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Author Topic: BroMikey's Capacitor Dump Circuit  (Read 45325 times)

Offline Farmhand

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Re: BroMikey's Capacitor Dump Circuit
« Reply #30 on: May 09, 2014, 10:24:43 PM »
Going by the schematics on the previous page, Bromikey seems to be using a high side switch between the caps + and the battery +, he would be better served to use a low side switch between the battery negative and the cap negative. That would simplify the driving circuitry.

If he has the mosfet drain at battery voltage he will have problems without a working level shifting driver circuit or something. I'll ask him.

Cheers

Free Energy | searching for free energy and discussing free energy

Re: BroMikey's Capacitor Dump Circuit
« Reply #30 on: May 09, 2014, 10:24:43 PM »

Offline Farmhand

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Re: BroMikey's Capacitor Dump Circuit
« Reply #31 on: May 10, 2014, 10:46:49 PM »
I've just recently downloaded the newer manuals for the picaxe chips and it tells me now that the M2 chips can all run at the default clock speed of 4 Mhz or they can be clocked up to 8 -16 and 32 Mhz, the old manuals only stipulated that the M2 chips could be clocked to 8 Mhz. Theoretically the M2 chips at 4 Mhz clock speed can output a 500 Khz PWM signal,
but at 32 Mhz the theoretical PWM signal could be 4 Mhz. Not bad for a cheap 8 pin micro.

What I like about the smaller chips is the price, the small size and low power draw along with many of the functions of the larger chips. For more outputs and inputs the 14M2 seems good and I intend to use those more.

..

Offline MarkE

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Re: BroMikey's Capacitor Dump Circuit
« Reply #32 on: May 11, 2014, 02:03:45 AM »
I've just recently downloaded the newer manuals for the picaxe chips and it tells me now that the M2 chips can all run at the default clock speed of 4 Mhz or they can be clocked up to 8 -16 and 32 Mhz, the old manuals only stipulated that the M2 chips could be clocked to 8 Mhz. Theoretically the M2 chips at 4 Mhz clock speed can output a 500 Khz PWM signal,
but at 32 Mhz the theoretical PWM signal could be 4 Mhz. Not bad for a cheap 8 pin micro.

What I like about the smaller chips is the price, the small size and low power draw along with many of the functions of the larger chips. For more outputs and inputs the 14M2 seems good and I intend to use those more.

..
An alternative 8 pin part is the Atmel ATtiny85.  64MHz PWM oscillator, ~$1.00 each.  The core runs 8MHz, 8MIPs.

Free Energy | searching for free energy and discussing free energy

Re: BroMikey's Capacitor Dump Circuit
« Reply #32 on: May 11, 2014, 02:03:45 AM »
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Offline Farmhand

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Re: BroMikey's Capacitor Dump Circuit
« Reply #33 on: May 11, 2014, 07:59:16 AM »
Mark those chips look awesome, I scanned this document below and found a lot of interesting functions it can do. And the price is much better than picaxe, however. How long would it take me to learn how to write code for it. I'm a complete novice, the way I see it trying to learn how to write programs for picaxe is difficult enough, and I also want to learn how to use Arduino.

Is there a language I can use to program them all ?

It has some good PWM features.

http://www.atmel.com/images/atmel-2586-avr-8-bit-microcontroller-attiny25-attiny45-attiny85_datasheet.pdf 

..

Offline SeaMonkey

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Re: BroMikey's Capacitor Dump Circuit
« Reply #34 on: June 17, 2014, 08:18:02 AM »
BroMikey's self-education project continues to reveal to him
interesting developments
just when he thinks he's got it
figured out.  His education has progressed to the point
where he "knows enough to be dangerous."

Not meant to denigrate BroMikey or his endeavors but it is
a phase that all aspiring electronics technicians work through
as they grow in their understanding.

Six paralleled MOSFETs should be more than capable of handling
40 Ampere pulses, or even more, if properly driven and properly
protected from flyback pulses which are capable of destructive
avalanche of the MOSFET Body diode.

Hopefully BroMikey will either find the answers to his dilemma through
his own research efforts or a knowledgeable tech will provide him with
the elusive technical details he hasn't yet found.

It is possible in his case that his MOSFET gate driver chips aren't properly
positioned as close to the MOSFETs as possible and with sufficient capacitance
to properly drive the Gates.

It is also possible, and very likely the case, that the length of the cables from
his capacitor bank to his battery bank results in quite a lot of stray inductance.
At the instant pulsing current switches off a very substantial flyback pulse will
be generated which, although very brief, will take the MOSFET's into avalanche.
Since avalanche characteristics are quite different amongst the parallel connected
MOSFETs it is probable that one of them will serve as a current hog for the avalanche
current.  In time this will destroy the MOSFET and result in its failure much as has
been the case so far with his setup.

