The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: "John Jardine"
Subject: Re: 2.5kW-3kW-SMPS-(cheap)-suggestions?
Date: Fri, 20 Dec 2002 23:44:04 -0000
NNTP-Posting-Date: 20 Dec 2002 23:52:19 GMT
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
John Woodgate wrote in message
> I read in sci.electronics.design that John Jardine
> wrote (in >) about '2.5kW-3kW-SMPS-(cheap)-suggestions?', on Fri, 20 Dec 2002:
> I think you are determined by tradition to paint yourself into a corner,
> but since I can see a mistake in your examples, I'll have one more try.
> >(Imagine a flat or circular inductor coil). Extra wire would change the
> >heating surface area or wiring pitch. These are rigidly defined by the
> >particular heat flow dynamics and final joint strength
> 'Rigidly defined' - I just don't believe zero flexibility in real life.
Nor do I. You've identified the crux of the situation but we're talking here
about the products of large European companies.
Any rework to an existing, expensive, process will cost a lot of money and
cause the accountants to be afraid, very afraid.
K's of km's of wire and 10k's of tonnes of polymers are involved, along with
£10's of millions of plant and tooling and 1000's of people. Marginal costs
multiply up rapidly. Adding 1p's worth of wire to a product that's runnning
at 100k/year is a profit loss of £1k easily visible to the bean counters.
Slightly modify a heater wire alloy for just 1 product?, that'll be £15k +
3months for retests and approval. Change a wire pitch to accomodate an extra
1mtr of wire?, that'll be £40k +4months for tool remodelling +£15k +3months
for retests and approval. Redesign an existing product to harmonise on a
wanted voltage?, £60-80k + 8months. Cost of a new product designed with
small reference to voltage, £1. Cost of same product when voltage was prime
Supply power at an oddball voltage?. That'll be a power unit resistor change
at 5 minutes design effort. I'm no bean counter but I know what I'd choose
> >Increasing this wire length also increases the linear wire
> Can you not conceive of any way of relieving this expansion? We're
> talking about a few cm at most, not metres.
Probably you can figure out more ways than I can!. I've read at least 50
proposals and patents on this subject. Some straightforward,some rubbish,
some markedly ingenious. There's a couple of methods actually in use. One
involves forming the element into zig-zag hairpin loops where mechanical
stresses are optimally absorbed. These methods, for a number of valid
manufacturing reasons, cost upto 20% more and used only for speciality
items. Hence a cheaper, existing, 'known to work well' process would be
> >2) So as our wire length is immutably fixed we simply need to replace it
> >a different diameter (x0.5) or use a different rho factor and diameter.
> Not x 0.5. See below.
> >Not that easy ...
> >(For demo I'm showing some numbers here from an old prog I wrote.)
> >Assume existing 50V design uses 15mtrs of wire at a 2mm pitch winding
> >spiral) rate. Resistance wire type is .52mm dia stainless steel. 50ohm at
> >23degC. 60.5ohm at 200degC. Max linear expansion 35mm. Cost say 22pence.
> >First option ... go for a lesser diameter of existing wire so ...
> >0.26mm SS gives 200ohms 50Watt, same material, same length, same tempco,
> >available at 1/2 the price!.
> >Useless! ... The machines can only handle wire down to 0.4mm . £15k to
> >and then problems start turning up with wire stressing, requiring a
> >expensive stress relief operation .
> No, you want half the AREA to go with double length. That gives you 0.37
> mm diameter. Oh, dismal failure; still below 0.4 mm. And, of course,
> it's unthinkable to use 32.5 m of 0.4 mm wire.
I've figured the dichotomy! ...
I simply am *not allowing* the length to be changed (for the previous
reasons). Which is why I settle on 25% of the CSA and not the 50% you
suggest. (for the doubled voltage test case)
> >Second option ... go for a different material that will work with the
> >machines .4mm min, yet still give the 50Watts at 100V using same length.
> >Useless! ... Out of 18 resistance wires none can get down to 50W at 0.4mm
> Oh, yes, dismal failure. Just 2.5 m extra works with the SS material.
> >We're buggered, so lets pretend we made a mistake and the example problem
> >really 50V, 2ohm, 25amps, 1250watts. Stainless would have been impossible
> >use for this design (>£5) and the >2.5mm dia is too big for the machines.
> >copper (or a copper mix) *would* have been used for size and cost.
> >So assume a particular copper mix (K3) heating wire ... gives 0.54mm
> >2.8ohm at 200degC, cost would be 27pence. 38mm max expansion.
> >So at 100V can't use K3 again as dia would be too small.
> 0.54 mm divided by ROOT 2 = 0.38 mm. Once again, a very small increase
> in length allows you to use the same material in 0.4 mm dia.
> I can't tell you in detail how to accommodate the longer wiring, because
> it depends entirely on the exact dimensions of each particular heater.
> But I don't believe it's universally impossible.
(Technically easy but £~70k to implement).
> >So look for
> >another. Most are u/s but a cheap copper (K10) mix looks promising ...
> >0.5mm, 9.1ohms hot, cost 25pence, a saving!.
> >Useless!. ... This time we suffer from a temco problem. The tempco is
> >9.1/8= +13% compared to previous tempco of 2.8/2=+40%. This will mean
> >too much power is being dumped into the plastics at the tail end of the
> >cycle. Heat flows and material degradation will suffer badly. (too many
> >total Joules per mm^2).
> But if these things are driven at present from generators through
> transformers, the voltage is not closely controlled, so it's difficult
> to see that tempco is a major factor. And if it is, your switch mode
> solution wants to be a **constant power** source, not a constant voltage
You are correct. Constant power offers a neat solution in these situations.
It does though bring its own unique problem concerning heat flow dynamics.
For quality joins, a particular product's two heated surfaces require a
specific number of Joules paced over a particular *time*. E.g 1MJ over
60secs. *Not allowed* is 1MJ over 50secs or 1MJ over 70secs. With a high
working voltage (no feedback) the required Joules energy accumulates too
early and the heatflows have not stabilised to the correct profiles before
the cycle finishes. (One low cost system did use this concept).
But ... . I never intended coming here in the first place :-). I was simply
pointing at an amorphous "40-50V" the "value" to be decided later. Whatever
power control mechanism is finally decided on is a minor detail. The hard
work is in the power switching aspects.
> >Stainless would work nicely in this 50V to 100V upgrade as it's tempco is
> >useable but the 1.31mm wire costs £1.42 a go, pitch spacing problems turn
> >and the accountants would (and should) veto the idea simply on cost
> I hope they include the cost of your transformers or switchers,
> including the cost of meeting EMC requirements. You can't avoid the
> requirements on radiated emissions with a label, and there must be at
> least some doubt that you can avoid the conducted emission requirements
> that simply, because the uncorrected emissions would be horrendous and
> would create havoc if the switcher *were* ever connected to a public
> mains supply.
Whoa, hold on! I've been out of it for three years and only know these
engineering details from 6 months helping on the design of a product range.
The PSU's are traditionally just 'bought in' items to be given away or sold
as required. Their design costs would not be factored in as they are
externally sourced and essentially trivial as compared to plant and machine
The stick on label "Solution to all EMC problems" is of course a tease.
The interesting suggestions I received from the guys give me a lot of food
for thought. A natural (and interesting) part of which is to reduce the
noise. It's just good manners!. (directives or otherwise).
> Regards, John Woodgate, OOO - Own Opinions Only.
> Interested in professional sound reinforcement and distribution? Then go
> PLEASE do NOT copy news posts to me by E-MAIL!
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup