Re: My Battery Monitor

[Victor Tikhonov <vtikhono@lsil.com>]

"Intrinsic" diodes may be built in as extra protection devices,
often on the same piece of silicon, but it's not a part of the transistor.
Most transistors don't have this diode (otherwise they can't work as
floating substrate switches). In fact, for small signal FET transistors
there is no distinction between drain and source (that's why arrow
on its symbol points to the middle of the gate). Pick any terminal as
a drain, the other becomes source and it works almost equally well
either way. Usually gate moved toward source a bit, which increases
the gain of the transistor, but that's about it.

power HEX a V-fets are little different (several drains are paralleled)
and you better not to switch them with source, but statement above
about diodes still holds true for them as well.

Victor

Richard Bebbington wrote:

> Ron Schroeder <rjs@bnl.gov> writes
> >At 01:59 PM 11/29/00 +0000, you wrote:
> >>If using relays to connect batteries to a central measuring
> >>device, please make sure your circuit CANNOT energise more than one
> >>relay at a time, or you'll short out part or all of your pack!
> >>
> >>Also, think of what might happen if the contacts weld closed....
> >>...you might have switched off the relay coil, but the contacts
> >>are still closed, and then you switch on another relay...
> >>...plasma frenzy!!
> >>Positively guided contacts would be a good idea, as well as using
> >>a multipole relays, so an active relay will lock all the others out.
> >>
> >>Just a little more fuel for the fire!  ;)
> >>--
> >>Richard Bebbington
> >>
> >
> >You must fuse each sense wire with either relays or solid state switches.
> >
> >Ron Schroeder
>
> Yep, fuses are a MUST anyway, but bear in mind that they all
> have to be rated to break full pack voltage, possibly at
> very high current. Standard automotive fuses won't work!
>
> Plus, if you do blow a fuse because your monitor turned on
> two relays at once, you've got to open up your battery boxes
> to change the blown fuses, and the relays might be damaged
> by the short-circuit current that blew the fuse.
>
> I feel it'd be better to avoid this situation by trying
> to make it almost impossible for the monitor to turn on
> two relays at once - the whole idea of this thing is to
> make battery maintenance easy for non-techies, right?
>
> Re: MOSFETs or other solid state devices - remember that
> FETs have an intrinsic body diode. If you put reversed
> voltage across it, it'll conduct.
>
> Bad ASCII art warning!
>
>   drain    =   positive
>     |
>     |
>     +--+
>     |  |
>  k  |  +-||
>     -     |
>     ^     |
>  a  |  +-|+--- gate
>     |  |
>     +--+
>     |
>     |
>   source   =   negative
>
> a and k are the body diode's anode and cathode.
>
> I have a feeling that this diode will cause a short circuit
> if you try to use FETs to connect one common measuring
> device up to many different points in a chain of batteries!
> Try drawing it out, remembering to draw in the body diodes that
> come with the FETs.... as soon as you turn on one FET,
> somewhere there's a diode that'll complete a short circuit....
>
> Lastly, you need to be careful with your gate drive for FETs.
> They can turn on very fast, and the gate is effectively just
> a capacitor, so if noise gets into your gate drive, the device
> could be turned on when you don't want it to be on!
>
> This is why I'd use relays - FETs are great, but they come
> with their own problems too.
>
> Regards
> --
> Richard Bebbington