REFLECTOR: [c-a] Re: COZY: 24 Volts VS 12 Volts
Dave Philipsen
velocity at davebiz.com
Mon Oct 7 10:15:47 CDT 2013
Ohm's law states that voltage divided by resistance equals amperage. If
you have a 12 volt 200 amp starter the resistance, in theory, is 60
milliohms (12 / .060 = 200). If you insert a 50 milliohm resistance
(say, for a starter solenoid) the total circuit resistance will increase
to 110 milliohms (the 50 milliohm resistance plus the 60 milliohms for
the starter). The entire circuit will now only draw 109 amps (12 / .110
= 109.09). This means the starter will see a voltage of 6.54 volts
across it and the solenoid contacts will see a voltage of 5.46 volts
across them.
The same scenario with a 24volt 100 amp starter would work out to a 170
milliohm total resistance (starter is 120 milliohms and solenoid
contacts are 50 milliohms). The circuit will now draw 141 amps and you
will see 16.9 volts across the starter and 7.1 volts across the solenoid
contacts. This is a better scenario than the 12v system but not as
exaggerated as the scenario you stated below.
Also, your 24v system may have a maximum current draw of 12 amps with
everything turned on but that works out to 288 watts dissipated whereas
a 12v system with twice the amperage (24 amps) works out to the same
dissipation of 288 watts. (All of this discussion is theoretical since
we're not taking into account the minimal resistances of wires and
switches and circuit breakers / fuses.)
Dave Philipsen
Velocity STD FG
N83DP
On 10/6/2013 8:45 AM, Douglas Holub wrote:
> I love my 24 volt system because it allows me to use a small gear
> driven alternator on the accessory pad instead of a big belt driven
> alternator next to the fly wheel on my IOX-360. With LED nav and taxi
> lights, my total electrical system current draw with everything on
> including the heated pitot tube is about 12 amps.
> I think 24 volts is easier for starting, too. How many times have you
> heard about guys having a hard time starting their engines? It is
> often poor electrical connections. If a connection has 50 milliohms
> of resistance, that will cost you 10 volts at 200 amps of starting
> current. With a 12 volt system, that only leaves you with 2 volts to
> crank the starter. The same load on a 24 volt starter will only pull
> 100 amps, so the voltage drop across the 50 milliohm connection is
> only 5 volts, and that still leaves 19 volts to crank the starter.
> Doug Holub
> Standard FG w/ electric nose lift
> 135 hours, hope to be flying again this month
>
> ----- Original Message -----
> *From:* Carl Hoffman <mailto:hoffman3 at rcn.com>
> *To:* Velocity Aircraft Owners and Builders list
> <mailto:reflector at tvbf.org>
> *Sent:* Sunday, October 06, 2013 4:49 AM
> *Subject:* Re: REFLECTOR: [c-a] Re: COZY: 24 Volts VS 12 Volts
>
>
> On Oct 6, 2013, at 1:02 AM, Dave Philipsen <velocity at davebiz.com
> <mailto:velocity at davebiz.com>> wrote:
>
>> But heat is measured in watts which is, basically, volts x amps
>> (ohm's law). So, essentially, you have the same amount of heat
>> generated whether you are running 12v or 24v. In theory, a 24v
>> device may draw one amp which means it dissipates 24 watts. The
>> same type of device designed to run at 12v would draw two amps
>> and dissipate the same 24 watts. As long as you use the proper
>> size wire I don't think there is greater danger of fire.
>>
>> Also, a 24v battery has essentially the same amount of reserve
>> power as a similarly sized 12v battery. The 24v battery would
>> supply 1/2 the amps of a 12v battery but the same amount of watts
>> (or watt-hours). I'm guessing that a 24v battery would descend
>> through its discharge curve at roughly the same rate as a similar
>> sized 12v battery arriving at the point of 'full discharge' at
>> about the same time if they were both delivering the same amount
>> of power.
>>
>> While it is true that the voltage drop across wires in a 24v
>> system may be a lower percentage of total voltage than with a 12v
>> system, there is a point of diminishing returns somewhere.
>> Otherwise, why wouldn't we all just go ahead and run several
>> thousand volts in our homes? After all, it's available at the
>> utility pole. But, instead, we transform it down to a more
>> manageable level.
>>
>> Most of Europe might argue the need for 240v wiring in their
>> homes but here in the US we, for the most part, are content with
>> 120v and Japan, for the most part, is content with just 100v.
>
> Actually US homes have 240v for mayor appliances. Your 240v dryer
> has a 120v light bulb, and will draw more current on 208v. Also,
> your dryer has a 3 prong plug, and doesn't need a safety ground.
>
>> In my opinion, 12v is sufficient for most experimental aircraft
>> and, as was mentioned earlier, keeps you in the realm of
>> inexpensive parts which are available for automobiles that, for
>> the most part, are content to run at 12v.
>>
>> Dave Philipsen
>> Velocity STD FG
>> N83DP
>> On 10/5/2013 7:48 PM, Izzy Briggs wrote:
>>>
>>>
>>> The biggest argument I read in favor of 24 volts was:
>>>
>>> Higher volts draws less amps (Ohms law) = less heat, less likely
>>> to have an electrical fire?
