Author Topic: BroMikey's Capacitor Dump Circuit (Read 52895 times)

SeaMonkey · « **on:** March 12, 2014, 05:38:02 AM »

BroMikey has built a CapDump Circuit which is in need of
some "fine tuning" to get it working efficiently. He's
begun a thread discussion at the "other forum" explaining
his problems and his thoughts on resolving them.

If anyone here is able to enter discussion at the "other
forum" perhaps you'd consider inviting BroMikey to come
over here to get some real top notch assistance.

The problems he's experiencing are very common among
experimenters who've not yet got up to speed on the
"care and feeding of MOSFETs" and how to make them
switch capacitive discharge currents safely and efficiently.

Many thanks to any who provide technical assistance.

MarkE · « **Reply #1 on:** March 12, 2014, 11:22:44 AM »

Quote from: SeaMonkey on March 12, 2014, 05:38:02 AM

BroMikey has built a CapDump Circuit which is in need of
some "fine tuning" to get it working efficiently. He's
begun a thread discussion at the "other forum" explaining
his problems and his thoughts on resolving them.

If anyone here is able to enter discussion at the "other
forum" perhaps you'd consider inviting BroMikey to come
over here to get some real top notch assistance.

The problems he's experiencing are very common among
experimenters who've not yet got up to speed on the
"care and feeding of MOSFETs" and how to make them
switch capacitive discharge currents safely and efficiently.

Many thanks to any who provide technical assistance.

Any capacitor to capacitor charge shuttling circuit will suffer losses for several reasons. The one that you cannot avoid without using a resonant topology is the N*(X/N)² problem. That problem basically says that you don't want much change in voltage between the two voltages in the shuttle.

Efficiency versus low side voltage / high side voltage at start of transfer for equal value capacitors:

0% 50% efficient
50% 90% efficient
80% 99% efficient
90% 99.7% efficient

So, keep the voltage above 80% on the low voltage side. The other trick is to limit the surge current. You can keep the MOSFETs cool by switching them reasonably fast and using an external impedance. If for example you use an inductor, then you can realize a resonant transfer (this was the basis of the original now expired Vicor patents) and reduce the loss. If pick a resistor that sets a time constant about 1/7th of the switching interval but that is significantly larger than the MOSFET resistance then it will absorb most of the energy that must be dissipated. The current load that you can impose on the circuit is going to be limited by the switching frequency and the size of the caps, and the loss you are willing to suffer. Or you can get clever and pick a piece of magnetics that will limit the initial current surge before saturating.

SeaMonkey · « **Reply #2 on:** March 13, 2014, 04:58:41 AM »

Aye, the capacitor discharge surge current can have
explosive power and it may be necessary in some
applications to limit the surge to a safe value in order
to protect the switching MOSFETs.

The capacitive discharge surge rises near instantaneously
to a peak which can be hundreds to thousands of Amperes.
This is why Tesla loved capacitors and disruptive discharge;
it enabled him to produce very brief pulses of incredibly
great "horsepower."

It is essential when using semiconductor switches for this
purpose to turn them fully "ON" in the shortest possible time.
The proven most reliable way to do this is to use a MOSFET
Driver Chip capable of providing the necessary Gate Charge
Current located as close as is practicable to the MOSFET it is
driving. Where a bank of parallel connected MOSFETs are used
which are each some distance from their adjacent MOSFET, it
is best to place a Driver Chip at each individual MOSFET. The
logic signal input to the driver chips which controls their ON
and OFF timing may by applied from a single chip or pulse
source.

If a resistor is used between the output of the MOSFET Driver
Chip and the MOSFET Gate, it should not be too large. In most
cases less than 10 Ohms. This resistor is ordinarily used only
when switching the MOSFET at high frequencies in order to
relieve the Gate Driver Chip of excessive power dissipation
and excessive heating. The Gate Drive Current at high frequencies
can be surprisingly high.

I've not yet seen the schematic diagrams BroMikey has made
up of his circuitry. It would be appreciated if someone were
able to post them here for all to see and evaluate.

