Can I use a resistor as a fuse?

[JPU]

Maybe a daft question but can I use a resistor in a circuit to act like a fuse. The circuit is powered by a 4.2V LIPO cell and the maximum current drawn (or should be) at 4.2v is 400mAh. Can I put a resistor into this circuit to act as a fuse if the current drawn exceeds 500mAh.

It's not that I am to tight to go and buy a fuse but I need this circuit sent off by tonight and I don't have any fuses at hand?

I was once in the presence of a cleverer man than me and he pointed out that a resistor had blown in a circuit and he said "The resistor had acted like a fuse in the circuit by burning out before anything else did". This got me thinking!

Thanks..

 

[Gryd3]

Don't do it. Bad idea

Your resistor will behave differently than a fuse, and if you even tried to use one, you would need to carefully choose the values.

Consider this:
-A resistor will drop voltage across directly proportional to the current through it... so if you had connected say a 1Ω, at 400mA your 4.2V battery would only supply 3.8V (The other 0.4V would be lost across the resistor) This also means that at 400mA, the resistor would be dissipating 160mW of heat.
-Resistors are rated at power dissipation as well as resistance, but the power dissipation is not so tightly controlled. A 1/4W resistor may still operate at an undisclosed power above this value, making your fuse almost useless. (It may eventually pop, but you wont be able to reliably control when)
-A fuse is designed, but not guaranteed to completely protect the circuit. Often times, silicone will fail before the fuse which is irritating. Fuses are commonly used to protect the wiring or power source as well. Automotive applications have fuses as close to the battery as possible, as this will prevent the battery from shorting out and violently failing or causing more damage.
-A resistor is not guaranteed to go 'open circuit' when it fails...

 

[JPU]

Thanks for that...Points taken. 500mah fusses just ordered with Farnell. I guess an extra day wont hurt..

Cheers

 

[KrisBlueNZ]

Fusible resistors are a thing. They're not suitable for all places where you would normally use a fuse, but they're a great idea for situations where the normal current is quite low and the fault current will be high, because they fuse very quickly and without making a mess. They're ideal for capacitor-fed power supplies.

They're not suitable for your application because they will drop too much voltage. In your situation a PTC resettable fuse might be suitable. See http://www.digikey.com/product-detail/en/MF-R050/MF-R050-ND/259965. This one has a resistance of about 0.4~0.8Ω so it will drop up to 0.3V under normal load.

 

[Gryd3]

Thanks for that...Points taken. 500mah fusses just ordered with Farnell. I guess an extra day wont hurt..

Cheers

You can always check your local electronic supplies for an inline fuse holder and a matching fuse.
This is not the first choice, but can get you going.

 

[shrtrnd]

Here's one to consider. I work on a lot of Hewlett Packard/Agilent/Keysight test and measurment instruments. They use 'zero ohm' resistors that act as fuses between circuits.
They're readily available, and the ones I see look like regular resistors, but have a single black band in the center of the resistor.

 

[Gryd3]

Here's one to consider. I work on a lot of Hewlett Packard/Agilent/Keysight test and measurment instruments. They use 'zero ohm' resistors that act as fuses between circuits.
They're readily available, and the ones I see look like regular resistors, but have a single black band in the center of the resistor.

If it's 0Ω, is it still a resistor?
If you see one on a board, at what point will it 'act' like a fuse and pop?

From the looks of things, the answer to this post is yes, but a fusible resistor would need to be purchased.

 

[KrisBlueNZ]

Here's one to consider. I work on a lot of Hewlett Packard/Agilent/Keysight test and measurment instruments. They use 'zero ohm' resistors that act as fuses between circuits. They're readily available, and the ones I see look like regular resistors, but have a single black band in the center of the resistor.

If you're talking about standard zero ohm resistors...



... then no, they're not fusible! Perhaps you have a part number to check?

 

[davenn]

If you're talking about standard zero ohm resistors...

View attachment 15747

... then no, they're not fusible! Perhaps you have a part number to check?

yup indeed, they are just used for links between separated circuit tracks, they use those instead of bits of wire as a component pick and place machine cannot pick up thin wires
as used in pic below





Dave

 

[shrtrnd]

Sounds completely electronically logical to me.
It makes perfect sense to me that HP engineers don't intend a zero ohm resistor to be fusible.
All I can tell you after 44 years of repairing their gear: Is that zero ohm resistor has fused a hundred times and saved me a heck of a lot of extra repair to previously connected circuits.
But the engineering explanation makes perfect sense to me.
jpu, I withdraw my suggestion of a zero ohm resistor for your application.
I am curious though davenn. If the zero ohm resistor is only intended to be used as a machine-insertable replacement for a thin wire, why they are used extensively in HP gear that actually has
wire jumpers between points on the boards, and also why I am seeing the surface mount version of the zero ohm resistor in boards that are totally surface mount. It seems to me that there
must be some other purpose for this component, besides machine-insertion replacement of a jumper wire.

 

[KrisBlueNZ]

Valid reasons I can think of for using a zero ohm link (THT or SMT) would be (a) to clarify the presence or absence of an option, (b) to take the place of an optional component whose position must be bypassed if it's not installed, (c) aesthetic reasons, (d) as a jumper wire with a body that can be properly gripped by a board stuffing machine during lead bending and insertion (THT) or part placement (SMT).

I'm not saying you're wrong, but consider that fuses have current ratings, and the only marking on a zero ohm resistor is a black band around the middle. So are you suggesting that all such "fuses" in all of these products have the same current rating?

