---------------------------------------------------------- AeroElectric-List Digest Archive --- Total Messages Posted Mon 12/15/14: 6 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 12:46 PM - voltage drop across relay (Ken Ryan) 2. 01:12 PM - Re: voltage drop across relay (Charlie England) 3. 03:09 PM - Re: voltage drop across relay (Robert L. Nuckolls, III) 4. 03:32 PM - Re: voltage drop across relay (Ken Ryan) 5. 05:03 PM - Re: voltage drop across relay (Robert L. Nuckolls, III) 6. 05:07 PM - Re: voltage drop across relay (Robert L. Nuckolls, III) ________________________________ Message 1 _____________________________________ Time: 12:46:23 PM PST US From: Ken Ryan Subject: AeroElectric-List: voltage drop across relay I have scanned chapter 11 but was not able to find the answer to this question: What would be the typical voltage drop across a relay in a 12 volt system using 10 gauge wire to carry about 20 amps? ________________________________ Message 2 _____________________________________ Time: 01:12:25 PM PST US Subject: Re: AeroElectric-List: voltage drop across relay From: Charlie England Should be in the range of a few millivolts, at most, if the contacts are in g ood shape. Wire size isn't relevant to voltage drop across the relay contact s. Sent from my iPhone > On Dec 15, 2014, at 2:44 PM, Ken Ryan wrote: > > I have scanned chapter 11 but was not able to find the answer to this ques tion: What would be the typical voltage drop across a relay in a 12 volt sys tem using 10 gauge wire to carry about 20 amps? > > ========================== ========= ========================== ========= ========================== ========= ========================== ========= > ________________________________ Message 3 _____________________________________ Time: 03:09:46 PM PST US From: "Robert L. Nuckolls, III" Subject: Re: AeroElectric-List: voltage drop across relay At 14:44 2014-12-15, you wrote: >I have scanned chapter 11 but was not able to find the answer to >this question: What would be the typical voltage drop across a relay >in a 12 volt system using 10 gauge wire to carry about 20 amps? Charlie's right . . . but I'm curious as to what prompted the question. In the best of all worlds, two pieces of metal brought together to carry current (switch and relay contacts) would produce a zero-ohms joint but alas, we don't live in a perfect world. A relay in the same 'class' as you describe Emacs! Emacs! . . . has a published maximum resistance of 0.030 ohms Emacs! according to specs . . . a drop of 20A x 0.030 ohms = 600 millivolts or about 1.2 watts of dissipation at the contacts. I can guarantee that contacts of that mass dumping that much heat would not be long for this earth. As a practical matter, any time you measure more than 100 millivolts across any set of switch or small relay contacts at these kinds of loads, it's worth your time to see if the critter is getting into trouble. Bob . . . ________________________________ Message 4 _____________________________________ Time: 03:32:15 PM PST US From: Ken Ryan Subject: Re: AeroElectric-List: voltage drop across relay The answer to "what prompted the question" as to typical voltage drop across a 12 volt relay is: The Rotax schematic has the regulator C wire (which measures voltage) going through a couple of relays on its way back to the battery. The text says that there should be no more than 0.2 volt difference between the battery and the C terminal. I was trying to get a handle on how much drop to expect from the two relays. On Mon, Dec 15, 2014 at 2:08 PM, Robert L. Nuckolls, III < nuckolls.bob@aeroelectric.com> wrote: > > At 14:44 2014-12-15, you wrote: > > I have scanned chapter 11 but was not able to find the answer to this > question: What would be the typical voltage drop across a relay in a 12 > volt system using 10 gauge wire to carry about 20 amps? > > > Charlie's right . . . but I'm curious as > to what prompted the question. > > In the best of all worlds, two pieces of > metal brought together to carry current > (switch and relay contacts) would produce > a zero-ohms joint but alas, we don't live > in a perfect world. > > A relay in the same 'class' as you describe > > [image: Emacs!] [image: Emacs!] > > . . . has a published maximum resistance of 0.030 ohms > > [image: Emacs!] > > according to specs . . . a drop of 20A x 0.030 ohms = 600 millivolts > or about 1.2 watts of dissipation at the contacts. I can > guarantee that contacts of that mass dumping that much > heat would not be long for this earth. > > As a practical matter, any time you measure more than > 100 millivolts across any set of switch or small relay > contacts at these kinds of loads, it's worth your time > to see if the critter is getting into trouble. > > > Bob . . . > ________________________________ Message 5 _____________________________________ Time: 05:03:47 PM PST US From: "Robert L. Nuckolls, III" Subject: Re: AeroElectric-List: voltage drop across relay At 17:30 2014-12-15, you wrote: >The answer to "what prompted the question" as to >typical voltage drop across a 12 volt relay is: > >The Rotax schematic has the regulator C wire >(which measures voltage) going through a couple >of relays on its way back to the battery. The >text says that there should be no more than 0.2 >volt difference between the battery and the C >terminal. I was trying to get a handle on how >much drop to expect from the two relays. Oh . . . yeah . . . Suggest you consider an architecture like Z-16 that places no switches or relay contacts in series with the regulator's output (B&R) or sense lead (C) and the bus. Bob . . . ________________________________ Message 6 _____________________________________ Time: 05:07:44 PM PST US From: "Robert L. Nuckolls, III" Subject: Re: AeroElectric-List: voltage drop across relay At 17:30 2014-12-15, you wrote: >The answer to "what prompted the question" as to >typical voltage drop across a 12 volt relay is:=C2 > >The Rotax schematic has the regulator C wire >(which measures voltage) going through a couple >of relays on its way back to the battery. The >text says that there should be no more than 0.2 >volt difference between the battery and the C >terminal. I was trying to get a handle on how >much drop to expect from the two relays. I'm surprised they did that. Here's an excerpt from another Rotax document that seems to avoid that pitfall nicely . . . Emacs! 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