Commercial capacitive discharge welding circuits often use
a Silicon Controlled Rectifier (SCR) as the switching device.
The SCR is much easier to drive than a MOSFET and some are
able to switch thousands of Amperes in very short pulses.

MarkE · « **Reply #3 on:** March 13, 2014, 05:20:25 AM »

Yes, and no. There are a couple of considerations: An unintentional resonant circuit will cause current to pass back and forth between the capacitors multiple times, subjecting the MOSFETs to extra heating. You want to avoid that. This is where either turning the MOSFETs on in a controlled manner can actually reduce total heating by preventing the circuit from passing current between the capacitors over multiple oscillations. The right amount of resistance either effected through the MOSFETs or applied externally can prevent a lot of loss.

An intentionally designed resonant circuit needs to have a mechanism to cut the current off when the capacitor voltages first match. In that case you want a driver that can turn the MOSFETs off very fast.

SeaMonkey · « **Reply #4 on:** March 13, 2014, 05:44:07 AM »

You may have a point MarkE. I'm not yet clear on how
BroMikey intends to make use of his Capacitor Dump
Circuit. Judging from the size of his capacitor bank and
the layout of his switching circuit from photos he's
posted at the "other forum" it appears that it is
probably for very low to low frequency pulsing. It rather
reminds me of the 30 Volt banks which were/are part of
impulse spot welding devices made for joining small parts
with a single pulse.

TinselKoala · « **Reply #5 on:** March 13, 2014, 02:05:54 PM »

It might be nice to see a schematic of the BroMikey circuit you are discussing. That other forum won't show you anything unless you are logged in.

ramset · « **Reply #6 on:** March 13, 2014, 02:20:16 PM »

I did post an invite at Energetic!

thx
Chet

totoalas · « **Reply #7 on:** March 13, 2014, 02:31:35 PM »

bro mickey latest ckt

MarkE · « **Reply #8 on:** March 13, 2014, 04:19:50 PM »

Aside from melting MOSFETs and damaging the battery plates, what is this circuit supposed to do?

totoalas · « **Reply #9 on:** March 13, 2014, 04:33:10 PM »

quote from Bro Mickey
I failed to include other info such as 80,000 uf 250vdc caps charging up to 90vdc.

Also the circuit that controls dump down voltage needed more resistance in the bases at 16K each. Just finished testing this. What happened was this is my first time using mosfet's for a dump so large and I trusted others who told me 200 ohms.

When I decided that the diagram I had was wrong my fets were toast so I started with 20k and nothing got through then 10k and 6k worked and 16k worked the best so my other pot control works with it well.

Another aspect of the dump is............ when i first hook the batteries to the dump they charge up the cap bank backwards through the fets I think, maybe this is smoking my junctions.

Again when I hook the 36vdc charge bank up to the dump with empty caps the power some how back tracks and I think to fast and this maybe damaging things.

So I thought like car audio caps maybe I better have those caps charged up first or think up some other protection. Who knows maybe I had 1 or 2 trashed fet's by then. I am in a whirlwind of study. Gotta think back and keep trying.

These are my first conclusions and will grow in the process of failures.

totoalas · « **Reply #10 on:** March 13, 2014, 04:35:12 PM »

Another quote from Bro Mickey
I charged up my 120,000 uF bank just for fun to 65-70vdc and pulsed that through one big mosfet to a battery and that thing only worked once.

Maybe I will end up will 12 instead of 6 AAA?

But 1 or two? it's little legs just can't carry it.

My cap bank is set to deliver massive amounts of inrush to the battery.

By the way I did go down to 10,000 ohms of resistance on the base to get a full On so as to dump the entire load but that fet gave up the Ghost.

Just had to see. That pulse also burnt a tooth off of my gator clip so maybe the shorting out added resistance smoked the fet I do not know these things.

But one thing is sure the Wiley will always be my friend forever after that stunt.(http://www.energeticforum.com/images/smilies/rofl.gif) Yes big big inrush amps enough to blow the sulfation right of a huge battery plate.

