From: "John Jardine"
Subject: Re: 2.5kW-3kW-SMPS-(cheap)-suggestions?
Date: Tue, 17 Dec 2002 20:42:06 -0000
NNTP-Posting-Date: 17 Dec 2002 20:29:18 GMT
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John Woodgate wrote in message
> I read in sci.electronics.design that John Jardine
> wrote (in k>) about '2.5kW-3kW-SMPS-(cheap)-suggestions?', on Tue, 17 Dec 2002:
> The SMPS design is FAR from simple, especially with the wide input
> voltage range and the need for intensive PFC. The obvious question is
> 'why are the heaters designed for 40 to 50 V?' You have no electrical
> problems, no weight, no cost, if you only can get 110 V heaters.
> 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!
Yes. Some heater systems can and do use 110V (or 220V) direct. So the PSU is
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 3kW
The heaters are fine wires embedded in plastic. Heat em up and the plastic
melts sufficient that a joint or weld can be made to other similar plastic
items (simple eh!).
The problem is one of the thermodynamics of the heat flows from the powered
heating element into the surrounding plastics. The odd heating voltages are
one of those engineering compromise things that is bound up with resistance
wire performance and costs, safety of use voltages, time-energy density
profiles and the enthalpy/expansion characteristics of particular polymers
... Sort of, "To hell with the electrics, let electronics people sort them
out, we've got much bigger problems with the material's structural
Heating energy is best being supplied at a constant rate for an optimum
time. This implies a constant voltage (stable to maybe 1V) or a constant
current. Too low a power flow for too long a time and the end of cycle
internal temperature profile from the hot wire feeding heat into the
surrounding material is too 'flat' to give the amount of localised focussed
heating needed. Too much energy=localised high temps with resulting material
degradation and little heat flow or spread into the surrounding material.
Hence an optimum heater voltage may end up in the 20V to 60V area.
AC,DC,Hf,lf, chopped, or any mix doesn't matter, it's just the controlled,
RMS energy equivalent that's required.