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Zener vs Fast-Act Diodes

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Maxwell

Wed Dec 12 2012, 09:25PM

Registered Member #8497 Joined: Tue Dec 04 2012, 06:24PM
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Posts: 74

I would like to propose a new thread dedicated to the discussion of eddy-current quenching with the use of Zener Diodes as opposed to standard anti-parallel coil diodes.

As background, we all understand that eddy currents destroy CG efficiency. The faster the eddy currents within the coil are quenched, the lesser the 'suck-back' effect will be experienced.

Yandersen

Wed Dec 12 2012, 09:43PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Um, Maxwell is trying to say, that if you put a zener diode in series with dempher diode, than current will dissipate faster, resulting in a shorter pulse time making it able to shot without suckback.
Okay, if you drive your coilgun from battery switching it on and off with IGBT or MOSFET, it will work the best way possible. But if cap is discharged to the coil via thyristor, than current will start dissipating as soon as cap will finish flushing it's energy into coil.
Whatever, isn't it better to recuperate unused energy back instead of just warming the world around you? Battery-driven halfbrige can do that, non-polar cap discharged to the coil through the SCR will recuperate energy back in inverse polarity, so adding a second stage driven from the same cap will not just reuse energy, but will inverse it back for a second shot. Just think recuperation-wise instead of "bite a little and throw away". Even in ideal conditions, just a small part of magnetic field energy may possibly get converted into kinetic energy of projectile. So the major part of energy is either wasted or recuperated. I choose recuperation as I'm half-jewish. :)

Maxwell

Wed Dec 12 2012, 10:29PM

Registered Member #8497 Joined: Tue Dec 04 2012, 06:24PM
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Posts: 74

I've been reading about your exploration into non-polar caps - it's quite intriguing and a new development in the CG world, very nice job.

While I love the energy recuperation method of the non-polars, I would like to keep this thread dedicated to the traditional CG development.

(I'll likely start a new thread with facts known about the non-polar cap CG's).

We should also hard-define the term 'dempher diode'.

Yandersen

Thu Dec 13 2012, 01:15AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Here is the schematic of the simpliest IGBT driven monkey-design coilgun:
D is a dempher diode,
DZ is a zener in series with dempher diode - once the IGBT is switched off, coil' current fastly depletes by heating up the zener.

Even it does not let energy to be recuperated back, at least it minimizes the suckback effect.

Maxwell

Thu Dec 13 2012, 02:35AM

Registered Member #8497 Joined: Tue Dec 04 2012, 06:24PM
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Posts: 74

Right.

I wouldn't call it monkey-designed though, I would call it armature. I'm not going to discourage any new folk from building a simple CG. Play nice.

Has anyone taken measurements of the current/voltage inside the coil after the IGBT is shut down?

What sizing diodes, both regular (dempher) and zener should be considered?

Yandersen

Thu Dec 13 2012, 04:32AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

So far I haven't seen zeners over 5W of power. Need to try this design to see how many of them should be paralleled in order not to fry from overrated surge current. Zener dissipates hundred times more enrgy than dempher diode (voltage drop over dempher is 0.8V, while zener may be up to 200V). I think, for single stage bazooka dozen of zeners will be fine. For hundred-amps multistage I would take 3 zeners in parallel.

Turkey9

Thu Dec 13 2012, 06:42AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

I don't think this circuit works the way you think it does. First off, what are you meaning when you say dempher diode? I've never heard of it and neither has google.

Why do you want to use a zener to dissipate the energy? It seems that the idea is that because the zener has a larger voltage across it when it is reverse biased, it will burn more energy as heat? This isn't true. The zener will simply clamp the reverse voltage to whatever its breakdown voltage is. A normal diode will do the same thing, but to 0.6V. The only thing different that your circuit does is change the point where the diode will stop dissipating energy. This isn't good as you don't want the diode to stop until all the current is gone.

Think of it this way; the fastest way to dissipate the energy stored in the magnetic field after the power has been cut off is to simply short the coil. A short has a voltage drop across it of zero.

I ran a couple LTspice sims to show my point. The zener combo doesn't dissipate enough energy to stop the coil current from oscillating. In real life, this reverse current will have no where to go and will build the voltage higher and higher until something breaks.

I'm not trying to be harsh and I'm sorry if it's come out like that.
If I'm wrong in my thinking, present evidence to the contrary and I'll happily admit I was wrong. The coilgun community definitely needs innovations as it's been pretty stagnant of late.

Something that might work well to dissipate the current as fast as possible is to use a MOSFET that turns on at the right time instead of a diode. The MOSFET has a very small voltage drop across it. This trick has been used for years in switching power supplies to reduce loss across the diode.

Ash Small

Thu Dec 13 2012, 11:45AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Yandersen wrote ...

So far I haven't seen zeners over 5W of power.

You can get zeners in a 'stud package'. They are certainly a lot higher than 5W. I'll try to find some links, and add them to this post later.

(A lot of 'old' motorcycles used stud zeners mounted on heatsinks for voltage regulation, sometimes dissipating huge amounts of power. I have a few stud zeners lying around in my parts bin.)

EDIT: here is a link to lots of 50W zeners:

I'll try and find some links to even higher wattage ones later.

EDIT: And here is a link to some 100W, 27V zeners on Ebay:

I'm sure you could find the voltage ratings you require if you do a bit of 'googling'.

High power zeners DO exist!

Yandersen

Thu Dec 13 2012, 11:51AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Turkey, In school, at 6th grade, we learned a simple formula for power which is

P=U*I,

where U is a voltage drop and I is a current. That is for zener. As for the coil' current change rate, another one exists:

dI/dt=dU/L,

which means that the rate of current change is proportional to the voltage drop across the inductor. Zener increases that voltage drop so current dissipates faster. Let's say, coil has 0.5Ohm of resistance and current running through it is 100A. The voltage drop will be 50V. Some time after current will decrease to 10A. Voltage drop will be 5V resulting in 10 times slower current' drop rate. In result, current dissipates for infinite time. Dempher diode adds 1V resulting in 51V in total for 100A and 6V for 10A. With addition of one 200V zener voltage drop is 251V and 206V, respectively. In other words, current dissipates almost linearly and with much higher rate. The more zener voltage is, the faster current will deplete. Most of the energy will be dissipated on zeners, and just a little on the coil.

Sulaiman

Thu Dec 13 2012, 01:49PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Dempher = Damper

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