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

realflow100 · 17th February 2013, 11:49 PM

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

john65b · 17th February 2013, 11:51 PM

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 · 17th February 2013, 11:58 PM

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 · 12:31 AM

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

john65b · 18th February 2013, 12:03 AM

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 · 18th February 2013, 12:05 AM

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.

gmphadte · 18th February 2013, 04:30 AM

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

sofaspud · 18th February 2013, 05:31 AM

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.

madtecchy · 18th February 2013, 07:15 AM

What happened to the original charger for your newer laptop??????

Spiny · 18th February 2013, 07:06 PM

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.

17th February 2013, 11:49 PM	#1
realflow100 diyAudio Member Join Date: Sep 2012	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?

17th February 2013, 11:51 PM	#2
john65b diyAudio Member Join Date: Jan 2005 Location: Chicago	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? __________________ All right, but apart from the sanitation, medicine, education, wine, public order, irrigation, roads, the fresh water system and public health, what have the Romans ever done for us? Life of Brian

18th February 2013, 12:01 AM	#4
DigitalJunkie diyAudio Member Join Date: Apr 2003 Location: Portland,Oregon Blog Entries: 4	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.

18th February 2013, 12:03 AM	#5
john65b diyAudio Member Join Date: Jan 2005 Location: Chicago	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... __________________ All right, but apart from the sanitation, medicine, education, wine, public order, irrigation, roads, the fresh water system and public health, what have the Romans ever done for us? Life of Brian

18th February 2013, 05:31 AM	#8
sofaspud diyAudio Member Join Date: Nov 2005 Location: San Antonio	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. __________________ It is error only, and not truth, that shrinks from enquiry. - Thomas Paine


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17th February 2013, 11:58 PM	#3
realflow100 diyAudio Member Join Date: Sep 2012	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.

18th February 2013, 12:05 AM	#6
realflow100 diyAudio Member Join Date: Sep 2012	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.

18th February 2013, 04:30 AM	#7
gmphadte diyAudio Member Join Date: Jul 2003 Location: Dona paula, Goa	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

18th February 2013, 07:15 AM	#9
madtecchy diyAudio Member Join Date: Dec 2010 Location: wigan	What happened to the original charger for your newer laptop??????

18th February 2013, 07:06 PM	#10
Spiny diyAudio Member Join Date: Feb 2008	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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