>>>
>>> Greater Reliability (Page 5 of Greg's document) - "A 24-volt
>>> system also has a LOT more reserve energy available for use than
>>> a 12- volt system. As in point #2, a failed alternator in a
>>> 12-volt system leaves you 2 volts from shutdown. It a 24-volt
>>> system you've got a lot more reserve before your avionics and
>>> FADEC drop offline. Look at the discharge curves, and you'll see
>>> that a 12-volt battery at 11 volts is only partially discharged.
>>> A 24 volt battery at 11 volts is very close to dead."
>>>
>>> Izzy
>>>
>>>
>>> ------------------------------------------------------------------------
>>> *From:* trcsmith <trcsmith at aol.com>
>>> *To:* INBRIGGS at YAHOO.COM; jerryskipschneider at gmail.com;
>>> n528dr at ca.rr.com
>>> *Cc:* cozy_builders at googlegroups.com;
>>> canard-aviators at yahoogroups.com
>>> *Sent:* Saturday, October 5, 2013 8:33 PM
>>> *Subject:* Re: [c-a] Re: COZY: 24 Volts VS 12 Volts
>>>
>>> 24v in a small homebuilt today is just so wrong. You won't save
>>> that much weight in wire.
>>>
>>>
>>> Tom Smith A&P
>>> Long-EZ N12TS
>>> Cell-707-592-0869
>>> KVCB
>>> KJ6PZN
>>>
>>>
>>>
>>> -----Original Message-----
>>> From: Izzy Briggs <inbriggs at yahoo.com>
>>> To: Jerry/Skip Schneider <jerryskipschneider at gmail.com>; Dick
>>> Rohaly <n528dr at ca.rr.com>
>>> Cc: <cozy_builders at googlegroups.com>
>>> <cozy_builders at googlegroups.com>;
>>> <canard-aviators at yahoogroups.com> <canard-aviators at yahoogroups.com>
>>> Sent: Sat, Oct 5, 2013 3:09 pm
>>> Subject: [c-a] Re: COZY: 24 Volts VS 12 Volts
>>>
>>>
>>>
>>> The first few pages seem to suggest a 24 volt sytem is better.
>>> Is this what you meant Jerry?
>>>
>>> Izzy
>>>
>>> ------------------------------------------------------------------------
>>> *From:* Jerry/Skip Schneider <jerryskipschneider at gmail.com
>>> <mailto:jerryskipschneider at gmail.com>>
>>> *To:* Dick Rohaly <n528dr at ca.rr.com <mailto:n528dr at ca.rr.com>>
>>> *Cc:* Izzy Briggs <INBRIGGS at YAHOO.COM
>>> <mailto:INBRIGGS at YAHOO.COM>>; "<cozy_builders at googlegroups.com
>>> <mailto:cozy_builders at googlegroups.com>>"
>>> <cozy_builders at googlegroups.com
>>> <mailto:cozy_builders at googlegroups.com>>;
>>> "<canard-aviators at yahoogroups.com
>>> <mailto:canard-aviators at yahoogroups.com>>"
>>> <canard-aviators at yahoogroups.com
>>> <mailto:canard-aviators at yahoogroups.com>>
>>> *Sent:* Saturday, October 5, 2013 5:16 PM
>>> *Subject:* Re: COZY: 24 Volts VS 12 Volts
>>>
>>> Have you read:
>>> http://www.eaa.org/experimenter/articles/0903_aircraft_wiring.pdf
>>>
>>> Greg makes a pretty good case for a 12vdc system.
>>>
>>> Jerry
>>>
>>> On Oct 5, 2013, at 5:10 PM, "Dick Rohaly" <n528dr at ca.rr.com
>>> <mailto:n528dr at ca.rr.com>> wrote:
>>>
>>> True and the wire size is smaller. Some Berkuts are running 24
>>> volt systems with great success.
>>> *From:*cozy_builders at googlegroups.com
>>> <mailto:cozy_builders at googlegroups.com>
>>> [mailto:cozy_builders at googlegroups.com] *On Behalf Of *Izzy Briggs
>>> *Sent:* Thursday, September 26, 2013 8:43 PM
>>> *To:* cozy_builders at googlegroups.com
>>> <mailto:cozy_builders at googlegroups.com>;
>>> canard-aviators at yahoogroups.com
>>> <mailto:canard-aviators at yahoogroups.com>
>>> *Subject:* COZY: 24 Volts VS 12 Volts
>>> I spoke with an Avionics expert the other day about an
>>> electrical bus for my Cozy. He's trying to convince me that a 28
>>> volt system for my Radios and other Avionics will be preferable
>>> to a 12 volt system because they perform better and the
>>> components are cheaper.
>>> Anyone ever try to do 24 volts in a Cozy? The last machine I
>>> ever drove with 24 volt system was my Grandfathers 1969
>>> Volkswagen Squareback. It was nothing but trouble.
>>>
>>> Izzy
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
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