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I bought an IMAX B6 LiPo Balance charger that needs as input 11-18V but I have an unused laptop charger (the laptop is gone) having 19V with 4.74A. What is the easiest method to lower the voltage to 18V or even a little lower? (if I connect it as it is to the IMAX B6 it says something about input voltage is too high).

Thanks!

Sorin

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3 Answers 3

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Connect four rectifiers in series, each rated to carry the DC input to the IMAX, and connect the string in the forward direction between the laptop charger output and the IMAX input.

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  • \$\begingroup\$ Would something like this work? 1N5822 ( fairchildsemi.com/ds/1N/1N5822.pdf ). I have 10 of them sitting in a small box. \$\endgroup\$
    – Sorin Trimbitas
    Aug 24, 2014 at 18:30
  • \$\begingroup\$ Yes, as long as the IMAX's average input current was less than or equal to 3 amperes, the diode's forward current rating. \$\endgroup\$
    – EM Fields
    Aug 24, 2014 at 19:03
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    \$\begingroup\$ You might have to use more than four diodes, though, since they're Schottkys and have a lower forward drop than silicon diodes of the same rating. Try six, and if the IMAX complains add one at a time until it stops complaining. \$\endgroup\$
    – EM Fields
    Aug 24, 2014 at 19:38
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    \$\begingroup\$ My pleasure :-) One caveat: the diodes have a negative temperature coefficient, so as they warm up their voltage drops will decrease, allowing the voltage on the input of the IMAX to rise, so if it complains about the voltage being too high after it's been running for a while, just add another series diode. \$\endgroup\$
    – EM Fields
    Aug 25, 2014 at 9:05
  • \$\begingroup\$ Thanks for the tip! I didn't know this about diodes. \$\endgroup\$
    – Sorin Trimbitas
    Aug 25, 2014 at 13:10
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If you're comfortable with circuits and soldering, you could open up the laptop charger, find the resistor divider or zener diode on the feedback circuit, and solder on an alternative value. I've done this before with good success.

Otherwise, I like the diode-drop solution mentioned by EM Fields (though I would just use diodes, not bridge rectifiers -- and I don't think you'd need more than one or two of them).

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  • \$\begingroup\$ I wouldn't really open the laptop charger as I'm not that good at electronics in order to modify their original circuit. Would something like this work? 1N5822 ( fairchildsemi.com/ds/1N/1N5822.pdf ). I have 10 of them sitting in a small box. \$\endgroup\$
    – Sorin Trimbitas
    Aug 24, 2014 at 18:30
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    \$\begingroup\$ A "rectifier" isn't a bridge rectifier, it's what a bridge rectifier is made from. \$\endgroup\$
    – EM Fields
    Aug 24, 2014 at 19:33
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    \$\begingroup\$ Yeah, a few of those in series will drop the voltage enough. EM Fields, sorry for misinterpreting your response. \$\endgroup\$
    – Jay Carlson
    Aug 24, 2014 at 23:26
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The easiest method is to buy ready-made buck regulator board from fleabay like this.

The proper method is to get another power supply with suitable voltage.

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