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View Full Version : Can a home circuit breaker protect on 12V DC?



shobley
02-10-2007, 11:47 AM
Can I use home circuit breakers to protect my 12V DC robot?

My wiring is rated 15Amps so I want to have a 15Amp breaker in the circuit at least.

Steve

Dave
02-12-2007, 03:09 PM
It should work, though I've never tried using an AC breaker in a DC application. Regardless, I'd recommend using an automotive "blade" fuse, just because they're small, light-weight, designed for DC systems, and they're dirt cheap. Of course, all this is assuming that the rest of your system can handle up to 15A.

edit: You want your fuse to blow before your wiring starts to melt, so you may want to drop the fuse rating to 10 or 12A.

Quirf
07-31-2008, 09:20 AM
Hi,

Electronics Technicians believe AC breakers can be used in place of DC breakers. Elecricians will
tell you that DC breakers are made for a reason and the two are not interchangeable. Since I am both - I can tell you the difference is safety and current interrupting capability (one in the same).

A quick source explaining the difference, and why you should choose the added expense of a DC breaker is: http://www.mrsharkey.com/busbarn/electrics/chapt9.htm . And since I am not affiliated with that site I hope you will consider it's advice.

Remember, a fuse is still cheaper, replaceable, and can be safe if used within it's voltage rating.

Please feel free to contact me with any questions you might have.

Bob

BillSlugg
09-16-2008, 06:00 PM
I wanted to add something that was not mentioned in mrsharkey's explanation of why AC breakers and fuses should not be used in high current DC applications. AC current has the property of passing through zero volts twice per cycle. When the AC contacts are opening, the arc will quench when the voltage passes through the zero point. This does not occur on DC. DC breakers (and large AC breakers) have provisions to interrupt the arc. They use blasts of air, submergence in mineral oil, sulfur hexaflouride gas, various auxiliary contacts moving a resistance in and out of the circuit, etc.

Quantum
09-18-2008, 11:58 PM
The other thing is the ac breaker is made for ac circuits just use a cheap fuse.

Paul

Quantum
09-18-2008, 11:59 PM
You could use a radio shack inline fuse which is a buck or two.

Adrenalynn
09-19-2008, 02:18 AM
The other thing is the ac breaker is made for ac circuits Paul

I think that was kinda what Bill was saying - he just didn't state it quite as succinctly. ;)

BillSlugg
09-19-2008, 02:34 PM
I did some testing today. I took a new, fully charged automobile battery and shorted it with a variety of fuses. I used auto tab type, glass ferrule cartridge type and edison base T-type all of a variety of amp ratings from 2 to 30. In each case, the fuse blew properly with no undue arcing. Examination of each under magnification showed no anomalies.

I dissassembled a used 15 Amp house panel type circuit breaker, removed the silver contacts, ground and polished them, reassembled it, tripped it 100 times in quick succession across the car battery, examined the contacts under 10x magnification and noted only moderate pitting.

I suspect that the problems with using AC breakers on DC occur only at higher voltages.

Adrenalynn
09-19-2008, 03:24 PM
That's a fascinating experiment, Bill! An automotive battery can deliver a TREMENDOUS amount of current through a short. I'm quite surprised to hear that the contacts on an AC panel breaker didn't vaporize after one or two shots.

Even a little Gel Cell can deliver 300A-1KA across a direct short. I accidentally shorted 6x (3x series in parallel) lawn tractor batteries (380 "cold cranking amps") and drew an arc that cut 1/4" mild steel angle frame for two inches "instananeously" - so paint me stunned.

I assume they weren't anything special, just like D-Types or something, right?

BillSlugg
09-19-2008, 03:52 PM
"I assume they weren't anything special, just like D-Types or something, right?" - Adrenalynn

What is "they" you refer to?

Adrenalynn
09-19-2008, 09:16 PM
Sorry - your AC breakers

BillSlugg
09-20-2008, 10:48 AM
Ah yes!, the breaker. A fine Square D, type QO-115, (single pole, 15 Amp), ca. 1995, slightly worn but servicable. A hint of ozone at the back of the palate and a nice burned plastic aroma belie its tender age. Rated to interrupt 10,000 Amps at 120 V ac.

