OU/COP>1 switched cap PS cct like Tesla's 'charge siphoning'

[CTG Labs]

Hi Sandy,

Yes, I also thought that the proportions were not the same.

The resistance of the coil is 0.5ohms and 1.886H.

I am currently using the printed values and will have to measure their capacitance, they are too large for my Digital LCR meter so I will have to do it the old fashion way.

I do not discharge C2 back through R1 (10 ohms), but simply calculate the energy stored in it.  So it could be "slightly off" as I used the nominal capacitance value.

The pulse on time is 50uS, off time 300uS and there are 10,000 repetitions, which drop from about 12.60v to 11.40v in 3.6 seconds.

During this time, the scope samples 1784 times, so there is a sample once every 2mS, hmm, this is no where near the sampling time specific for this scope 

Regards,

Dave.


PS. I can also hear the coil "sing", guess its around 3khz.

[nul-points]

hi Dave

thanks for the info

your coil value of 1.9H seems very large for a 0.5ohm winding - approx how many turns with what diam wire is it?

my coil is 0.5ohm DC with approx 60-80 turns of .5mm wire

or is it a typo for mH?

measuring the discharge energy of C2 isn't critical - i usually find there's a good correspondance with the calculated value from the final stored voltage - it's just a cross-check of one of the results

as a quick check of the relative cap values you could swap C1 & C2 & see if you still get a similar efficiency result - if you do, then the cap values aren't out too much - if the efficiency values changes significantly then it would be a good idea to measure them (old method = time to discharge to 37% of Vstart, with a measured shunt R?)

might be worth trying a longer 'off'' time  - see if you're starting each new pulse before the flyback energy from previous one is all returned to C2

if your coil really is 1.9H, then 50us may be on the short side - you could probably increase the pulse width significantly & reduce the number of cycles - you might reduce some switching loss overheads this way?

very encouraging!

regards
s.

[CTG Labs]

Hi Sandy,

Sorry for confusion!  The inductance is around 1milliHenry, measured on a digital LCR meter, the resistance 0.4ohms measured on a Fluke 177.

I hear what you are saying about matching the pulse duration to the rise time, I fine tuned this on the scope.


Regards,

Dave.

[CTG Labs]

Hi Sandy and all,

I have now tested the capacitors using timed discharge method.  They are different to the labeled value (of course), but the overall result leaves with around the same apparent COP.

Same calculations taking in to account measured capacitances:

C1: 111,201uF
START: 12.60v, 1.4q, 8.827j
END: 11.42v, 1.27q, 7.251j
INPUT ENERGY: 1.576j

C2: 108,961uF
END: 2.53v, 0.275q, 0.348j

R1: 2.822j

OUTPUT ENERGY: 2.822 + 0.348 = 3.170j

COP = 3.170 / 1.576 = 2.011

So, I don't think cap values being off at this level will cause much of an overall error given the large apparent amount of extra energy.  If we were seeing say COP = 1.1 then it could very well tip the balance.

I am still not seeing the equal share of energy in R1 and C2 though like you show.

How are you intending to prove the energy in R1?  This is not big enough to perform any heat tests?  I thought about the motor example but that is also flawed.


Regards,

Dave.


ERM>.... Time for me to have a big red face, I messed up the energy calculation for R1.  In fact R1 has only 0.223j, so even with this test the over all COP is: 0.360.  There was an error on my excel sheet, after adding all the voltage samples and then finding an average voltage, I neglected to square the voltage over the load resistance before mulitplying that by the time to get the energy.  So of course the figure was much larger.  This is my fault for staying up so late and posting too early.

[nul-points]

hi Dave

> after adding all the voltage samples and then finding an average voltage, I neglected to square the voltage over the load resistance before mulitplying that by the time to get the energy

i'm relieved to learn that i'm not the only one who sometimes finds that a set of good results were only achieved by 'creative accounting'! 


before looking more closely at possible losses in the system, i think you'll also need to alter your Excel calcs to average the sum of the instantaneous powers - not the instantaneous voltages (ie. calc the power for each reading first, then sum all, then average)

you can confirm this if you performed the same calcs on the discharge from C2 as you do on the charging energy in R


here's an example from my results:

Average of the Sum of the Instantaneous Power readings:
  0.119W

duration:
7.81ms

Energy in discharge:
0.119 * 7.81 = 0.929mJ

Vmax C2:
3.07V

E stored in C2 (196uF):
0.924mJ


so the above calcs from the scope measurements match the stored energy calc for Vmax on C2


however, if we used the average of the instantaneous voltage readings we'd get:

Vav:
-0.809V

P:
  -0.809 * -0.809 / 10 = 0.0654W

E:
0.511mJ


so getting Excel to calc the Av volts & THEN squaring will give you an answer approx 50% of the actual


i think the next thing to do is to look for possible losses in your setup

i notice that your results show a charge gain imbalance around 10% - it should be possible to achieve values nearer 30-50%

before attempting to do a raft of calcs, i usually confirm that i'm getting a sufficiently high output voltage on C2 - if i keep to a set input volt drop with a known input energy then i can very quickly calc & compare the energy on C2

i check & modify my circuit parameters until the max volts on C2 represent a stored energy around 70% or more of the input - then i make a fully measured test-run


your switching schedule gives 10000 charge pulses - could this be reducing the efficiency due to switching-loss overheads?

your coil has a relatively low inductance which will limit the max pulse 'on'-time you can use before you start getting additional I^2*R loss per pulse in the coil - maybe you could try doubling the inductance? (additional ferrite packing might help here?)


hope these suggestions help

all the best
sandy
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