ac adapter: how to step down output voltage slightly?

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  #1  
Old 09-04-09, 08:54 PM
D
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Question ac adapter: how to step down output voltage slightly?

Hi everyone,
I hope this is the right place for this question. Anyway I have a special battery charger that requires 12-18v to work and in the past I was using an old laptop ac adapter (12v 5a) to power it but I am now using it to charge some significantly larger batteries and therefore need a larger power supply. I picked up an 18.5v 6.5a laptop ac adapter off of ebay but it turns out my charger has a built in safety feature that won't allow it to accept any voltage greater than 18v so I'm wondering if there is a relatively simple way to step down the ac adapters output voltage just slightly to bypass my chargers safety feature? I do have some basic electrical knowledge and should be able to handle a relatively simple project. If this is more effort than its worth does anyone have some suggestions on where I can get an affordable power supply that can output 12-18v and supply at least 5a at 18v?
Thanks!
Dan
 
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Old 09-04-09, 11:18 PM
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Welcome to the forums.

If you can find a 13-amp or greater diode, installing it in series with the positive side of the power supply feed to the charger should knock down the voltage by about .7 volts. In your case, that would bring the 18.5 down to 17.8.

Why not build your own power supply? A transformer, a bridge rectifier, and a couple of filter caps is all you'd need. Less than $100.
 
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Old 09-05-09, 08:44 AM
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Thanks Rick! Where might I be able to find a diode capable of this? I checked Radio Shack's site and wasn't able to find anything. I could buy another AC adapter that will work for much less than $100, I just want to try and use what I already have first. Also how were you able to estimate the voltage drop, is there a formula to calculate this?
Thanks!
Dan
 
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Old 09-05-09, 11:30 PM
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A typical diode will drop .7 volts to pass current. I don't have time this weekend to search suppliers, but post back if you don't get any hits on your search for
"13 amp 18-volt diode"
 
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