I am going to pick up some 500 ah 4vdc cells in a few hours.

So I need this thing working come hell or high water.

Yeah 10k on the base gave me a "FULL ON" condition and it smoked that fet in a New York mil second.

Mike

MarkE · « **Reply #11 on:** March 13, 2014, 04:38:51 PM »

Quote from: totoalas on March 13, 2014, 04:33:10 PM

quote from Bro Mickey
I failed to include other info such as 80,000 uf 250vdc caps charging up to 90vdc.

Also the circuit that controls dump down voltage needed more resistance in the bases at 16K each. Just finished testing this. What happened was this is my first time using mosfet's for a dump so large and I trusted others who told me 200 ohms.

When I decided that the diagram I had was wrong my fets were toast so I started with 20k and nothing got through then 10k and 6k worked and 16k worked the best so my other pot control works with it well.

Another aspect of the dump is............ when i first hook the batteries to the dump they charge up the cap bank backwards through the fets I think, maybe this is smoking my junctions.

Again when I hook the 36vdc charge bank up to the dump with empty caps the power some how back tracks and I think to fast and this maybe damaging things.

So I thought like car audio caps maybe I better have those caps charged up first or think up some other protection. Who knows maybe I had 1 or 2 trashed fet's by then. I am in a whirlwind of study. Gotta think back and keep trying.

These are my first conclusions and will grow in the process of failures.

What does he hope to get from this combination MOSFET and battery eradicator? Depending on how many pulses his MOSFETs survive, he is going to drive big pits into his battery plates. He may also induce a hydrogen explosion in the batteries. Lead acid batteries make a big mess when they explode. Flying H₂SO₄ and burning metal are not the kind of stuff that you want to find yourself dodging. I hope he is keeping all of this inside an explosion proof enclosure.

Charging a big capacitor bank to two or more times the battery voltage means that he is going to through a lot of energy away in the wiring and the MOSFETs. A little work with Ohm's Law will determine a more appropriate voltage for the capacitors. I don't care what he does with his MOSFET drive, the combination of a fat capacitor bank charged to 30V or more above his batteries is going to smoke the transistors. That's about his only protection against dumping enough energy into the battery to create an explosion hazard.

There are lots of good pulse circuit based battery chargers that desulfanate out there. He really should take a step back and reconsider the wisdom of the fire that he is literally playing with.

Hoppy · « **Reply #12 on:** March 13, 2014, 05:03:37 PM »

Quote from: MarkE on March 13, 2014, 04:38:51 PM

There are lots of good pulse circuit based battery chargers that desulfanate out there. He really should take a step back and reconsider the wisdom of the fire that he is literally playing with.

Looks like he's doing a an ultra Berdini cap dump!!

wattsup · « **Reply #13 on:** March 13, 2014, 07:01:46 PM »

@totoalas

hehehehehehehehehehehehehe. Man you iz a crazy bugger.

I think you will have to invent a new mosfet called Samurai Battery Pulsing Mosfets. You have to take a course in Harakiri first to get the full impact of a one shot pulse. hehehe

Pulsing is a bitch.

wattsup

SeaMonkey · « **Reply #14 on:** March 13, 2014, 07:36:54 PM »

Quote from: RamSet

I did post an invite at Energetic!

thx
Chet

Thanks for doing that!

Totoalas,

Thanks for posting the diagrams - they're
very helpful.

In order to charge/desulfate a 36 Volt battery
bank as BroMikey hopes he'll need to add some
additional devices to his circuitry. Gate Driver
Chips will be needed as well as a precise Pulse
Generator circuit (a 555 may do the job) which
produces a driving pulse of not more than 100
microSeconds. Applying very short pulses to
the battery bank may result in a low enough
average power to avoid destroying the MOSFET
switches.

Caution is certainly necessary as a capacitor bank
charged to 90 or more volts is lethal.

News:

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Author Topic: BroMikey's Capacitor Dump Circuit (Read 52895 times)

SeaMonkey

BroMikey's Capacitor Dump Circuit

MarkE

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