Kerry Wentworth wrote:
> V G wrote:
>  =20
>> On Thu, Sep 1, 2011 at 11:25 PM, Kerry Wentworth <
>> kwentworth@skunkworksnh.com> wrote:
>>
>>  =20
>>    =20
>>> FETs conduct equally in either direction when on, but you have left out
>>> a lot of details.
>>>
>>>    =20
>>>      =20
>> Do they also block in both directions when off? Or do they only block wi=
th
>> the inherent diode, blocking drain to source?
>>
>>
>>  =20
>>    =20
>>> Is the "high side" going to go negative?  How far negative?
>>> How much current do you expect to conduct?
>>>
>>>    =20
>>>      =20
>> No negative voltages are expected with respect to ground. All positive.
>>
>> I'm going to conduct a maximum of 2A in either direction. I'll choose th=
e
>> MOSFETs accordingly. The "solid state relay" is going to be used to cont=
rol
>> the charging/discharging of a battery pack and will need to conduct in b=
oth
>> directions and it's relative position between "high side" and "low side"
>> will therefore dynamically change based on whether the battery is chargi=
ng
>> or discharging.
>>
>>
>>  =20
>>    =20
>>> Some FETs, particularly high current ones, have a diode between source
>>> and drain, so although they conduct equally in both directions, the
>>> don't block equally in both directions.
>>>
>>>    =20
>>>      =20
>> So I guess I'll need to use two MOSFETs back to back. The problem with t=
hat
>> is, what do I connect the common "source" pins to? I can't connect them =
to
>> GND, because then the current will be shunted to GND when the MOSFET is =
on,
>> instead of going through the two MOSFETs.
>>  =20
>>    =20
> Do you want to turn on the 'switch' and allow charge or discharge, or do=
=20
> you want to turn 1 FET on to allow charge or turn the other one on to=20
> allow discharge?
>
> If you tie the sources together, then the one with the drain to the=20
> battery needs a low gate voltage to turn on, and the one to the=20
> circuitry needs a high gate voltage to turn on.  Or you could just use 1=
=20
> FET from the battery - to ground, if you want to turn charge AND=20
> discharge on/off together.
>
> Kerry
>
>  =20
Forget I said that!  Please!  A high voltage turns both on, and a low=20
voltage turns both off!
But the 1 FET between battery - and ground should work.

Kerry


--=20
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