[AX-O]

Please pardon my ignorance. As of right now I am thinking about using a 12V system on my aircraft. I am also leaning toward using circuit breakers vice toggle switches. My question is the following: I have access to 24V circuit breakers for a very reasonable price. Can I use 24V circuit breaker on a 12V system? Is it as easy as saying a 1 amp (24V) circuit breaker is equivalent to a 2 amp (12V) or is there more to it? I can?t find anything on books about it.

 

[gmcjetpilot]

Actually its a good question, common misunderstanding

Please pardon my ignorance. As of right now I am thinking about using a 12V system on my aircraft. I am also leaning toward using circuit breakers vice toggle switches. My question is the following: I have access to 24V circuit breakers for a very reasonable price. Can I use 24V circuit breaker on a 12V system? Is it as easy as saying a 1 amp (24V) circuit breaker is equivalent to a 2 amp (12V) or is there more to it? I can?t find anything on books about it.

Not a dumb question at all and a common misconception. The circuit breaker's job is to cut off the circuit whenever the current jumps above a safe level.

Logically your math sounds correct but that is not how CB's (or fuses) work. CB's are fuse are totally different in that a fuse will disintegrate to protect the wiring but the job is the same. CB's (like the ones in our planes) do their job with mechanisms, springs and bimetallic or magnetic latches. The current either heats a bimetallic strip or in other types increases an electromagnetic to the point it trips a lever, that than releases a "mouse trap" type spring loaded switch, opening the cuircuit, "POP" or "SNAP!". The strong spring force moves or breaks the switch contacts open FAST, reducing arching.

There are volts limits in CB's but it does not affect the AMP rating. Most CB's (quality aircraft CB's) are rated to "interrupt" at say 30 volts max. Less than 30v, voltage is not an issue and does not affect the rated AMP.

The only time voltage is critical to the CB (within reasonable limits) is when it actually has to "interrupt" or trip. A voltage well above the 30 volts rating you can cause extream arching inside the CB, which could affect its ability to trip or damage it. Below the max interrupt voltage, amps are amps as far as the CB goes.

The rated volts of CB's are for endurance test specs for repeated trips. It can handle more volts but not for 50,000 cycles (trips) which they usually are rated for. I recommend Texas Instrument Klixon or Eaton brand CB's.

Think of it this way, you can run say 1000 volts at 1 amp thru a 20 awg wire it is not a problem, but try to run 100 amps at 12 volts in a little 20 awg wire, you will have heat, smoke and sparks. At 1000 volts and 1 amp the wire is cool. There are practical limits in volts but it has more to do with the insulation than the wire area. It is all about the amps. The CB is protecting amps not volts in other words.

Using the water analogy, AMP is the flow or volume of the water, VOLT is just the pressure or force potential behind the water. You can have all the force you want but with out the actual flow or water its not an issue.

A static spark can be 1000 volts but it does not kill you, because it has lots of potential but does not have any flow or volume (amps) behind it. A static spark will not kill you or trip a CB.

In fact a CB will work with AC current as well as DC, but can work at even higher voltage. The reason is it is easier to interrupt AC (alternating) than DC (direct) current. In a jet with DC and AC equipment the CB's are the same.

Bottom line you just need to worry about amps. Volt rating (as long as its less than 30 volts) is not critical. Now there are other important issues between 12 volt and 24 volt equip that are important but not the CB's.

PS Unlike the CB's discussed above which are mechanical devices, there are now semiconductor or solid state CB's. The whole voltage issue is similar (does not matter) but for different reasons.

 

[pczar3]

Just to further clarify

The current rating is the actual current that the breaker opens at after a typical delay. If you need a 1 amp breaker in a 24V system, you will need a 1 amp breaker in a 12V system. The voltage rating is the maximum voltage that the breaker can open without failing. In short use the current rating you need and a voltage that is higher than you system voltage.

Paul
N694BP - reserved

 

[G-force]

I'm no expert, but arnt aircraft CB's thermal circut breakers, that work off the amount of current passing through them? GMC...are you saying that a 24V 5 amp breaker that is designed to trip at 120 watts will also trip when you put 60 watts throug it? Thats what a 12v at 5 amps would put through it. I'm confused, unless aircraft CB's are not thermal and work off heating a bimetalic strip of metal.

 

[Kevin Horton]

If the CB is rated for 24v DC circuits, you can safely use it in any circuit of 24v or less. It will work just fine in a 12v circuit.

The amperage that the CB will open at does not change. If it takes 5 amps to open the CB in a 24v circuit, it will still take 5 amps in a 12v circuit.

 

[G-force]

I'll bring this back to the top because I still dont understand how a CB can get around Joules Law and/or Ohm's law when we are talking about a circut breaker that functions by turning current into heat and thus tripping a bimetalic thermal CB. It has been said that voltage has noeffect on tripping a CB...I don't see how this is possable...unless the CB's can self compensate somehow? Where are the electrical engeneers when you need one...

