7
\$\begingroup\$

I want something for my power that acts like a diode, but without the voltage drop, to protect the circuit from damage if the batteries are connected the wrong way. Is this even possible?

\$\endgroup\$
  • 5
    \$\begingroup\$ I like Microsoft's mind-blowing invention, here. \$\endgroup\$ – tyblu Dec 12 '10 at 21:56
  • \$\begingroup\$ @tyblu, Interesting how it has been invented already: US Patent 5431575. \$\endgroup\$ – Thomas O Dec 12 '10 at 22:44
  • 4
    \$\begingroup\$ @Thomas O, Must have either sold his soul to MS or conveniently disappeared. \$\endgroup\$ – tyblu Dec 12 '10 at 22:51
  • \$\begingroup\$ @tyblu Either that, or Microsoft didn't do a thorough check on possible prior art - OR - Microsoft knows it's prior art, but knows the small company or individual doesn't stand a chance against their small army of lawyers. \$\endgroup\$ – Thomas O Dec 12 '10 at 23:24
  • \$\begingroup\$ @tyblu are you sure you haven't sold your soul to MS since this is the second time you have linked it? :-) @ThomasO That patent is for the mechanical aspects of the holder and not the electronics. I am sure MS is capable of designing a different holder or licensing it. \$\endgroup\$ – Kellenjb Dec 13 '10 at 6:04
10
\$\begingroup\$

Put the diode across the supply connections so that it is non-conducting, with a fuse ahead of it. The diode will conduct if the supply is reversed, and blow the fuse. There won't be any voltage drop.

alt text

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ how does this protect the circuit? it just tells me that i've plugged it in the wrong way. \$\endgroup\$ – Dean Dec 12 '10 at 22:27
  • 3
    \$\begingroup\$ @tyblu The fuse is in series with the battery line and the diode is in reverse across the supply. It will work. I think you're thinking of putting the fuse in series with the diode and the load across the battery, which wouldn't serve any purpose. I have done this before and it works, but it's obviously annoying to have to replace fuses. \$\endgroup\$ – Thomas O Dec 13 '10 at 0:03
  • 1
    \$\begingroup\$ @Leon Heller I hope you don't mind, I added a picture since it seemed some people weren't fully clear what was happening. Feel free to remove or replace if you aren't happy with it. \$\endgroup\$ – Kellenjb Dec 13 '10 at 5:57
  • 2
    \$\begingroup\$ I believe this would be called a crowbar. \$\endgroup\$ – XTL Dec 13 '10 at 15:43
  • 2
    \$\begingroup\$ You could use a Resettable Fuse (aka PTC or polyfuse), then no need to replace blown fuses. en.wikipedia.org/wiki/Resettable_fuse \$\endgroup\$ – davr Dec 14 '10 at 17:26
10
\$\begingroup\$

A MOSFET can be used to protect a circuit with a low voltage drop. See the following for more information:

Reverse Battery Protection with Hexfets doubles Battery Life

Advanced Power Switching and Polarity Protection for Effects

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ Careful with these circuits though: if you attempt to use a reverse supply voltage over the maximum voltage of the MOSFET gate (usually ±20V) it won't be pretty. \$\endgroup\$ – Thomas O Dec 12 '10 at 22:14
  • 1
    \$\begingroup\$ If you need to survive larger voltages, you can either add a resistor divider or zener. \$\endgroup\$ – Nick T Dec 13 '10 at 19:21
5
\$\begingroup\$

You might try what's called a Schottky diode. A normal diode has a drop of around 0.6 V, but Schottky's are around 0.2 V. At very low currents, the drop can be even lower.

| improve this answer | |
\$\endgroup\$
  • 1
    \$\begingroup\$ I would suggest this as a comment on Leon's post. \$\endgroup\$ – Kortuk Dec 13 '10 at 22:16
  • 2
    \$\begingroup\$ I should have been more clear-- I'm suggesting using a Schottky diode in series with the battery, not in parallel. I think that's different from what Leon is suggesting. (I think what Leon suggests is pretty good, but blowing fuses annoys some people.) \$\endgroup\$ – pingswept Dec 13 '10 at 22:32
  • \$\begingroup\$ I thought that might be what you meant. I Would suggest editing that into your answer. \$\endgroup\$ – Kortuk Dec 14 '10 at 1:26
1
\$\begingroup\$

I am planning to use the LTC4365, but that's overkill, because it has overvoltage and undervoltage protection built in (which I have a use for because it may be used in an automotive environment, but you might not.)

| improve this answer | |
\$\endgroup\$
  • 1
    \$\begingroup\$ Linear has many "ideal diode" type things IIRC. \$\endgroup\$ – XTL Dec 13 '10 at 15:42
  • 1
    \$\begingroup\$ @XTL, Indeed, although some of them are overkill for this application. \$\endgroup\$ – Thomas O Dec 13 '10 at 20:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.