[Diy_efi] current sense MOSFETs

[bcroe at juno.com]

phil, 

That could work.  But note that simply by going to an FET 
regulator (possibly with lower resistance than your example) 
you might as well use it for both voltage and current control. 
You have now gotten rid of 1 1/2 to 2 volts of loss.  That is 
already enough.  You could now reduce the resistor by 2/3s 
and get another volt if you really need it, without even using 
that current sensing FET.  But this just transfers more heat 
to the FET; I'd rather heat the resistor some.  

I think the FET is best placed in the negative leg of the heater 
load, makes measurements and gate drive a lot simpler.  It 
will not be as simple as the DIY-WB, but there is a trade off 
here.  I use such a circuit.

Bruce Roe

On Thu, 01 Aug 2002 19:42:39 -0700 Phil Hunter 
<diy-efi at t-n-e.com> writes:

> The basic idea is you can get some MOSFETs that have a 
> small, accurate percentage of the total current "bled off" to 
> a fourth lead, from which you can determine how much 
> current is flowing between the source and drain leads. 
> The design "win" is the reduction in power to measure
> large currents, and probably less complexity also.
> 
> Imagine a transistor made up of 100 identical MOSFETs 
> on a chip. All 100 drains are tied together to one lead. 
> Ditto 100 gates. 99 sources are tied together w/ the third 
> lead, and 1 source goes to the fourth.

> Looking at the heater control section of the DIY-WB, measuring the
> current thru the sensor's heater is done w/ a 1 Ohm resistor having
> about 1 Amp flowing thru it, thus a 1 Volt drop across it. If this
> resistor could be eliminated, the DIY-WB could work on 1 less volt
> of power, less still if you eliminate the voltage regulator also.
> In theory, a current sense MOSFET could be a way to do it.

> > philh

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