exploding wirewound resistors

John Larkin wrote on 9/15/2017 4:16 PM:

I think that is 75 joules, no?

Really? You need to borrow a $50 battery?

Right, 75.

No, not constant energy. A survey of datasheets shows about a P ~ t^(1/x) trend, for x = 2 or 3.

At time scales short enough where only the wire is heating up, it's probably also short enough that skin effect is relevant, and so the trend continues (give or take a hiccup as it crosses over from thermal to skin-effect diffusion) down to very short timescales indeed (ns).

There's also a sweet spot of energy capacity, with respect to resistance value. A high value wirewound has less metal than a small one, because the wire is thinner and the winding fill factor is the same. On the other hand, really small ones don't have a high fill factor (just a few turns), or a single wire link is used.

If you need a shitton of energy, fast, there are bulk resistors. Ohmite makes some that are frequently stocked at the usual suspects, or the really big ones (often in puck form) are special order, from who knows where.

On the other hand, if you have the pulse time and budget to do it, you can use a switching circuit to chop up the pulse and pump it into a capacitor (switching is necessary to implement an equivalent resistive input), then bleed that off over time. Uh, needless to say... this is more feasible in a controlled surge situation (say, inrush current limiter) than not (say, lightning).

Tim

The hard plastic case TH Ohmite's go pop when over stressed. Wear safety glasses, seriously, that plastic can fly.

Cheers

Use of an electrolytic / high voltage capacitor for energy source is the classical "WOW" method (and seem a preferred way also).

Yeah, it looks like the best way to get hundreds of amps in the millisecond down time frame is to parallel a bunch of electrolytic caps. Maybe a sheet of copperclad FR4 with a lot of caps soldered or bolted to the connections on both sides. Where's my Dremel?

I have a lot of 22,000uF 50V parts in stock, surplus from a discontinued product. Each one can store 27 joules. Even 100 J makes an impressive bang.

A cap bank, and a pulsed series mosfet or so, maybe some current-limiting resistors, could explore the power-time destruction envelope of my wirewould resistors, or of other parts. Might be handy to have around.

More voltage might be good. I'd have to buy some caps.

Isn't such a sudden discharge likely to damage the caps?

-- This message may be freely reproduced without limit or charge only via the Usenet protocol. Reproduction in whole or part through other protocols, whether for profit or not, is conditional upon a charge of GBP10.00 per reproduction. Publication in this manner via non-Usenet protocols constitutes acceptance of this condition.

How? If you don't heat up the contents of the capacitor all that much, how is the sudden discharge going to mess anything up?

I don't think so. You can fully charge an electrolytic cap and then short it with a screwdriver (or a flashtube) and it doesn't seem to mind.

I'd have to use a lot of caps in parallel, to get enough coulombs and low ESR, so no one cap would see all the load current.

A cap bank like this can magnetize anything ferrous. Just a few turns of fat wire, and a couple thousand amps, makes a lot of field.

Don't you think you should find out for sure? Maybe you'll be fine doing a one-off, but multiple tests I'm not so confident about.

-- This message may be freely reproduced without limit or charge only via the Usenet protocol. Reproduction in whole or part through other protocols, whether for profit or not, is conditional upon a charge of GBP10.00 per reproduction. Publication in this manner via non-Usenet protocols constitutes acceptance of this condition.

Shorting the caps are normally not a problem, just buy the best you can fin d

Cheaper manufactors can have problems. We have seen that in motor drives, t hat failed when the powermodule discharged the caps to fast

To create you bank, use leaded caps with long wires. Mount the cap into an PCB with unbroken copper foil, with one of the leads into an oversized hold , so it has no contact. Then add another PCB with the same holes and solder only the lead that was not soldered in the first PCB. Then you get two PCB s with very low inductance

Cheers

Klaus

Nowadays we have this newfangled "two-layer" thing. ;)

Cheers

Phil Hobbs

il.com:

it is hard to solder the pin connected to the bottom side unless you get pcbs made with plated holes ;)

and with a string of caps connect to opposite ends of the string to get even similar pcb resistance to all the caps

it is hard to solder the pin connected to the bottom side unless you get pcbs made with plated holes ;)

Well, so you do know about some of the modern developments, I see.

Seriously, if you have top and bottom pours, why on earth do you need two boards?

Cheers

Phil Hobbs

I've been shorting charged electrolytic caps, and banks of same, since I was a kid, and never broke one. I'm suggesting a one-off setup to see how many joules various parts can absorb, not a production unit.

We have sold lots of NMR gradient amplifiers that output up to 120 amp/17KW pulses, from energy stored in electrolytic caps. I don't think we had any caps fail.

I'm going to Dremel my cap board from double-side copperclad FR4. I think I'll just carve an interdigitated pattern on the bottom side. I could use both sides, but I can't solder the radial caps on the top very well, so I'd have to make some little islands on the bottom and via them up to the top side. A nut+bolt makes a pretty fat via. Soldering a bunch of little pieces of wire as vias is a nuisance.

il.com:

;)

boards?

if you have PCBs made you don't, I was guessing that John would just drill some holes in a piece of FR4

Gotta love the "I haven't seen it so it can't exist" fallacy.

And the manufacturer, let's not leave them out. Y'think they might know a thing or two about their product?

Tim

When you build thousands of high power things, over several decades, and no elec caps fail from discharge pulses, that's a pretty good sign.

Did you notice, on my sketch, the parts labeled CURRENT LIMITERS ?

Why are you being such a smarmy jerk?