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The problem is simple: I have bought an UBEC, which should accept 7,6V from my battery and give me 5V to power my electronics. But the one I bought does not give me 5V, but 5.3V - that could cause trouble as I am going to power a Raspberry Pi as well, which could be damaged by this voltage...

So I tried using a diode to drop the voltage a bit. It did indeed, but I am having trouble finding a diode, that would drop it maximally for 0.5V at 3A, so that the drop is not too big (Raspberry Pi demands 4.75V - 5.25V, if I remember correctly and I'm planning on using more TTL electronics, like a webcam, which could be sensitive to input voltage as well).

The question is: Could I use more diodes in parallel to increase the maximum current and decrease the voltage drop or would this not work? I yes, could anyone recommend a good diode to use? I need to have maximum current at least 3A (for some motors and such).

I was thinking of using these: http://www.gme.cz/dokumentace/223/223-001/dsh.223-001.1.pdf

Thanks!

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    \$\begingroup\$ 5.3V is probably the open load voltage. Put a load on it and measure again; it may drop within spec with only a few hundred mA. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Jul 11, 2013 at 17:00
  • \$\begingroup\$ I actually tried hooking up some motors, LEDs and stuff, but still measured pretty consistent 5.3V :( \$\endgroup\$
    – Matouš Vrba
    Aug 11, 2013 at 14:03
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    \$\begingroup\$ the PDF link seems broken - do you have the schematic of the UBEC? it seems like something that should be adjustable (perhaps by adding a resistor if it does not contain any variable pots) - also, assuming you are powering the Pi via it's USB port, there exists a polyfuse which will drop the voltage by enough to get it into the safe USB range (and thus usable for all the 5v USB ports on the Pi) - depending on the model of Pi there is then a linear regulator or a switching regulator that drops it to 3v3 for everything else, so it won't be affected by higher voltages. \$\endgroup\$
    – user2813274
    May 7, 2015 at 15:37

2 Answers 2

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Diodes have characteristic forward voltages: Connecting multiple in parallel will not eliminate this forward voltage. Silicon diodes typically will have a forward voltage of 0.65-0.7 Volts, and there is no getting around that.

In order to drop a smaller voltage, use a Schottky diode instead: Look for one with forward voltage of 250 to 300 mV, and rated for perhaps twice the maximum current - so 6 Amperes or better in your case.

An example of such a Schottky diode is the Vishay VS-95SQ015, rated for 0.25 Volts at 9 Amperes. You can find others at sites like DigiKey, by using their parametric search.

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  • \$\begingroup\$ That is what I was afraid of... I have failed to find a diode with these characteristics - only diodes I could find with the voltage drop this low were for about 0,5A. Guess I'll have to either slap six of them in parallel or search better. Thanks for your answer, anyway! \$\endgroup\$
    – Matouš Vrba
    Jul 11, 2013 at 17:06
  • \$\begingroup\$ @MatoušVrba There you go, found you one suitable Schottky, if a bit of an overkill. \$\endgroup\$
    – Anindo Ghosh
    Jul 11, 2013 at 17:09
  • \$\begingroup\$ I see, thanks! Hope I'll find some local supplier for these... \$\endgroup\$
    – Matouš Vrba
    Jul 11, 2013 at 17:11
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But the one I bought does not give me 5V, but 5.3V - that could cause trouble as I am going to power a Raspberry Pi

There is nothing to worry about. The USB spec allow up to 5.25 Volt on the host ports. Usual 5 Volt TTL logic allows VCC up to 5.5 Volt in most cases.

The Raspberry Pi has on-board voltage regulators, and uses the bare 5 Volt rail very little.

You don't need a diode in your case, but the voltage drop of a Schottky diode probably would not hurt much either.

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  • \$\begingroup\$ Yeah, that's what I thought as well, thanks for reassuring! But I'll be more confident with the power closer to 5V as I won't have to recheck the specs of all the parts used on the robot about input tolerance... \$\endgroup\$
    – Matouš Vrba
    Jul 11, 2013 at 17:49

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