using an smt resistor as a fuse

I've placed 22-ohm 0805 resistors in series with the HV supply lines in the latest version of my easy-to-build AMP-62 high-voltage amplifier board. This version is often made with aggressive output current-limit values, e.g., 150mA. But it's intended that such currents last only a short time, e.g., to rapidly charge a capacitive load (during the rapid output slewing, the 22-ohms only drops 3.3 volts). By comparison, the original version of this board, as described in our x-Chapter book, has a wimpy 5mA current limit. But we assume that users of this amplifier version who need its extreme slewing capability understand the risks, when they select the 150mA current-limiting resistor values.

If the output were to be shorted, and draw 150mA from the supply, firstly it's hoped the supply voltage will soon fall dramatically, due to its own say 20mA current limit. Meanwhile the 22-ohm 0805 resistor would dissipate 0.5 watts, or

4x its rated power, and hopefully after a while it would fail open. But if it doesn't fail fast enough, one of the high-voltage MOSFETs, dissipating 40 watts, might short. At that point the current would increase further, and the 22-ohms would certainly fail, stopping the high power dissipation and removing the load from the supply.

OK, I see I'm going to get roasted for this one.

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 Thanks, 
    - Win
Reply to
Winfield Hill
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onsdag den 9. januar 2019 kl. 22.44.00 UTC+1 skrev Winfield Hill:

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0805 fusible resistor fusing power 2.5W !
Reply to
Lasse Langwadt Christensen

An 0805 will probably survive 0.5 watts for a long time. If the end caps are heat sunk to pretty big pads or traces, it will usually survive forever.

Your resistor will act like a fuse, at (guessing) several watts or so.

(Didn't somebody do some tests like that recently?)

Given good end cap heat sinking, most resistors (from 0603 to 1206) will have about the same central hot-spot temperature at a given power.

It's a time race between the 20 mA supply current limit (plus any capacitive energy storage) and the SOAR of the fet. The dominant thermal time constant of a fet is usually in the 100 ms ballpark.

Here's a 1206:

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At 30 volts, 18 watts, it lasts about 60 ms with no conductive cooling, and about 300 ms soldered to huge copper pours.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

What are you fusing? To what end?

"High voltage" is particularly onerous because said fuse will continue arcing the whole time the supply is draining down.

The transistor protects the fuse, not the other way around!!

Tim

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
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Reply to
Tim Williams

OK no more x-chapter teases, unless I get to proof early copies*, or it's being published soon. But we assume

Hmm well my first idea would be to try and fry some, with a bigger fet if necessary. (It doesn't have to be fast.) How about one of those resettable polyfuses? The ones I've tested fail earlier (lower currents) with each fail.

George H.

*I'd most likely make a terrible reviewer of circuits, with way more questions than 'real' errors.

Reply to
George Herold

I've connected 0805 resistors across the AC line. Try it.

They usually just go PIP.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

We usually use the output resistors to limit the load current eg protect the load. I would put current protection on the MOSFET to protect the MOSFETs. Why let the driver burn up if the resistor fails closed, which there is good chance it arc's over and turns into a carbon blob.

But thats just me.

Cheers

Reply to
Martin Riddle

I like to compute fet power dissipation, rather than current limiting. You can safely push a fet a lot harder that way.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

I'm happy to send a draft of the HV amplifier section to anyone who wants to look at it. It's a good example of the advanced material in the x-Chapters. (We're aiming to be out this year.)

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 Thanks, 
    - Win
Reply to
Winfield Hill

If the transistor is not properly selected for fusing, one may also try to augment it with the Soviet LED technique:

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Best regards, Piotr

Reply to
Piotr Wyderski

John Larkin wrote

DC is different and sustains the arc.

Reply to
<698839253X6D445TD

I hear you can get them in white as well as red.

Re using a jellybean resistor for fusing, there are some general issues. Voltage rating is low for sm parts Carbon resistors sometimes hot tunnel when overloaded. This means R drops precipitously. General purpose small resistors can also initiate a fire. Carbons are also prone to arcing. BTDT - suffice it to say the arc has no ability to limit the short current to a sane value.

Fusible Rs address some of these issues.

NT

Reply to
tabbypurr

And anything in between.

There are really robust HV SMD resistors, good to 10kV (although I prefer the VR37 THT series), but they are way more expensive than a proper fuse, so there is no point.

There's even a negative resistance region over a significant fraction of the I-V curve, so the glowistor will be happy to slurp a bit more. And more.

Fusible transistors do that too, LOL. "Im an arcist"...

Best regards, Piotr

Reply to
Piotr Wyderski

precipitously.

Carbon resistance elements have negative temperature coefficient of resista nce, so they are very likely to hot tunnel.

One of my bosses had party trick where he could persuade a 10k carbon film resistor to pass roughly an amp indefinitely, and it still measured 10k aft erwards - all you could see afterwards was a dark line in the paint above w here the hot channel had been. He was on the "intrinsically safe" technical review committee, and that trick had been part of the case against allowin g any carbon-based resistors in intrinsically safe equipment.

ability to limit the short current to a sane value.

Nor a hot channel in a carbon composition or carbon film resistor.

But it does pay to read the data sheet carefully.

--
Bill Sloman, Sydney
Reply to
bill.sloman

OK, I'll say just use a fuse because they're available, in a variety of specifications.

Three things the fuse has to do: carry the current, break the connection on overcurrent, and recover from the fault afterward. A metal-film resistor will get hot enough in places to kill its metal film, or ceramic core, or endcaps, when it fuses, and THAT can damage the PCB underneath it. Using a non-fuse-rated component risks the heat damage (potentially, fire) that a fuse was intended to prevent.

It's that third part, repairing the damage and returning the gizmo to service, that is the question: is it OK to throwaway the board on an overload (potentially, just a stray wire) event, or would you want to repair/rebuild? And, is heat damage limited to the resistor/fuse, or does it scorch the printed wiring?

I've never greatly admired surface-mount fuses, but you can also get fuseholders. I HAVE repaired boards with failed SMD fuses a few times, and found surface-mount to be only slightly annoying (tweezer-type irons help).

Reply to
whit3rd

If the fuse or fuse able resistor is connected to the mains it needs UL approval

It will then be listed as a Critical Component

Cheers

Klaus

Reply to
Klaus Kragelund

Could you /ensure/ that happens?

What's the *benefit* of not using a "proper" fuse or equivalent?

Reply to
Tom Gardner

Sure, it pops...

How many hundreds of times was the transistor dead before it cleared?

Tim

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
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Reply to
Tim Williams

Good thing I wasn't drinking coffee!

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

OK, I'd be happy to read it. (gherold-at-teachspin.com) The technical book i'm reading now is Feynman's gravity text... kinda hard slogging, so something a little lighter would be welcome.

Oh dear, well now you've done it, don't you know that predicting how long something will take immediately causes God to multiply that time by pi. :^)

George H.

Reply to
George Herold

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