Simple 50mA fuse, but not using a Fuse.

[GreenTea]

Hi chaps,

I am wanting to make a circuit that I can "blow" on purpose - think of it as an expiry mechanism. I don't want something to reset (non-reset type fuse).

I was just thinking I could use an LED and drive the current high, but Ive noticed that the LED is still in-circuit after driving it well over its rated forward current limits.

What other devices could I use as a "fuse" because I don't want a Fuse as a component (and even then its hard to buy a cheap one that will blow at 50mA.

The voltage drop across the component needs to be either around 2.5V or 0.5V, and it should be able to handle current through the component up to 50mA, but will go open circuit (blow) when driven above 50mA (up to 100mA).

I was considering BJTs and FETs.... any advice would be grand

Cheers.

 

[Mitchekj]

The problem with semiconductors is that their failure mode isn't always an open. Many components will fail into a short, as a matter of fact. A good fuse this does not make.

I'd look into fusible resistors, or failing that, a small "calibrated" wire can work. I've seen PCBs layed out with a fuse built right into the copper, which is just a very thin bottleneck in the trace.

If you're actually trying to fuse the circuit for safety, I would not advise any of those suggestions and go with a properly rated and recognized fuse.

 

[Militoy]

I wouldn't try to re-invent the wheel on this one, unless accuracy and reliability are absolutely unimportant. At 50 mA, even a time-lag fuse, like a Littelfuse 37200500411, is under US$1 from Digi-Key in a single-piece quantity. I'll bet you can beat their price, if you shop around.

 

[(*steve*)]

Yeah, there are surface mount fuses that don't look to the inexperienced like fuses. Also they're a little harder to replace than a standard fuse.

Here is a 62mA fuse that looks a little like a capacitor. If you remove the markings it may well be mistaken for one. And if you measure it and it's open circuit, you would kind of be expecting that. A little cleverness with the routing of the tracks and you could make it look (at first glance) that it was connected differently to how it really is.

 

[Militoy]

Good idea - wiping out the markings - especially if the goal is concealing the fuse. I didn't pick up on that angle. The through-hole part I looked up actually looks like a little radial lead cap - so I suppose you could even stamp a little + on one end..

 

[GreenTea]

Thanks for your responses thus far.

I may need to elaborate a litte.

I really can't use a Fuse, and need to use something very small (equal to or less than 1206 size). So, please do not suggest a Fuse. Plus, a Fuse costs too much. My design is less than $1 already and I want to keep the cost below 20c.

Yes, the out of spec operation for a lot of components is unknown, and some do short instead of open, hence the question. We are all use to making sure we opererate in spec, but I want someone that can help that understands out of spec operation that could mimic a fuse with a very small rating.

This component is not being placed onto a PCB, but instead flexi-PCB or in conductive fabric. The small wire solution was not practical when flexing is likely - I need something a little more solid. I even tried glue around the wire - still unreliable.

There must be someone out there with experience with returned devices that knows the hows and whys of out of specification functionality.

 

[(*steve*)]

Perhaps you need to elaborate a little more...

Why do you need this fusable link? The more you can tell us, the easier it will be for us to suggest what your options are.

Are these to be made in quantity (sounds like you're thinking that way)?

edit: there are lots of components that are relatively easy to damage, but it's rather hard to think of something that will actually be destroyed in the way you require.

 

[GreenTea]

Oh, and another thing, Fuses are too tall! I need a low profile component

 

[GreenTea]

Perhaps you need to elaborate a little more...

Why do you need this fusable link? The more you can tell us, the easier it will be for us to suggest what your options are.

Are these to be made in quantity (sounds like you're thinking that way)?

Yeah, high volumes!

Think of it as an electronic expiry indicator! (Patent Pending ) Once the "consumable" product has been used too many times, a component "blows" to indicate that you need to use a new "consumable" product. And, NO, its not to take advantage of warantees.... thats something Apple may have already done in their products (sorry Apple lovers out there)

 

[(*steve*)]

It seems best to do that in a microcontroller. It could also serve a useful function in the device and change a bit in the eeprom memory to indicate it's expired.

In quantity you're looking at prices that make even those fuses seem expensive.

It is the same sort of trick employed in cheap printers to keep making you buy expensive ink.

 

[Militoy]

Yeah, high volumes!

Think of it as an electronic expiry indicator! (Patent Pending ) Once the "consumable" product has been used too many times, a component "blows" to indicate that you need to use a new "consumable" product. And, NO, its not to take advantage of warantees.... thats something Apple may have already done in their products (sorry Apple lovers out there)

Not sure I follow the connection between the number of times the product has been used, and reaching the 50 mA limit to burn the "fuse" - but it certainly sounds like you know what you're looking for. If the product is disposable - which it must be, at those kind of $ figures - then I advise building a fusable link right into your flex circuit board - as suggested above. In the kind of quantities you're suggesting, it shouldn't be much of a problem holding your PCB manufacturer to tight tolerances in trace width and thickness. Beyond that observation - I draw a blank. Working in quantities higher than about a thousand is above my pay grade.

 

[GreenTea]

It seems best to do that in a microcontroller. It could also serve a useful function in the device and change a bit in the eeprom memory to indicate it's expired.

In quantity you're looking at prices that make even those fuses seem expensive.

It is the same sort of trick employed in cheap printers to keep making you buy expensive ink.