Thankfully, these problems are rather routine in high current switching systems
and the remedies are well established,  BroMikey will find success if he keeps
searching.

Free Energy | searching for free energy and discussing free energy

Re: BroMikey's Capacitor Dump Circuit
« Reply #34 on: June 17, 2014, 08:18:02 AM »
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Offline TinselKoala

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Re: BroMikey's Capacitor Dump Circuit
« Reply #35 on: June 17, 2014, 09:02:51 AM »
Faugh. I looked at that post and I can see that BroMikey is terribly confused and is clearly doing something wrong.

The IRFP460 has Rds 0.27 ohms (when switched fully ON; higher if not), Id max 20 A reducing to 13 A at 100 degrees C, and 500 Vdss. Its max _power dissipation_ is 280 Watts when it's on a good heatsink.

He is thinking that 3 amps at 80 volts is getting close to the mosfet's limit! Because 3 x 80 is 240 Watts and the data sheet says 280 Watts at 25 C!!!
But this is derated 2.2 w/degree C, so at 110 C the power dissipation limit is actually 280 - (2.2 x 85) = only 93 Watts. Hence the need for cooling at high power levels.

But really, if the  mosfet is carrying 3 amps it is dissipating I2R or 3x3x0.27 = about 2 and a half Watts !!!! 6 amps boosts it up to nearly 10 Watts. The mosfet can easily handle this even if not on a heatsink or fancooled. You'd think he'd notice the room getting a bit warm, if his bank of mosfets was really dissipating nearly 3 kW.

Let's say the mosfet is carrying 20 amps.  How much power is it dissipating? 20 x 20 x 0.27 = 108 Watts, exceeding the max power limit at higher temperatures. How about that. It must be kept cool in order to operate at that power level. So again we begin to understand the relationship between mosfet power dissipation, the maximum rated limits of Id and Pd, and the need for proper cooling at high power levels.

But he's pulsing at 3 Hz with 250 ms ON time for a duty cycle of 75 percent, so that lowers the actual average power dissipation accordingly.

So if his mosfets are running hot and failing it most probably means he isn't switching them properly _and_ he is getting huge dissipation from avalanching on switch-off, not failing from carrying too much current per se. They are nowhere near their actual power handling capacity when properly switched.

Quote
When reconsidering I would have to say that each fet might be capable of 3 amps of surging high voltage. This fet is rated at 280 watt max burnout.
 
 I have not put my thinking cap on much till things burnout.
 
 I am passing 35 amp pulses using 6 parallel IRFP460 FETS.
 
 This divided up evenly = 6 amps per FET and since each Fet is also passing voltage with it we must multiple 80vdc X  6 Amps = 480 watts in 250 mS. That is 480 watts EACH so X 6 =2800 watts.

Insert facepalm here.



ETA: That "also passing voltage with it" phrase indicates to me that BroMikey's mental model of electricity is wrong, and he is misleading himself because of it.

Cure #1: put an ultrafast, high current diode reverse biased across each mosfet. MUR1560 for example, anode to Source and cathode to Drain. These diodes may also need to be heatsunk.

Offline Farmhand

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Re: BroMikey's Capacitor Dump Circuit
« Reply #36 on: June 17, 2014, 11:06:38 AM »
Maybe if he is dumping enough capacitance charged to a high enough voltage into a battery he might be exceeding the maximum pulsed current rating for the mosfet, and with several in parallel one mosfet might be taking most of the abuse, as a cap discharge can produce a very high initial peak current.  :D That is why it's done, isn't it.

And coupled with poor switching and high "on"resistance then "Pop goes the mosfet".

He also mentioned at one stage he w is using quite long cables, like meters in length.

I've made a drawing for him with a store bought transformer run from the wall to give a 12 + 12 volt supply so he can rectify 12 volts for switching supply and 24 volts for charging the caps with a Large inductor between smoothing caps and dump caps to partially isolate the dump cap from the supply when dumping.  Maybe a MOT primary would work there, (just remove the secondary for safety XXX).

..

Free Energy | searching for free energy and discussing free energy

Re: BroMikey's Capacitor Dump Circuit
« Reply #36 on: June 17, 2014, 11:06:38 AM »
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Offline TinselKoala

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Re: BroMikey's Capacitor Dump Circuit
« Reply #37 on: June 17, 2014, 07:08:29 PM »
Some reading material.