 

[gmcjetpilot]

Not sure, but it works

I'm no expert, but arnt aircraft CB's thermal circut breakers, that work off the amount of current passing through them? GMC...are you saying that a 24V 5 amp breaker that is designed to trip at 120 watts will also trip when you put 60 watts throug it? Thats what a 12v at 5 amps would put through it. I'm confused, unless aircraft CB's are not thermal and work off heating a bimetalic strip of metal.

Not sure what you are asking or if I have the answer, but a CB does not work on I*V, which is P = power (watts). As I think we agree a plane's DC system, 12v or 24v, CB's are the same. Also whether it uses heat (which I do believe they do) or magnetic field I can say they are very clever devices. In fact the Klixon are thermal and temperature compensating. They are like little watch works.

I understand the confusion. However at some point you accept it or find more info. I have either reached comfort in how they work or reached my level of incompetence. The do work. How they do it is clever for sure.

Again with the water analogy, the CB is "looking" at the water FLOW regardless of the pressure. How they celebrate the CB to work on amps and be somewhat insensitive to voltage, I don't know; however the "POP" happens in a very narrow AMP range over a wide range of volts. I am sure there are limits to the working voltage range.

The best way to satisfy yourself is get a CB and test it. Get two 12v batteries. For 12 volts use one battery or two in parallel or for 24v use two in series. Get different loads (maybe on of those carbon-pile battery testers) and an amp-meter.

NOT A BAD QUESTION. Part of building is learning.

 

[iansrv6]

CB's tripping

The CB's are thermal and trip by heating. The mistake being made here is that the CB does not see the voltage that is on the system until they open or trip. The heat is generated by the current passing through them and the small internal resistance they have. The heat is generated by this power loss according to the equation (I squared) times R.

As previously stated the voltage rating is maximum interupting voltage they are designed to take. Our CB's would happily work in a 1000V system until such times as it tripped at which time it would be destroyed, probably quite dramatically.

 

[Kevin Horton]

I'll bring this back to the top because I still dont understand how a CB can get around Joules Law and/or Ohm's law when we are talking about a circut breaker that functions by turning current into heat and thus tripping a bimetalic thermal CB. It has been said that voltage has noeffect on tripping a CB...I don't see how this is possable...unless the CB's can self compensate somehow? Where are the electrical engeneers when you need one...

Imagine two similar circuits, one in a 12v system, and the other in a 24v system. They both have the same type of CB.

Ignoring the internal resistance in the battery, the resistance in the switch, and the resistance in any of the connections, the total resistance in the 12v circuit is 11.5 ohm in the load, and 0.5 ohm in the CB, for a total of 12 ohm. Ohm's law tells us that I = V/R, so we have 1 amp of current flowing in the circuit. The voltage drop across the CB is V = I * R, or 0.5v.

Now, for the 24v circuit, the total resistance in the circuit is 24 ohm, so the current is 1 amp, and the voltage drop across the Cb is 0.5v. From the CB's perspective, it has no way to know what the system voltage is.

If there is a short, the current going through the CB increases significantly, and the CB will trip shortly after the current reaches the design value for that CB. I'm not sure what physical mechanism is used to detect the current value in typical GA CBs.

If the system voltage is too high, an arc may form inside the CB when it opens, which could possibly damage it, and might melt enough stuff so current still flowed through the CB, even though it had tried to open. This is why you don't want to put a CB in a system that has higher voltage than the CB is rated for.

 

[cytoxin]

low voltage

the only way less voltage should affect it is if the voltage is lower than designed system voltage. 12 volts should be availiable (9-14.5 or so). if not you will sometimes find c/b or fuses blown due to rush-in currents staying high too long.
an amp is an amp

 

[jcoloccia]

Expanding on Kevin's explanation

I'll bring this back to the top because I still dont understand how a CB can get around Joules Law and/or Ohm's law when we are talking about a circut breaker that functions by turning current into heat and thus tripping a bimetalic thermal CB. It has been said that voltage has noeffect on tripping a CB...I don't see how this is possable...unless the CB's can self compensate somehow? Where are the electrical engeneers when you need one...

edit: By the way, now that I look this is basically the same explanation that Kevin gave except with the tie-in to Watts.

It's always current that causes a temperature rise. Power is related to current but in a roundabout way through the resistance. It's also related to voltage. They're ALL related to each other, though, and to understand this you need to look at everything as a whole to really see what's going on. Here's the math:

V=IR

P=IV

Substitute for V: P=IIR = I squared * R.

Voltage goes away. Notice that an R shows up, though.

Let's say you take a circuit breaker and accurately measure it's resitance. For the sake of argument, let's say it's 1 ohm. Lets also say this breaker is rated at 5 amps. In this case, we could just as easily say this is a 25 watt breaker. P = I*I*R = 5*5*1 = 25W.

So let's see if this works. Let's say you have a simple series circuit with one breaker and a 1 ohm resistor in it. The total resistance of the circuit will be 2 ohms (1 for the resitor, 1 for the circuit breaker). Let's now put 10 Volts through it.

V = IR, so I = V/R = 10/2 = 5amps. The breaker will pop. Let's see if this works out with the math: P=IV = 5*5 = 25W. Wait! Were'd the 5volts come from?? Well, if you want to figure it out in watts, now you need to look at the voltage drop across the breaker. Since it's a 1 ohm resistance on the breaker, and I=5, V=IR=5*1 = 5 volts.

