How do I use an old 19v laptop transformer for a laptop that needs 18.5v?

The title pretty much says it all, I have power 'brick' that is sitting unused in a drawer that has an output of 19v and 3.95A. The brick that came with the laptop I want to use this on has an output of 18.5v and 3.5A. I want to modify the older one so that it can be used at home, or as a backup, and the one that came with the laptop will continue as the primary supply when at work and on the go. Would it be as simple as adding a resistor(s), or am I going to need to figure out voltage regulators and capacitors? I kinda fall into the category of being experienced, but untrained...I can follow directions and do the work, but the more research I do, the more confused I get.

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jeff-o7 years ago
Well, it could be as simple as adding a diode in series. A standard silicon diode has a voltage drop of about 0.7V, which would reduce the output to 18.3V. Some have a lower forward voltage drop. The laptop should be happy with that. Just make sure you use a beefy diode - it has to be able to handle at least 3.5A at 19V (and ideally, twice that).

Check out this one.  It has a forward voltage drop of 0.55V at 5A, and costs just 50 cents.  It should do the job just fine.  But again, as long as you grab a standard diode that can handle the voltage and current, it should work fine.
vphreeze (author)  jeff-o7 years ago
Hey guys, thanks so much for the info. If this was any other gadget in my house, I probably would have just tried the other power supply, or gone straight to a resistor and chalked the results (good or bad) up to a learning experience. This computer, on the other hand, is my life and I didn't want to take ANY chances. A couple quick questions:

1. if I did test the 19v brick 'as-is', what danger signs would I be watching for? just excessive heat from the laptop? or would we be talking components acting wonky?

2. Once I get the diode hooked up, I can use a multimeter to test for the proper output, right?

If I can get the diode this weekend, I may be able to try this next week. Anyway, once I do get it done, I will post back with my results. Again, thanks.
jeff-o vphreeze7 years ago
1. Well, the power from the laptop is used to charge the batteries, so I'd watch the battery pack to make sure it doesn't get hotter than normal. 2. Yup, a multimeter should be all you need. Remember that the rated output voltage may be with some sort of load attached, so if you're measuring higher than expected that could be why.
NachoMahma7 years ago
. While jeff-o probably has the safest answer, I'm with steveastrouk - that extra half volt probably won't make any difference. I'd monitor the laptop closely for a few days, just in case.
It's true, it probably won't make any difference at all. ;)
I must admit to being tempted to try it on 19V....