Offline TinselKoala

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Re: BroMikey's Capacitor Dump Circuit
« Reply #38 on: June 17, 2014, 07:30:59 PM »
Maybe if he is dumping enough capacitance charged to a high enough voltage into a battery he might be exceeding the maximum pulsed current rating for the mosfet, and with several in parallel one mosfet might be taking most of the abuse, as a cap discharge can produce a very high initial peak current.  :D That is why it's done, isn't it.

That's true. A capacitor discharge can produce very high currents, the more capacitance the more current ... into low impedance loads. The max pulsed current for the P460 is 80 amps and that's under pretty narrow pulse parameters. So if he's charging to 80 volts, then dumping the caps through a single mosfet due to poor paralleling, he needs at least one whole ohm of impedance in the circuit after the mosfet, to keep the max current below 80 amps.  R = V / I  so 80/80 = 1 ohm. Oh, wait, the mosfet itself has 0.27 ohms minimum resistance, so the rest of the circuit needs less than three quarters of an ohm impedance (resistance) to keep the maximum surge current below 80 amps. No matter the capacitance.

I'm still having my first coffee of the day, so please check my math and reasoning.

Dumping a cap through a mosfet into a load is a lot less problematic than charging an empty cap through a mosfet switch or crowbarring a cap bank with a mosfet. The empty cap looks like a dead short, so if the supply voltage is there, the surge current in the mosfet can be very high. But dumping a cap into a resistive or inductive load with a mosfet is much less problematic because the load impedance limits the surge current. I think.


Quote

And coupled with poor switching and high "on"resistance then "Pop goes the mosfet".

He also mentioned at one stage he w is using quite long cables, like meters in length.


Then he should put the diode I mentioned above, at the load end of the cables, and supplement with an additional similar diode right at the mosfet pins.

Quote
I've made a drawing for him with a store bought transformer run from the wall to give a 12 + 12 volt supply so he can rectify 12 volts for switching supply and 24 volts for charging the caps with a Large inductor between smoothing caps and dump caps to partially isolate the dump cap from the supply when dumping.  Maybe a MOT primary would work there, (just remove the secondary for safety XXX).

..

Is the complete actual circuit he is actually presently using right now, available for study? I'd like to take a look but I can't be arsed to sift through page after page of nonsense at EF.

Free Energy | searching for free energy and discussing free energy

Re: BroMikey's Capacitor Dump Circuit
« Reply #38 on: June 17, 2014, 07:30:59 PM »
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Offline Kator01

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Re: BroMikey's Capacitor Dump Circuit
« Reply #39 on: June 17, 2014, 07:58:46 PM »
Hello,

this subject has been covered by diifferent physicists starting with Heinrich´s first publication 1985, so I wonder why this topic is brought up anew again and again.

Here are some publications, starting with Heinrich´s work:

http://freenrg.info/CAPTRET/Entropy_Change_when_charging_a_cap.pdf

http://xputers.informatik.uni-kl.de/conferences/patmos/patmos98/desoete.pdf

http://arxiv.org/pdf/1201.3890.pdf

Regards

Kator01


Offline SeaMonkey

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Re: BroMikey's Capacitor Dump Circuit
« Reply #40 on: June 17, 2014, 09:36:54 PM »
Aye, BroMikey still has a ways to go before he acquires full
and complete technical understanding of what he's up
against with his circuit.

Good education always seems to come with some sort of
pricetag - be it in dollars or burned out MOSFETs.  But the
rewards can be enormous if the "student" has the gumption
to stick with it until he is finally able to make it do what it is
supposed to do reliably and without fear of premature failure.

And know the reasons why...

Free Energy | searching for free energy and discussing free energy

Re: BroMikey's Capacitor Dump Circuit
« Reply #40 on: June 17, 2014, 09:36:54 PM »
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Offline Farmhand

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Re: BroMikey's Capacitor Dump Circuit
« Reply #41 on: June 18, 2014, 01:42:31 AM »
Tinsel he is charging the capacitor bank from a series set of batteries to get the higher voltage "through a resistor" to limit the current when he discharges the capacitor, I explained how much power the resistor is dissipating when he does that, then he is dumping the huge cap bank through the paralleled mosfets to the batteries. I think he is using a large capacitance 10,000's of uf and charging the dump caps to more than double the battery voltage, maybe triple.

When we dump caps the caps end up not discharging to zero volts they end up at battery voltage, when the cap is first connected it charges to just under charge battery voltage.

I suggested he use a mosfet to isolate the supply from the load when dumping or use a series inductance coil to partially isolate the supply for at least part of the dump.

A better way would be to use a transformer to charge the caps. And prime the caps if the bank is high uF.

Or if using a series batt set to charge the bank he really needs to isolate the supply series string of batts from the batt he is dumping into.

What would be the peak current of 36 volts in a 200,000 uf cap dumped into a paralleled set of four 12 volt batteries in good condition ?

