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" To: 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. > > _________________________________________________________________ > Let the new MSN Premium Internet Software make the most of your high-speed > experience. http://join.msn.com/?pgmarket=en-us&page=byoa/prem&ST=1 > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics > -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics