Post by thread:[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

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 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}>From: James Nick Sears <spam_OUTjsears2027TakeThisOuTHOTMAIL.COM> >Reply-To: pic microcontroller discussion list <.....PICLISTKILLspam@spam@MITVMA.MIT.EDU> >To: PICLISTKILLspamMITVMA.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 removed}

> > ----- Original Message ----- > From: "al smith" <.....micro_eng2KILLspam.....HOTMAIL.COM> > To: <EraseMEPICLISTspam_OUTTakeThisOuTMITVMA.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 > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

----- Original Message ----- From: "al smith" <micro_eng2spam_OUTHOTMAIL.COM> To: <@spam@PICLISTKILLspamMITVMA.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 -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email KILLspamlistservKILLspammitvma.mit.edu with SET PICList DIGEST in the body