From the Square D technical specs (http://ecatalog.squared.com/pubs/Circuit%20Protection/Miniature%20Circuit%20Breakers/QO-QOB%20Circuit%20Breakers/0730CT9801R108.pdf):

"QO ® (plug-on) and QOB (bolt-on) one-, two- and three-pole thermal-magnetic circuit breakers provide overcurrent protection and switching on ac and dc systems. QO and QOB circuit breakers are rated for use in the following voltage systems [including]:

• 48 Vdc (10–70 A for 1 and 2 pole circuit breakers)"
(Emphasis mine)



The only derating is the maximum current it can interrupt. The QO series has a maximum current interrupt of 10,000 Amps per Table 1 on page 5. For dc this is derated to 5,000 Amps. But there is a caveat according to "The Real Goods Solar Living Source Book" by John Schaeffer, Gaiam Publishing, 1995, pg 490:


"The AIR [Ampere Interruptible Rating of the Square D QO series breakers in a dc application] is 5,000 Amps, so a current limiting fuse (RK5 or RK1 type) must be used when they are connected on a battery system. (NEC - 690.71(c))


However, according to The University of Hawaii Ham Club, (http://www.chem.hawaii.edu/uham/bat.html) an automobile battery can only produce about 300 Amps when short circuited. This would negate the need for an inline fuse unless you have more than 16 car batteries in parallel, an unlikely scenario for a robot.


So, go ahead and use Square D type QO or QOB breakers for dc service in your robot. Just keep your voltage at 48 or lower and keep your short circuit amps below 5,000, or else put an RK5 or RK1 fuse (of the same amperage as the breaker) in series.

Shozbot
10-11-2009, 08:49 PM
the way to protect switch contacts in high current DC circuits is the commutation diode.
commutation refers to the reversal of voltage polarity or current direction

when you run a current through a wire you generate a magnetic field. when you run a magnetic field past a wire you generate a current.

when you STOP a current through a wire the magnetic field collapses. This collapsing magetic field is in motion, inwards toward the wire. it generates a current in the opposite polarity of the current that created it. this is how the coil in a car works. the spark occurs when the points open ( on your fathers Buick )

this reverse current is what pits points. it can be shunted to ground through a diode. just get a big honking diode with a robust PIV - peak inverse voltage. connect the diode to the switch contact on the load side, and ground. cathode to ground, anode to circuit in a negative ground system

http://www.allaboutcircuits.com/vol_3/chpt_3/9.html

Orac
10-12-2009, 11:29 AM
Take extreme care, I used Lead acid batteries for Robot wars and was demonstrating my robot to a manager at work.

We smelt a nice, cooking pork smell and I looked down at my wrist and noticed that my watch strap was glowing a nice shade of red.

Partially cooked my wrist through before I noticed and burnt the fingers of the other hand I used to try to flick the watch off.

All that for only a 7.5ah 12v SLA battery !!

Adrenalynn
10-12-2009, 12:09 PM
Funny thing - electricity will heat metal objects that you short across terminals with.

In this case, you were lucky you were using an SLA. They can deliver 300, 350, 400A into a load making them ideal for welding and cooking. If you'd been using something with less potential, it would have cooked your wrist, then either exploded showering you with acid, or burst into an incredibly hot fire that would have instantly ignited all your clothing and hair.

"7.5Ah" doesn't refer to the battery's ability to deliver instantaneous power on demand...

The real moral to the story is "don't wear jewelry when working with electricity", right? :) [Glad you survived it though!]

ooops
10-12-2009, 04:31 PM
One thing that was suggested to me by a design engineer when I was wanting to convert a DC fuse to a DC breaker on some manufacturing equipment some years ago. The fuses are fairly precise in their range of breaking the circuit, where as breakers of the same rating are somewhat variable, therefore if I replaced the fuse holders with breakers I would void the warranty.
For the record the fuse panel held 3 fuses and when any of the three failed it would kill the whole machine, so countless production minutes were lost figuring out which fuse failed, all three fuses were replaced with comparable breakers and nothing ever "smoked".
Lynn or someone here may have an opinion of what the workable range differences are?

BDP
10-13-2009, 02:57 AM
If you are using single use fuses, an easy way to find the blown one is to add an LED across the fuse terminals in your design. I've also seen blade fuses with an integral LED inside them for the same purpose.

aj4mq
06-18-2010, 11:49 AM
Sounds like you guys are in the right vessel..

Breakers for 12V systems are found in 12V stores..

Check out ATV, Camper and Boat stores.

Boat stores usually have a nice large collection of 12V DC Breakers. I have used them in my Amateur Radio designs.

Enjoy. :cool: