how do I use resistors of certain values to get 19vDC down to 18vDC? - diyAudio
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Old 17th February 2013, 11:49 PM   #1
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Default how do I use resistors of certain values to get 19vDC down to 18vDC?

What kind of resistors would I need? And what would their values be?
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Old 17th February 2013, 11:51 PM   #2
john65b is offline john65b  United States
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You need to know the current for the required 1 volt voltage drop. V=I*R. So your Resistor = 1 volt/your current...

Why are you dropping one volt?
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Old 17th February 2013, 11:58 PM   #3
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the device can put out around 2 amps exactly on the mark and no higher.
I need to drop it one volt to run my laptop because the original laptop is gone and I don't want to buy a new charger for it.
the charger I have runs fine and I wish to use it to run my newer laptop. but the laptop requires 18 volts DC and 1.5 amps and since I don't have a charger for it I wish to use the one I already have. if I can use some simple resistors to drop it by one volt that would be great.
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Old 18th February 2013, 12:01 AM   #4
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Find a large 3A diode,and wire in in series with the positive lead. It will drop ~0.7V. That should be close enough. (~18.3V)

My last laptop was rated for 16V,but it would run on anything in the 14-18V range.

Edit:typo

Last edited by DigitalJunkie; 18th February 2013 at 12:31 AM.
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Old 18th February 2013, 12:03 AM   #5
john65b is offline john65b  United States
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I actually would not bother for 1 volt. the laptop should be able it take it with no issues...it will only take the amps it needs...
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Old 18th February 2013, 12:05 AM   #6
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I tried it before but the laptop doesn't turn on. I see the lights flicker madly when its plugged in. not a good thing. it scares me. so I want to try dropping the voltage by one volt to see if it makes a difference.
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Old 18th February 2013, 04:30 AM   #7
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Use two doides in series. The theoretical forward drop of a diode is 0.6 volts but it can be anywhere from 0.4 to 0.7 volts.

Gajanan Phadte
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Old 18th February 2013, 05:31 AM   #8
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Are you sure the laptop and supply are compatible?
I never really looked into it much, but I assumed the supplies were 3-conductor, with the 3rd being part of the charge management.
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Old 18th February 2013, 07:15 AM   #9
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What happened to the original charger for your newer laptop??????
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Old 18th February 2013, 07:06 PM   #10
Spiny is offline Spiny  United Kingdom
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sofaspud is right, many have 3 conductors so they will not work without the correct brands powersupply. The plug has a central spike and the plus and minus are the inner and outer of the barrel. Dell are infamous for this - the actual charger could be standard but for the lockout from the data line.
dell-laptops-reject-third-party-batteries-and-ac-adapterschargers-hardware-vendor-lock-in/

If this is not one of the trapped brands then a couple of 5Amp diodes in line will drop 1-1.2 v or so.
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