something similar to run it but a safety fuse at the battery would be a great idea. An MDL-10 is a slo-blo 10amp fuse and would be a good starter fuse to try. I wouldn't exceed a 15amp fuse.
 
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Old 10-10-13, 08:38 PM
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Q: can I put a 14 V car battery instead of 12V ?
Will it harm the motor ?
Also, what if the Motor draws too much current ? I am worried it will burn the motor out.
I like to experiment and modify tools and equipment, sometimes for the fun of it, sometimes out of necessity but in this case, I would just buy a new corded trimmer. You can get a very decent one for $30-$40.
 
  #7  
Old 10-10-13, 09:50 PM
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You are right toolmon. Point taken.

But why pay $30 when you can fix it for $10 with a proper transformer and enjoy your own creativity.
I am in the camp that believes we need to consume less and stop wasting stuff just because there is an easy way out.

The plastics work, the motor works. All I need is to get the power to it.

R.
 
  #8  
Old 10-10-13, 09:53 PM
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Very good suggestion PJmax.
I will give it a try and see what is the maximum current it takes.
Hopefully less than 10A.
R.
 
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Old 10-11-13, 12:10 AM
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I'm wondering if the "problem" experienced with the wall wart power supply is because it may be a switch-mode unit? Using a cheap transformer, full-wave bridge rectifier and a big capacitor to smooth out the ripple a bit may work better.
 
  #10  
Old 10-11-13, 05:27 PM
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Ok guys

I did measure the current and voltage with a car battery.

The voltage across my car battery is 12.84 V
when I connected the trimmer the measured current initially jumped up to as high as 7-8 A and then down to a steady 2.5 Amps.

So my question now is why the 12v, 7A transformer wouldn't work ?
Could the transformer be faulty ?
Or is it that I need slightly more than 12 V ?
(I find the marginal difference to be little but hey I am no expert).

Any comments ?
Thanks

Raminee
 
  #11  
Old 10-11-13, 05:56 PM
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You need enough amps to start. The transformer didn't have it and that may have dropped the voltage and that increased the amps even more. Bottom line the power supply triped out. You don't need more volts. You need more amps. A 10 amp battery charger with a capacitor in series might do the job. I'd probably just use a motor starting capacitor. (Capacitor in this case not used as a start capacitor but a filter capacitor to smooth the DC.)

Of course the cost of a 10 amp battery charger may be more then a new weed eater.
 
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Old 10-11-13, 06:25 PM
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Ray
why can I not just use a 10A 13V transformer ?
Assuming there is such a transformer.
Raminee
 
  #13  
Old 10-11-13, 06:29 PM
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You never said what type of power supply you originally purchased but it obviously doesn't like the motor starting load. Switch mode supplies like Furd mentioned don't like heavy motor loads that are close to its max rating. Those types of supplies see the motor as a short and go into momentary shutdown.

A capacitor added to any power supply would help. You would need something like 4700mfd at 25v. The larger the mfd rating the more it will help.

To answer your previous question.....you can't just use a transformer. You need to convert the AC to DC and then filter it. You would need a 25amp 50vdc bridge rectifier and the cap I mentioned above........ 4700mfd at 25vdc. Be advised if you use a 12v transformer you will end up with approx 15vdc.

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  #14  
Old 10-11-13, 10:18 PM
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Ok I made a mistake in my previous question.
I should have said ac/dc adaptor and not transformer.

Can I do that with an adaptor ?

Raminee
 
  #15  
Old 10-11-13, 10:37 PM
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When I hear AC/DC adapter I think of a plug in wall transformer.
Are you referring to an actual power supply ? That may work.
 
  #16  
Old 10-12-13, 08:51 AM
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YES!

I think I may have damaged my initial ac dc adaptor since the motor takes a huge current to begin with which the adaptor was not rated for. Recall I was using a 12v 7A version. In further tests I have seen the current go over 10A right at the beginning before it settles down to say 2.5A.

My ac dc adaptor or power supply as you call it is very similar to those used for monitors or laptops.

So to me the problem seems to be at the initial stage when the motor demands more from the power supply and this i am sure damages the power supply. How can I limit the initial demand to stay within the specs of the power supply ?
 
  #17  
Old 10-12-13, 10:12 AM
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It's pretty hard to limit the startup current on a DC motor. You have to keep in mind that it's not only the startup current that is an issue but as you use the trimmer and load the motor... the current will also go back up.
 
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