Presently, the power supplies that I am using are Linear Tech eval modules,
not the final units.  I just had several of these sitting around so put them
to good use.  If what you are saying, with the FETs turned on, then I should
see a drop in the source voltage?  Thats not the case.  I'm reading around
1.196 reguardless of the FET's on or off.


>From: James Nick Sears <jsears2027@HOTMAIL.COM>
>Reply-To: pic microcontroller discussion list <PICLIST@MITVMA.MIT.EDU>
>To: PICLIST@MITVMA.MIT.EDU
>Subject: Re: [EE:] Switching voltages with FETs
>Date: Tue, 13 Jan 2004 19:40:18 -0600
>
>How are you powering this circuit?  Each FET circuit is conducting in
>excess
>of 1A when turned on.  Losing 88mV sounds like power supply source
>impedance
>to me.
>
>Measure your power supply voltage with both FETs off, with one on and with
>both on.  If it drops appreciably then you have found the reason for the
>lower voltage.  If this is a problem then you will need a power supply more
>capable of handling larger loads
>
>Nick
>
>
>
>----- Original Message -----
>From: "al smith" <micro_eng2@HOTMAIL.COM>
>To: <PICLIST@MITVMA.MIT.EDU>
>Sent: Tuesday, January 13, 2004 4:19 PM
>Subject: [PICLIST] [EE:] Switching voltages with FETs
>
>
> > First off.....yes it works, but not behaving exactly how I expected them
>to
> >
> > Using a IR  IRLML2502 to switch 1.2V, driving the gate with +5 (direct,
>no
> > reisistor, etc)
> >
> > In an equivelent circuit, each FET can be represented by the Ron, so
>adding
> > a load resistor will affect the current draw thru the device.  Since Ron
>is
> > constant, the voltage drop across the FET will change based on the
>loading.
> >
> > So, taking some measurements, if I add a 1ohm resistor I read 1.070V
>across
> > the resistor and on the other FET seeing 1.125V (open circuit)
> >
> > If I add a 0.75ohm resistor on the other part, I have two parallel
>circuits
> > that have the RDSon and the resistor.
> >
> > Shouldnt the 1ohm circuit remain constant?  Or does the fact they are in
> > parallel now have a direct effect on it?  I sort of expect the latter
>since
> > the voltage on the 1ohm fell to 0.982 volts.
> >
> > What this tells me is that voltage switching applications, unless the
>load
> > is constant (and what load is ever?) that using FETs is not really a
> > preferred method.
> >
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