Rosemary Ainslie COP>17 Circuit / A First Application on a Hot Water Cylinder

[otto]

Rosemary,

MOSFETs are just fine!! Im using IRFP 450 MOSFETs and this are the only one I almost cant blow.
A lot of times I have connected them in wrong ways and they all "survived" my "out of mind" connections. They are good and fast enough.

It seems that I have to throw my TPU into water to see how fast the water would get hot and not to forget, how highly energized particles would "love" a water bath.

Yes, Im a little bit crazy because I want to SEE!!

You said you want more power?? Something like this. Hmmmm. you also said that your setup is not frequency depend. Of course your setup depends on the used frequency because this frequency is pulsating the battery. And from the battery you get the voltage!
So, use 2 oscillators - carefully. Because in 1 moment we have a oridinary battery and in another moment we have an electrolyser!! A very powerfull one.

Otto

[Rosemary Ainslie]

Hi Otto, 

I think we used IRFPG50's  - I could try that 450 number.  But here's the actual problem.  In our experiments we used a R10 Ohm resistor - (inductive number) and the best we could get is between 28 and 34 watts of heat from the circuit using it.  Notwithstanding which the actual measured voltage in the CEMF exceeds 1100 which is very nearly at the FET's limit.  So - that's the problem.  I think you guys refer to it as 'rise time'.  We need to switch it to get the required resonance - and in switching it we restrict the current flow required to get enough power into the system.  The good thing is that the high voltage gives a huge punch to return that voltage through the battery.  And it definitely recharges the battery which means that the energy delivered is almost zero.  And there's still that 28 odd watts of heat at the load which is way in excess of the energy delivered by the source.  But ITS NOT ENOUGH.  We now have to heat 8 litres of water.  It calls for way more energy.  Which also means that we need a transistor with a voltage tolerance in the 10's of thousands.  The hope is that maybe an IGBT can cut it.  But then the next problem is to establish the return path for the current flow from the CEMF to recharge the batteries.  So... We're thinking a diode across the switch may do it?  It could take the place of that internal body diode that we depend on in the FET? 

Not sure how it will pan.  The hope is rather focused on using that circuit design on that 'back to front' battery system with a return path of both cycles for the CEMF.  Maybe it will add to the heat and - provided we can return this to the battery to recharge - then this may compensate for the losses we're expecting from switching the IGBT.  I'm in unchartered territory here Otto.  I just don't know.

Kindest as ever,
Rosemary

edited irfpg50

[nul-points]

hi all

ok, now there is the beginning of a specification taking shape: 8L water!

the requirement list will need to grow from there


is it required to boil the water?  or just reach a certain temp?

is the next device just for testing or will it also eventually have to be practical (eg. for domestic use?)


let's consider a domestic kettle (just an example);

  - in a 220V consumer supply region, a 2KW kettle would handle approx 9A

  - the heating element would be need to be approx 24 ohms

  (max. volume of water is likely around 2L; heats water to 100*C in, say, up to 3 mins)


if the development of the circuit being proposed here (in this thread) is to continue using batteries then its likely that the supply volts will be much lower and the current much higher than the kettle example

we can see already that a heater impedance near 10 ohms is unlikely to be close to target - if we used a 24V battery supply for the kettle application  then we've divided the supply voltage by approx. 10 so we'd need to multiply the current by a similar factor (to around 90A in this example!)

we'd need a heating element with an impedance of around 0.25 ohm


so, one approach might be to 'divide-and-conquer' - have multiple 'inductive resistor' elements which can be safely handled by available MOSFETS; each sharing a fraction of the total current


are there other switching devices which might be considered?
(eg. the SCR - or a related device)

  - supply voltage (and current also?) might be less of a constraint

  - but would an alternative switch - an SCR, say - have the required switching-speed characteristics?


so - some more requirements needed:-

- target temp. for the 8L water
- preferred supply voltage
- practical constraints on the circuit
   (eg. would 10 elements + 10 switch devices, say, be acceptable just for tests, etc)

more (or tighter) requirements will help achieve a closer-bounded solution space

...sorry, i'm starting to sound like Prof. 'Lead Out'!   my meds must be starting to wear off!! 

'nuff from me for now, regards
sandy

[Rosemary Ainslie]

ok, now there is the beginning of a specification taking shape: 8L water!
is it required to boil the water?  or just reach a certain temp?

Hi Sandy.  The water does not need to boil - but I'd like to test it to boiling point.  We'll be fitting the cylinder with pressure valves so it probably won't get much over 80 degrees centigrade.

is the next device just for testing or will it also eventually have to be practical (eg. for domestic use?)

