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From: "John Jardine"
Subject: Re: 2.5kW-3kW-SMPS-(cheap)-suggestions?
Date: Wed, 18 Dec 2002 02:44:00 -0000
NNTP-Posting-Date: 18 Dec 2002 02:33:15 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 Tue, 17 Dec 2002:
> >Yes. Some heater systems can and do use 110V (or 220V) direct. So the PSU
> >simply a relay and timer (oh! if they were all 110V :-). Others have more
> >precise needs which throws complexity onto the power supply. Hence the
> >SMPS aspect.
> OK, but there's 'complexity' and 'five-dimensional chess'. (;-) Since
> the wires are 'fine', there must be some sort of paralleling to get the
> whole thing to take 55 A. Changing to a different network should get to
> a 110 V rating without changing so very much as far as the heating
> effects are concerned.
> As a thought-experiment, cut a heater network in half and wire the two
> halves in series. You now have four times the original resistance. Apply
> double the original voltage. Power = V^2/R, and (2V)^2/4R = V^2/R, so
> the heating effect is the same.
Surely ... Network =R : R/2 =0.5R : 0.5Rx2=R (I know what you are saying
> Think of the time, effort and money you'd save if you could get the
> heaters made that way!
> 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!
Well spotted :-). I used "fine" to avoid connotations with heavy industrial
processes. Think of plastic pipe joining bits and pieces, plastic
air-conditioning ductings or joining plastics assemblies together in
Heater wire(s) can be moulded in, or seperate (like a cooker ring). The
needed heater wires come in a number of arrangements from 'fine' (say 0.1mm)
wire series/parr mesh mats to single/dual/quad spiral wound and flat pad
heaters using to 2mm wire dia. Currents can range from 1amp to 200amps and
feed voltages from 20V to 230V. Total powers can range from say 20watt to
8-10kW. Depends on the makers and range of products and country.
Having said that. It is a massive R+D and process problem to standardise on
a 'really useful' voltage like 110V. The problems tend to hinge on a) Cost.
b) Cost. and c) Cost.
Fixing the (say) voltage and saying V=sqrt(PR) or V=IR is a theoretical
solution only. In practise ...
The reasons are not immediately obvious and are related to (not in priority)
1) Changing national and international standards.
2) Limited ohmic range of useful resistance wire alloys (about 12 acceptable
3) Resistance wire tempcos (can be harnessed or hated but not ignored )
4) Chemical resistance of wires in particular areas of use.
5) Cost per metre of particular resistance heating wires alloys.
6) Strength of particular wires from a machine handling POV.
7) Ease of adding reliable terminations.
8) Surface area needing heating.
9) Site available power source capability.
10) Heating dynamics of the plastics used. (a 'boss' variable)
(No 5 is top priority. No's 8,10 are prime heating voltage deciders).
Mixing and matching (design!) of these 10 variables (there are more) for
each product worldwide would be very, very, difficult without the massively
helpful extra degree of freedom that a variable input voltage (say) can
give. About 3 or 4 of the constraints are sortable as a matrix of
simultaneous equations but that's as far as it goes. The process variables
have only recently been able to be simulated by FE methods.
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