fuse for 24vac

[djacobow]

I'm designing a project powered by a 24Vac wall-wart. I'd like to add circuit protection.

I really like the idea of auto-style blade fuses, like ATO type. They're easy to replace and have the current rating clearly marked for olde eyes. But they are made for 12Vdc, not 24Vac.

Now, my impression is that using an AC fuse in a DC application is dangerous, as the fuse might not be able to extinguish an arc. But using a DC fuse in an AC application seems ... OK?

But using a 12V fuse in a 24V application is probably not ok, right?

My fallback will be regular 250Vac 5x20mm fuses, which I don't like for convenience, but they should be fine operationally.

[james_s]

It'll be fine. I don't know exactly what the voltage rating of this type of fuses is, but it's very likely higher than 12V. I've seen them used quite a bit in HVAC applications where 24V is the norm. 24V is still low voltage.

[Gyro]

Your assessment of the difficulty of breaking DC vs AC is correct, for the reason you indicated (arc extinction).

I suspect that the main reason for the 12V rating is to prevent people using them in commercial vehicles with 24V DC supply - that would probably cause problems.

Given the amount of de-rating needed for an AC fuse on DC, my feeling that using one, particularly a low current one, on 24V AC would be fine. You're well within the SELV limit to, so touchability of the pins wouldn't be an issue.

[T3sl4co1l]

I would worry more that your wall wart is unable to provide fault current, i.e., it is "impedance protected".  But not necessarily well enough by itself to avoid melting or smoking or burning up, for which the fuse would be nice.

Tim

[james_s]

Here, I just looked it up. ATO fuses are rated for 32V.

http://www.littelfuse.com/products/fuses/automotive-aftermarket-products/blade-fuses/ato.aspx

[djacobow]

Here, I just looked it up. ATO fuses are rated for 32V.

http://www.littelfuse.com/products/fuses/automotive-aftermarket-products/blade-fuses/ato.aspx

Thanks, this is good to know! I think these will work, then!

[coppercone2]

You want to look what type of fuse it is, regardless of the rating (slow, fast, time, etc) and make sure the trip characteristic is not unique compared to what we are used to in regards to conventional fuses.

[coppercone2]

I would worry more that your wall wart is unable to provide fault current, i.e., it is "impedance protected".  But not necessarily well enough by itself to avoid melting or smoking or burning up, for which the fuse would be nice.

Tim

do they have magentics that can be mismatched to the fuse trip characteristic like this? What would be the solution then, thermal fuse on the transformer? I de-rate by habit but in a sense by de-rating your making the fault current worse.

[djacobow]

I would worry more that your wall wart is unable to provide fault current, i.e., it is "impedance protected".  But not necessarily well enough by itself to avoid melting or smoking or burning up, for which the fuse would be nice.

Tim

Yeah, I've noticed this already. I have a 5V rail powered by an MC34063 which is powered from rectified 24Vac. I inadvertently used a rectifier diode that could not handle the negative swing, failed short, exposed the elco filter caps to negative voltage, they exploded, the MC34063 was hosed, as was the Raspberry Pi downstream on the 5V rail.

And the 2A fuse just sat that whole misadventure out. :-(

I wasn't standing over the board when it failed, so I don't know what the wall wart did in the circumstance, but I suspect that the voltage dropped under the near short, but it didn't cut out or do anything useful.

The lowest ATO rating seems to be 500mA, which may still be too high to provide useful protection for this part of the circuit.

[Gyro]

The ATO fuse characteristics from james_s's link look pretty respectable actually... 100ms min 5s max at 200% of rating, 600s max at 135%. I think most transformers would be comfortable with that.

[Paul Moir]

FWIW I've seen ATO fuses used in 24vac control applications before.  An air conditioning system comes to mind, but I think I've seen some furnaces too.

[Zero999]

I would worry more that your wall wart is unable to provide fault current, i.e., it is "impedance protected".  But not necessarily well enough by itself to avoid melting or smoking or burning up, for which the fuse would be nice.

Tim

do they have magentics that can be mismatched to the fuse trip characteristic like this? What would be the solution then, thermal fuse on the transformer? I de-rate by habit but in a sense by de-rating your making the fault current worse.

It's a tricking one. A fast blow fuse such as the glass cartridge type might work, but it could blow due to the surge taken when the capacitor charges.

I'd be inclined to measure the short circuit current of the transformer using a clamp meter. The transformer should be able to handle the short circuit current for the few seconds it takes to take a reading.

[drussell]

It seems james_s beat me to it above, but I as going to mention that the fuses used in typical automotive applications are rated for at least 32 volts (AC/DC).  This includes both the plastic body "blade" type ones, including standard ATO, the mini, micro and MAXI variants, as well as the traditional glass type ones.  Some of the newer high current "plug-in" plastic cartridge types like are used for larger loads are mostly rated 58 or 60 volts.

It would be silly to have a fuse that were only suitable to exactly 12 volts, especially since standard "12V" vehicle electrical systems often see voltages at least several volts above the nominal 12 volts.