Recently, I had occasion to plug a radio in backwards. Luckily, the inline fuse in the power cord saved the radio. This got me thinking, though.
The inline fuse was rated at 4a 250v. Since the fuse was inline with a battery (12v), does this mean it opened at a different amperage than specified, or will all 4a fuses blow when 4a goes through them regardless of voltage? If not, is there an easy way to convert?
I think I've got religion when it comes to fuses now - all my power cables in the future are going to have them. Losing 20 cents worth of fuse is nicer than losing $50 worth of radio.
Fuses blow due to current, not voltage. The "250V" voltage rating simply means the fuse is capable of interrupting 250V when it blows. So if you apply 1000V, and the fuse blows, too bad: the fuse will just explode or conduct current or catch fire, etc.
But it's still not true that all 4A fuses blow at 4A. In fact, probably none of them do. A fuse is a very coarse device....likely your 4A fuse will blow at 8A-10A in a few milliseconds, faster at higher currents, slower at lower currents. At 4.1A it will probably never blow, at 5A maybe in a few minutes, etc.
Final bad news: fuses prevent fires -- that is all. They generally take so "long" to blow relative to how long electronics takes to become damaged that you shouldn't think of them as protecting the electronics. They simply prevent CONTINUOUS current from flowing through the electronics so long that they get hot enough to catch on fire. The fact that your radio was saved by a fuse is a testament to the robustness of the radio design and/or components.
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The Ruggeduino: compatible with Arduino UNO, 24V operation, all I/O's fused and protected
They generally take so "long" to blow relative to how long electronics takes to become damaged that you shouldn't think of them as protecting the electronics.
While that is generally true, I have known cases where fuses have prevented reverse polarity damage which mainly occours due to excess current causing excess heat in the components.
Thanks very much for the useful information. In my case I think the fuse blew before the diode and/or traces on the SMT circuit board acted in the fuse's place... which is as good a reason as any to have a fuse in circuit!
(And nothing beats dumb luck when you're ignorant. I didn't even realize my cable was fused... now I'm checking that every time.)
As a warning to others: if you cut a trailer wiring extension cable in half, the red wire is hot on one end and ground on the other:
That happened, Mike because there was a shunt reverse polarity diode installed. Fuses ARE ONLY GOOD FOR THE PREVENTION FOR FIRES... according to the Fuse Mfr's semiconductors explode in uS time Fuses explode id Seconds Even @ 400% overload.
Let's check:
I gleaned these data from time-current curves on Cooper Bussman's website. These are operating times for fuses that Cooper classifies as "Electronic PCB and Small Dimension." I've selected 400% of rating, 200%, and 150%, for four types of fuses. Operating times are in seconds, and all of these data are for a fuse rated at 1 amp:
with his characteristically elegant economy of words:
However what I actually meant was
That happened, Mike because there was a shunt reverse polarity diode installed.
You don't have to have a shunt reverse polarity diode for a fuse to blow and protect a circuit.
I once wrote in an article back in 84
A semiconductor is a device designed to blow in order to protect a fuse.
And it is true, but mainly if the excess current is caused by excess voltage. With reverse polarity powering of components the damage is done to most of the components through the heat generated by excessive current. If you can get a fuse to blow fast enough this will provide protection. This is certainly possible as tmd3 has shown.
What case? My response is based on Using Fuses and the results of their destruction... I never discussed anything else but "Common Fuses". In this post or the last one I posted. Sometimes there is a series diode like the Arduino... a REALLY Good Idea, If the presence of the diode causes no issues of it's own and there ius a Shunt diode one that is connected cathode to V+ and anode to V- with a fuse inline with the power. With a Fuse and either a series or shunt diode nearly 100% protection is possible for fires either where the device is physically or for the power source. Over voltage or improper power (AC instead of DC) is shielded by the regulator more or less... Spikes in the KV range are common on Mains wiring and it is up to the rectifier filter to trap those components however those types of issues are most common with internal transformer rectifier arrangements. Fusses Fast enough to control current are commonly available, difficult to work with (real easy to blow with a slip on a meter probe) and VERY expensive. I own a Fluke 179 DMM it has 2 fuses one @ 11A 1KV and 1 440 mA 1KV the pair of fuses cost $18.+ dollars (about 21.00 with tax. For 2 fuses. Fast fuses are available and work well. The fuses installed in my meter aren't there for anything except to protect the shunt resistors used for current measurement. a Transistor is relatively hard to destroy compared to an IC and that was the basis for my statements about fuses. An Ic can and will destroy itself in the time it takes for any fuse to respond including current limiters and fuses do NOT protect inputs or outputs of IC's just the supply to them. My statements about current limiters is from my own experience. Unless the PSU "Crowbars" the output at over current and even then... there is still enough current available from the filter caps on the board to finish any device failure. IN MY DIRECT EXPERIENCE. It is likely that I am just too Old to tell you youngsters my experiences, likely that things in your world work differently for you. however you weren't kind enough to post all of why I was wrong and so rather than deffeend my view point, I think I will just keep my long and boring messages to myself. Yes undoubtedly I was wrong, Won't be the first timree and won't be the last but from now on unless a question is directed AT ME I will keep my bloody thoughts to myself.