Hi Steve,

Thanks for your feedback but maybe Im still not clear enough.

I have a high consumable good. Say its one of those Nestle Nespresso capsules. You have a very small circuit on the capsule that:-
1) Lets the coffee machine know the size of the capsule,
2) Lets the coffee machine know that the capsule is new.

When you insert a new capsule, the coffee machine detects its new and the size of the capsule and starts making the coffee. During the end phase of making the coffee, the circuit on the capsule is changed - a component is blown to create an open circuit and hence subsequent uses of the capsule - the coffee machine knows it has already been used.

So, I have 2-3 connections electrically (one 3V rail, one for reading, one for blowing something up). I want to multiplex the size of capsule and new/old information using three lines. I am trying to do this with a resistor and a component that I can blow up (open circuit). One component is less than 1c, the other would also be great to be less than 10c.

I hope I have made a clearer picture.

PS - I don't work for Nestle and its not a coffee machine I am dealing with

 

[(*steve*)]

Does the object get hot while operating?

You might be able to have two pieces of metal held together with wax, the heat melting the wax and breaking the circuit.

Is it a food related application? If it is, then you don't want something that will emit any "magic smoke" as it fails.

 

[GreenTea]

Does the object get hot while operating?

You might be able to have two pieces of metal held together with wax, the heat melting the wax and breaking the circuit.

Is it a food related application? If it is, then you don't want something that will emit any "magic smoke" as it fails.

Steve,

It aint a coffee machine! It was just an example.

My application does not get hot, no.

I want to control the burn-out electronically. Magic smoke is ok. We like magic! At 50mA, I don't think there will be much magic though

Cheers (and thanks for replying)

 

[(*steve*)]

Where does your limit of 50mA come from?

Is it possible for the main device to store a charge in a capacitor then dump it into the disposable device giving a brief pulse of very high current. It's possible that if you had an appropriately fragile diode in the disposable part that the current would destroy it.

It may become open circuit, short circuit, or just highly degraded. But you should be able to detect that. And you could always apply multiple pulses until it finally took up one of those states

 

[NickS]

I think you need to do some testing. If I were trying to blow out a part I would do some testing on a 0201 size SM resistor in say 49.9ohm I think they are rated for about 1/20W or 50mW.

So for a 2.5V drive it would draw 50.1mA which is (~125mW) until it starts derating. In my experiance SM resistors have always failed open. But I would stay away from semiconductors, those have always failed short for me.

 

[(*steve*)]

If you're looking at 0201, maybe look at 01005 as well. All other things being equal, their max dissipation is likely to be around 31 mW

They're seriously small, and I guess (like 0201 resistors) they come 10,000 to 15,000 per reel.

check out some specs here.

Oh, and get a microscope...

edit: 0201 are much cheaper though -- less than a cent each here.

edit2: Actually, check on ebay. You'll find reels of stuff going very cheap there. And considering that you're not after reliability, quality may be less of an issue. I've seen 0201 components on ebay (can't see any now), in fact I got a partial reel of some resistor (just checked it was 75 ohms) just to see if it was possible to use them at home! I got about 2000 left on a reel for under $2 US -- so if price is your major concern... (my problem is that I can't actually see them)

 

[NickS]

...(my problem is that I can't actually see them)

I think the preferred procedure for putting them down is solder paste and a salt shaker.

 

[GreenTea]

keep to 1206 and at minimum 0806 footprints

OK, I also have a footprint minimum. 1206 is what I am designing with, only due to the substrate it is going onto.

I tried using resistors, like a 1/16W and I could crank it up to 1W and yes, there was smoke, but it was still in-circuit!!!!! Ahhhhh.

I want something that will just pop. Not heat up and smolder for a while with a little smoke signal.


This problem is a bit challenging, especially when everyone thinks that a component will instantly die beyond the absolute maximum rating - its just not the case.

Keep trying guys, cos its not an easy nut to crack.

If you're looking at 0201, maybe look at 01005 as well. All other things being equal, their max dissipation is likely to be around 31 mW

They're seriously small, and I guess (like 0201 resistors) they come 10,000 to 15,000 per reel.

check out some specs here.

Oh, and get a microscope...

edit: 0201 are much cheaper though -- less than a cent each here.

edit2: Actually, check on ebay. You'll find reels of stuff going very cheap there. And considering that you're not after reliability, quality may be less of an issue. I've seen 0201 components on ebay (can't see any now), in fact I got a partial reel of some resistor (just checked it was 75 ohms) just to see if it was possible to use them at home! I got about 2000 left on a reel for under $2 US -- so if price is your major concern... (my problem is that I can't actually see them)

 

[(*steve*)]

This problem is a bit challenging, especially when everyone thinks that a component will instantly die beyond the absolute maximum rating - its just not the case

I think that pretty much everyone here has been telling you that.

There's another thread where we're talking about popping balloons and Resqueline has done some tests with 1/8W resistors dissipating well over a watt without smoking or terminal (as in killing) damage.

I still think your best bet is damaging a small diode by pulsing high current through it. It may not go open circuit, but the damage should be detectable.

However you're in uncharted waters. Most components are actually quite a lot more rugged than their datasheets might suggest. Absolute maximum ratings can be exceeded -- the device may stop working at that point, but it may "recover", albeit slightly degraded. Slightly degraded will take it out of spec (probably) but may not render it non-functional. Your problem is that you need to make it very obviously out of spec.