Now let's crank the voltage to 20 volts. Let's also change the resitor to be 3 ohms, though, so R for the whole circuit = 4 ohms. I = V/R = 20/4 = 5 amps. The breaker will pop. Now let's calculate this using watts and see if we come up with 25 watts:

Figure out the voltage drop across the breaker: V=IR=5*1 = 5V. P=IV = 5*5 = 25W. Yep...

So there's no magic going on. If you rated the breaker in Watts, though you would have to also give the internal resistance of the breaker so you could calculate how many amps will trip it. In our case, our breaker is a 25 watt, 1ohm breaker.

P=IIR = 25 = I*I*1
I= sqrt(25/1) = 5 amps.

You could also call it a 25 Watt, 5 volt breaker if you wanted to but then you'd be forever calculating voltage drops.

Same for resistors. If you have a 1ohm, 10 Watt resistor you could just as easily call it a 10 amp resistor. It's more convenient to have it in watts, though, because often you're interested in heat dissapation with a package, not just how much it will take to blow it up.

This was a VERY good question. I'd never thought of it like this before and it's not immediately obvious what's going on here. I hope someone will correct this if it's wrong.

edit: for extra credit, someone else can explain why op-amps rolloff @ 20db/decade. LOL...I spent a while on that one too.

 

[pczar3]

Well Done

Well done John, but maybe it is 6db/octave ! lol
Yep, I'm one of those EE types and pretty long in the tooth.

Paul Czarapata
694BP reserved

 

[G-force]

Ok, I figured out where my confusion lies. More than one post stated that 12 or 24 volts, the CB does not matter. That is what tripped me up because I know 5 amps at 12v is less energy than 5 amps at 24v. In kevins and johns explinations the amount or resistance changed, as is necessary with the voltage change, to keep the CB at 5 amps. My point in my first post, assumed the resistance stayed the same, for example lets say your are converting a 24V, 100 watt landing light to a 12V, 100 watt landing light. You will not be able to use the same CB...the load is different. The same style CB, sure, but not the same value due to different load.

 

[cytoxin]

higher voltage

higher voltage can do more with less. the 100 watt 24volt lamp uses 4.16 amps and the 100 watt 12 volt lamp uses 8.33 amps. isnt this is the reason larger a/c have higher voltage systems? with less amperage used the wires can be of smaller gauge therefore weigh less. when building a 747 you save quite a bit of weight..

 

[pczar3]

Breaker breaker in the panel!

Ok, I figured out where my confusion lies. More than one post stated that 12 or 24 volts, the CB does not matter. That is what tripped me up because I know 5 amps at 12v is less energy than 5 amps at 24v. In kevins and johns explinations the amount or resistance changed, as is necessary with the voltage change, to keep the CB at 5 amps. My point in my first post, assumed the resistance stayed the same, for example lets say your are converting a 24V, 100 watt landing light to a 12V, 100 watt landing light. You will not be able to use the same CB...the load is different. The same style CB, sure, but not the same value due to different load.

The way to clear it up once and for all in your head is to realize that the way the breaker trips is due to the heat generated in ITS INTERNAL resistance (heater). Therefore it has the same voltage drop across it (and thus same power being dissipated) no matter what the voltage of the external circuit. To have even more fun, some of the other breakers use a magnetic circuit (coil) as an auxilliary way to trip the circuit. If you Google for Klixon you can find some good explanations about the different types.

Paul
N694BP reserved

 

[David-aviator]

Not sure what you are asking or if I have the answer, but a CB does not work on I*V, which is P = power (watts). As I think we agree a plane's DC system, 12v or 24v, CB's are the same. Also whether it uses heat (which I do believe they do) or magnetic field I can say they are very clever devices. In fact the Klixon are thermal and temperature compensating. They are like little watch works.

I understand the confusion. However at some point you accept it or find more info. I have either reached comfort in how they work or reached my level of incompetence. The do work. How they do it is clever for sure.

Again with the water analogy, the CB is "looking" at the water FLOW regardless of the pressure. How they celebrate the CB to work on amps and be somewhat insensitive to voltage, I don't know; however the "POP" happens in a very narrow AMP range over a wide range of volts. I am sure there are limits to the working voltage range.

The best way to satisfy yourself is get a CB and test it. Get two 12v batteries. For 12 volts use one battery or two in parallel or for 24v use two in series. Get different loads (maybe on of those carbon-pile battery testers) and an amp-meter.

NOT A BAD QUESTION. Part of building is learning.

Agree with George 100%.

With one of my projects, while still working for a living I had frequent Seattle lay-overs, which always provided for a trip to the Boeing Surplus Store, time permitting. They sold everything from airline seats to to used drill bits (by the million if desired) to circuit breakers and miles of wire. I bought a lot of stuff there, including cb's which went into that airplane and worked fine.

Volts don't matter, amps do.

 

[AX-O]

Thanks for all the replies. I am certainly glad that I asked the question.