Indeed, both assertions - fuses take seconds to blow even at 400%, and "not always" - are supported by the data, depending on which fuse is in the circuit.
I can only conjecture, but I don't think that the data support the notion that even the fastest fuse can protect electronic circuits generally. In my view, they can save a beefy component that would otherwise fail due to VI2 heating - like, as was mentioned, a shunt resistor in a current measurement circuit. They're surely not fast or sensitive enough to keep excessive voltage from punching through a delicate junction. And that's just another way of saying, as was said earlier, that fuses are good at preventing fires, but not protecting electronics.
In the case described in the original post, a reverse-voltage protection diode apparently limited the reverse voltage until the fuse blew. The fuse might have protected the diode from destruction, or maybe not - the diode's opinion isn't available to us. But, in terms of protecting the electronics, the diode did the heavy lifting.
Nobody picked up on this tidbit from reply # 4: "As a warning to others: if you cut a trailer wiring extension cable in half, the red wire is hot on one end and ground on the other:"
That is NOT a picture of a "trailer wiring extension cable". It is a pair of connectors that are designed to connect a load (such as trailer lights) to a supply (such as a car battery). The connectors are sold that way so they do not get separated before use. You are supposed to cut the cable in half and connect the end with the covered red lead (the lower end in your picture) to the car wiring. The red goes to the battery and the white goes to the car frame. You connect the end with the exposed red lead to the trailer lights. The red lead goes to the lights and the white goes to the trailer frame.
Mike I said youngsters to include the principal recipients of our combined knowledge. As to my comment about what I call "Fuse Poppers" is a fuse from Vin to ground for reasonable reverse protection, not flawless as a big enough fuse will send the diode to nirvana and then the lot with it... Better of 2 poor ideas IMO as the series element is a source of intermittent behavior from 2 standpoints, 1 the diode 'pellet' is a poor design and the multi pellet (1N4007-8) are really bad 1 in 10 WILL fail at some time... that old saw ... "If I had a pence for every one I replaced I'd be rich... er" and 2 there is the I/V curve of the diode that sorta predicts that at some minimal current it will truly become a 'semiconductor' causes regulators to oscillate If there isn't a good bypass besides even with the best of schottky's it still represents a .5 V Drop in the source voltage. I used to use a series diode and a "Popper" but instead of the expected ground on the anode I places one of those really annoying 'sonalerts' the beeping kind, reversed in polarity so as to be an annoying warning that the battery was the wrong way around. holes in a board are cheap...
I'm surprised: the cable I bought was being sold as an "extension cable", and was labelled that way on the box. I'd expect that a cable intended to be cut into two would have twisted the leads so each distinct pin would have the same colour wire going to it.
Interestingly, if I'm wrong, then others are as well:
I'm learning way more in this thread than I though!
As for caravan vs trailer - I understand these kinds of cables are used more for the type of vehicle that tows boats, rather than rooms in which to camp. I'd call that a boat trailer - not sure what others would
I'm surprised: the cable I bought was being sold as an "extension cable", and was labelled that way on the box.
Well, I suppose it could be used as an extension but you would need a whole lot of them to do any real extending.
I'd expect that a cable intended to be cut into two would have twisted the leads so each distinct pin would have the same colour wire going to it.
No. When mated you want the same color leads connected to each other so one red lead has to be female and the other has to be male. The same goes for the black lead.
Think about the situation where you have finished using your trailer/caravan and have disconnected it. You now have two connectors dangling, one from your car and the other from your trailer. The car connector is still connected to the car battery but since the red (hot) pin is insulated nothing will happen if that connector accidentally hits the frame. The trailer has no battery so there is no problem if its uninsulated red pin hits the frame.