It's intended for use - as is.  There may be a market for this as our rural communities are off grid and their only access to hot water is by lighting fires.  Modified versions of this would be ideal to cook with - but that's definitely phase 2 - down the line.

let's consider a domestic kettle (just an example);

  - in a 220V consumer supply region, a 2KW kettle would handle approx 9A
  - the heating element would be need to be approx 24 ohms
  (max. volume of water is likely around 2L; heats water to 100*C in, say, up to 3 mins)


if the development of the circuit being proposed here (in this thread) is to continue using batteries then its likely that the supply volts will be much lower and the current much higher than the kettle example

At this stage we're aiming for not less than 100 volts dc (battery supply) and possibly as high as 200 depending on whether or not we use the nickle metal hydride in conjunction with the lead acids.

we can see already that a heater impedance near 10 ohms is unlikely to be close to target - if we used a 24V battery supply for the kettle application  then we've divided the supply voltage by approx. 10 so we'd need to multiply the current by a similar factor (to around 90A in this example!)

we'd need a heating element with an impedance of around 0.25 ohm

I agree.  We need low resistance and high inductance in the resistor.  But we're planning to test a variety of these resistors to see which work best.  Again.  The problem is to determine the 'switching speeds' to generate the required 'preferred oscillation' and yet retain enough power to ensure that there is some realistic level of efficiency in getting that water hot.

so, one approach might be to 'divide-and-conquer' - have multiple 'inductive resistor' elements which can be safely handled by available MOSFETS; each sharing a fraction of the total current

We considered this option.  The down side here is that multiple units is possibly clumsy and expensive.  Hopefully we'll get around this option.

are there other switching devices which might be considered?
(eg. the SCR - or a related device)

  - supply voltage (and current also?) might be less of a constraint
  - but would an alternative switch - an SCR, say - have the required switching-speed characteristics?

I have no idea.  You guys would know the answer.  One proposal made is that we use an IGBT as mentioned.  But then we'd need to introduce that diode across the switch to get a path for the CEMF induced current flow.  Still to be tested.

So that's where we're at.  But we still need to get some thermostat's and pressure gauges installed in the cylinder.  It's only just been plumbed in.  I'll take a photo of the set up either over the weekend or on Monday.  And we're still debating the best switch - and we've still got one more resistor to build before we can actually start those tests.  But I think it's a week away - at the most.  Can't wait.  I'll keep you posted.  And I'd be very glad of any advices that anyone can offer regarding switches.  Sandy?  Perhaps you can explain that SCR number.  I'm afraid I've never even heard of them.  I also want to explore a reed type switch but have been advised that it may be too 'sticky' and that there's a problem with arcing.  It seems we'll be obliged to use a transistor of some sort.  But not such a bad thing as we can at least retain that switching speed.

Thanks for the focus you've given us with these questions Sandy. 
Kindest regards,
Rosemary
http://www.scribd.com/aetherevarising

[nul-points]

hi Rosemary

apologies, i didn't mean for you to have to field all those questions

obviously you need to provide the requirement-related answers, but the more implementation-oriented ones are thrown out for consideration by members with power-application experience

your application sounds intriguing and very worthy - could this type of development herald the start of a new-technology steam age?

SCR is 'silicon-controlled rectifier'  - like a switchable diode - usually found controlling power devices on the mains supply - hence they can have pretty good voltage and current rating

however, i'm not so sure their switch-off behaviour would be suitable for initiating the inductive 'kick' of field collapse - but i'm sure folks with power-switching knowledge will advise

as you say, IGBTs have been suggested - but was there still a current-drive issue with them?  over to the heavy-current gurus!

i would expect reed switches to be discounted on two scores: 

- they're not at all suited to any kind of current drive, especially with inductive loads (arcing issues, as you say)

- they have very limited upper switching frequency - low hundreds Hz at best - i think your basic waveform is around a few kHz - and i don't think they'd be able to cope with your aperiodic or parasitic oscillation drive

a regular power relay would be better at the current drive - but equally dismal with the frequency response i feel

SSR - solid-state relay - might be a runner, again need input from those with power experience


more than enough** from me!

all the best
sandy


**oops... not quite...

  Rosemary, you *have* to see this:
      http://www.youtube.com/watch?v=wOv0AkphLhE&feature=pyv&ad=4232609694&kw=magnet&gclid=CNefjon5kaMCFYeY2Aodini3nA

...zipons, or what?!?