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'[EE:] Switching voltages with FETs'
2004\01\13@172049 by

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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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

Nick

{Original Message removed}
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.

{Quote hidden}

>{Original Message removed}
>
> ----- Original Message -----
> From: "al smith" <micro_eng2HOTMAIL.COM>
> To: <PICLISTMITVMA.MIT.EDU>
> Sent: Wednesday, January 14, 2004 7:18 AM
> Subject: Re: [PICLIST] [EE:] Switching voltages with FETs
>
>
> > 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.
>
>
> Maybe it is resistance in the leads to and from your power supply.  What
> size/length power and ground wires are you using from your power supply to
> your circuit?  Did you measure directly at the power supply before?  Try
> doing the same test measuring right where the 1.2V is fed to the FET
circuit
> (+ on load resistor and - on FET source).  Dropping 88mV at 2A only
requires
> .044 ohms which is roughly equivalent to 11 feet of 16AWG stranded copper
> wire ( or about 4' of 20AWG or <2' of 24AWG).  Wire resistances googled
> from: http://www.cirris.com/testing/resistance/wire.html
>
> Nick
>

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----- Original Message -----
From: "al smith" <micro_eng2HOTMAIL.COM>
To: <PICLISTMITVMA.MIT.EDU>
Sent: Wednesday, January 14, 2004 7:18 AM
Subject: Re: [PICLIST] [EE:] Switching voltages with FETs

> 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.

Maybe it is resistance in the leads to and from your power supply.  What
size/length power and ground wires are you using from your power supply to
your circuit?  Did you measure directly at the power supply before?  Try
doing the same test measuring right where the 1.2V is fed to the FET circuit
(+ on load resistor and - on FET source).  Dropping 88mV at 2A only requires
044 ohms which is roughly equivalent to 11 feet of 16AWG stranded copper
wire ( or about 4' of 20AWG or <2' of 24AWG).  Wire resistances googled
from: http://www.cirris.com/testing/resistance/wire.html

Nick

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