I haven't taken the time to verify what circuit he is using right now, I'm just going by his descriptions.

I haven't destroyed a mosfet for a long time now, I mustn't be trying hard enough.

..

Fairly sure a NC relay could be used to rout the transformer output through a resistor to initially "prime" the large cap bank, then when the caps are charged the relay would turn on to give a low resistance charge path from Xformer to caps..


..







Offline TinselKoala

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Re: BroMikey's Capacitor Dump Circuit
« Reply #42 on: June 18, 2014, 04:02:48 AM »
Quote
What would be the peak current of 36 volts in a 200,000 uf cap dumped into a paralleled set of four 12 volt batteries in good condition ?

The peak current depends on the total resistance of the circuit plus load. The capacitance _does not enter into it_.

The peak current into a _direct short_, when switched by the mosfet, is found by applying Ohm's Law: I = V / R. 
Since v = 36 volts and R = 0.27 ohms, the mosfet's minimum on-state resistance, the maximum current is just over 133 amps. Into a dead short, from whatever you are using that provides 36 volts, whether it be a capacitor of infinite capacity or a battery or a hydroelectric power plant.

For _how long_ can that peak current, or any current, be sustained? NOW the capacitance matters. Higher capacitance, more energy, more time to discharge it. As soon as the discharge from the cap begins, the voltage will begin to go down and the current will drop from its peak value, and as the voltage decays exponentially so does the current. More capacitance, longer time constant. More _voltage_ more peak current.

Now let's imagine a more realistic load impedance. Say it is one single ohm. Now solve I = V / R, and find that you can only push a bit over 28 amps through the circuit, no matter what the source is, no matter what the capacitance you are dumping is. That solution to Ohm's Law is the _maximum_ current that 36 volts will push through your circuit.

Now I don't know what other elements he has between the mosfet switch and the load, or what the equivalent internal resistance is of his battery banks, but I will bet that the total circuit resistance, even with all six mosfets properly paralleled, is more than 1.27 ohms. Which means that the _maximum pulse current_ from a 36 volt source will be less than 28 amps.

However in his post he says he's using 80 volts and is attaining 6 amps per mosfet, or 36 amps total. Again, solving Ohm's Law, we find that using those numbers at face value, R = V / I gives us a total circuit resistance of a little over 2.2 ohms.

So going back to the original question, if your capacitor of a million billion Farads is charged to 36 volts, what will be the peak current when it is discharged thru the mosfet (or thru a simple mechanical switch closure) into BroMikey's 2.2 ohm circuit? I = V / R = 36/2.2 = less than 17 amps. A single mosfet will work safely for a 36 volt supply, IF the backspike is handled properly.

Offline Farmhand

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Re: BroMikey's Capacitor Dump Circuit
« Reply #43 on: June 18, 2014, 05:35:59 AM »
Yes I understand that the peak current is limited by the voltage / resistance. basic stuff I'm not sure exactly why I wrote the uF value  except to say it is a high uF bank of several paralleled capacitors, won't that affect the resistance of the current path ? And if the batteries were paralleled then less resistance as well. I understand that no matter what the capacity of the supply it is voltage and resistance that determine current in DC.

Anyway it seems the problem is improper switching like most cases and since I don't have the time to help him, Matt over at EF will have to continue to do it. See no one helps anyone over there and he has been invited over here so, it's up to him.

With so many mosfets in parallel they probably are not turning on properly, I had to explain to him that there is a high peak current in a coil discharge to a battery, he was led to believe there was none, and also that to drive a mosfet requires current when some nong posted that mosfets do not require current for switching. Meaning I had to try to explain displacement current to him.

With the mosfets not turning on properly he has no chance.

..

To put it very simply if he cannot do it with one mosfet he is doing it wrong. Not much point trying to help people that want to parallel up hand fulls of mosfets.  :) And he is charging the caps from a battery series set through a resistor. Wow.

..

The other thing I meant to mention was he may be exceeding the Gate to Source voltage limit. Gate abuse maybe.

..

Offline SeaMonkey

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Re: BroMikey's Capacitor Dump Circuit
« Reply #44 on: June 18, 2014, 07:23:28 AM »
BroMikey is struggling with some internalized misconceptions and still
lacks full understanding of some critical details.  He's still in the embryonic
stages of his technical development.  Ways to go yet.

And yes, BroMikey you're reaching some of us.

The good news is that we've all been there; it too will pass. Then the
real understanding will congeal with eye-opening results.

Don't stop the quest!  True understanding is built upon numerous
failures which are temporarily disappointing but serve to fuel the
fires of desire.  The desire to know with certainty.


 

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