Try scratching the coating off one of these HP parts sometime to see where and how it has fused.

If these zero ohm resistors have fused "hundreds of times", what did you replace them with? A genuine HP part? What was the part number description for that part?

 

[shrtrnd]

Apparently, I am wrong. I was explaining the experiences I have had with them. Apparently not what they're designed for.
I mostly see zero ohm resistors rated at 5% tolerance, though I have seen some rated at 1% tolerance. So they do have resistance, and in the situations I've seen, they do open-up and need to be replaced; not like a wire,
even though it appears that it's just supposed to be a replacement for a wire.
And yes, I replace HP zero ohm resistors with exact replacement parts sold by them (Keysight,now). I gotta go check my tech library and get the HP replacement part number, I'll post it here later.
I did open one years ago see what it was, physically. The HP ones are wirewound.
I talked to one of our design engineers a while ago. He says he installs them in prototype boards where he's not sure if he'll need decoupling caps or not until he physically tests the boards. If he needs the decoupling caps,
he replaces the zero ohm resistor with a cap. If he doesn't need them, he just leaves the device in place. That doesn't explain why they'd deliberately install zero ohm resistors on production boards.
So it does sound like I'm wrong, and that the zero ohm resistor is what KrisBlueNZ & davenn said it is. Just a jumper wire that is machine insertable.
I'm going to fire-off a query to Keysight to see what they say, just so I'll know for sure why they use them in T&M instruments.
Oh, and my "hundreds of times" is no exaggeration. The damned things blow quite frequently. I keep a few dozen of them in my drawer for that reason. Some of the instruments I repair have upwards of 20 of them installed on
the boards inside of it. Most leading from one board in the instrument to another one.

 

[KrisBlueNZ]

How is it meaningful for a 0Ω resistor to have a 5% or 1% tolerance? Are you talking about 0Ω resistors or low-value fusible resistors?

There is no reason for a 0Ω resistor to be wirewound. I've never dismantled one, but I imagine it's just a piece of wire with something attached around it.

The explanation about replacing them with decoupling caps makes no sense because (a) you don't determine whether to use decoupling caps or not by testing the board, and (b) if you connected a 0Ω resistor in the position of a decoupling cap, you would short out the supply.

 

[shrtrnd]

The HP/Agilent part number is 8159-0005 Resistor Fixed- zero ohms.
Yeah, I didn't get the 1% or 5% Tolerance Rating on those zero ohm resistors either. I don't know how you determine a tolerance on zero ohms.
I don't build 'em, I just broke one open.
I also don't design with 'em, I just fix T&M instruments with 'em.
Right about now, I'm glad I'm just a tech. They cost about a dollar apiece, and I don't want to have to explain to my boss why I replaced a 8159-0005 Agilent part with a wire, if something goes wrong.

 

[KrisBlueNZ]

That's interesting and odd. I'm interested to hear the response from Agilent/Keysight, if you get one!

 

[shrtrnd]

Nothig from Agilent/Keysight yet. I expect them to tell me what KrisBlueNZ and davenn alreay did; I just want to hear what they say.
I've still got email contacts from old HP engineers I used to talk to a lot (retired now). I'm going to drop one of them a line about this.
I never really thought much about the zero ohm resistors before, and so this is something new I Iearned this week.
I always just assumed HP didn't want to install actually fuses at those places, but it was a way to last-ditch protect the adjacent boards from catastrophic damage
when the circuit on one board failed. (By opening the circuit using those zero ohm resistors).
They have saved me a lot of extra work by opening, and confining damage to one board.
I'll post what the HP/Agilent/Keysight response will be. My company spends a lot of cash with them, I'll get some kind of an answer out of them.

 

[shrtrnd]

Update: Keysight sent me an email saying my query was routed to Germany, but that they forwarded it back to the U.S. So I'm still waiting on that.
Talked to an old HP Engineer who retired. He said the zero ohm resistor is simply a machine-insertable jumper wire. Then he got engineer-technical on me.
For the T&M equipment I repair, the resistance doesn't matter. From his perspective on the older HP T&M gear they had to take the zero-ohm resistance into consideration in their designs.
There's no such thing as a zero ohm resistor. Most zero ohm resistors nowadays are typically .003 to .004 ohms. Early versions were 3 to 30 milli ohms. While most manufacturers don't
specify a tolerance, for his purposes they used manufacturer spec'd tolerances of 1 or 5%. Then he talked about problems with temperature coefficients and issues wth the construction
materials used by different manufacturers. What I came away with is that I was wrong, the zero ohm resistor is a jumper wire. Only the engineers considered technical issues with them.
When/If Keysight answers my query, I'll post what they say.

 

[BobK]

What is 1% of 0? What is 10% of 0? Tolerance for zero Ohms cannot be specified as a percentage.

Bob

 

[shrtrnd]

BobK, I know, I know, I know. I'll say what I found out: most mfgrs don't specify a tolerance. BUT SOME DO. I don't know why, I assume it's a percentage of that 3 milli ohm value. For 99% of the electronics community, a zero
ohm resitor is zero ohms. For 1%, it isn't. I'm tired trying to justify something I don't really know anything about.
The reason I'm back, is that I thought anybody following this disasterous post, might be interested to know the following:
I got a reply from Keysight. They are unaware of anything called a 'zero ohm resistor' in any of their instruments.

 

[KrisBlueNZ]

I'm still following this discussion and interested in what Keysight say. So is that their final answer - they're unaware of "zero ohm resistor" in their products?