# how do I use resistors of certain values to get 19vDC down to 18vDC?

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#### realflow100

What kind of resistors would I need? And what would their values be?

#### john65b

Paid Member
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?

#### realflow100

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.

#### DigitalJunkie

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

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#### john65b

Paid Member
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...

#### realflow100

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.

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.

#### sofaspud

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.

#### Spiny

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.

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.

#### realflow100

the power cord chargher going into the laptop only has 2 wires.
one negative one positive.

#### Kjeldsen

and you have connected the positive and negative correctly?

A new compatible power adapter cost next to nothing on ebay or Aliexpress. 10\$ - 12\$ for many types is normal price including freight

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#### sofaspud

the power cord chargher going into the laptop only has 2 wires.
one negative one positive.
Are you sure? 3 of the 4 laptops around here have 3 conductors. There's probably an embedded controller at each end, brick and battery.
Otherwise, using diodes or an LDO regulator would be the way to go. A resistor's voltage drop will vary with the current through it so makes a poor regulator.

#### JMFahey

Agree that 1V difference is no difference.

#### Kjeldsen

Asus and Acer use 2 wires

#### realflow100

it is a toshiba laptop very sensitive to voltage changes in power supply sadly.
And all the chargers I find online are in the 100-200 dollar+ range

#### realflow100

i can't find the right plug size.. mine is an odd size and it's hard to find the one that fits..

#### JMFahey

FWIW my 2011 Toshiba NB505 has happily charged with any generic charger I have plugged it into, at least so far.
My son has a 2012 Acer labelled one, which probably was made by the same giant OEM manufacturer, same considerations.

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