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Diodes in anti-parallel

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Berni

Mon Jan 26 2009, 04:56AM

Registered Member #1132 Joined: Mon Nov 19 2007, 06:15PM
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Posts: 84

100V that is a aful lot of kickback. And you will need some beefy diodes to clamp that kickback. With my first coilgun i had a 300A peak diode explode, so then i used 3 of them parallel.

Barry

Mon Jan 26 2009, 02:54PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

hotcrazyfruit wrote ...

i measured -100v only once. be, again, is there any way to reverse the damage? im thinking like memory on batteries, they lose their ability to store their charge if you keep charging them, but i don't think its the same concept. but any ideas?

You can repair or at least improve the dielectric insulation layer by reforming the capacitors. Just apply a low dc voltage and monitor the current. Whenever the current drops to near zero, increase the voltage again. This usually takes 6 - 8 hours.

A link to an example of how I did this last weekend:

Cheers, Barry

hotcrazyfruit

Tue Jan 27 2009, 04:10AM

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how to i know what the peak amount of current the diodes are rated for? if its 60 amps continuous. somewhere i read its about 10 times its continuous rating. not sure what to believe and what not to believe

hotcrazyfruit

Tue Jan 27 2009, 04:23AM

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also, will diodes in anti parralell lengthen the pulse? and thus even out the current with a lower peak? :\

Barry

Tue Jan 27 2009, 06:09AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
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Posts: 301

hotcrazyfruit wrote ...

how to i know what the peak amount of current the diodes are rated for? if its 60 amps continuous. somewhere i read its about 10 times its continuous rating. not sure what to believe and what not to believe

A reliable rule-of-thumb for estimating the peak short-duration current is a factor of ten (as you say).

For example, if a diode is rated for 60A continuous, then we can reasonably expect it to handle 600A for any pulse that lasts less than 10 msec.

It is a good thing that virtually all coilguns use pulses less than 10 msec. Our typical coilgun will have 1 - 5 msec firing time.

hotcrazyfruit wrote ...

also, will diodes in anti parralell lengthen the pulse? and thus even out the current with a lower peak? :\

These protection diodes will indeed lengthen the pulse. The peak is just as high, because the diode doesn't turn on until it the discharge reaches peak current. Then it simply takes longer for the current to ramp down (compared to having no diode).

The extra time is generally not a big concern, because you can tune the coil for a faster discharge by, say, removing turns to reduce inductance.

Cheers, Barry

hotcrazyfruit

Tue Jan 27 2009, 05:57PM

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Thanks alot :)

KLH

Tue Jan 27 2009, 06:47PM

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Myke wrote ...

I think the resistance put in series with the diode should be large enough to keep the peak current under the peak current of the diode (it's more than 60A).

No matter how much resistance is placed in series with the diode, the peak current is always going to be the same until the coil discharges. Inductors are current sources, not voltage sources. They will change their output voltage to maintain a specified current. Higher resistance will simply make the coil discharge faster, at the expense of a higher voltage pulse. (The voltage can be calculated by: Coil Current * (Resistor Value + Diode Voltage Drop).) Adding a resistor will also keep some of the power (but not all) from dissipating in an expensive diode.

Barry wrote ...

The extra time is generally not a big concern, because you can tune the coil for a faster discharge by, say, removing turns to reduce inductance.

That's possible but can only be done at the expense of a higher peak current, which could mean a much bigger SCR.

hotcrazyfruit

Tue Jan 27 2009, 09:39PM

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killah573 wrote ...

Barry wrote ...

The extra time is generally not a big concern, because you can tune the coil for a faster discharge by, say, removing turns to reduce inductance.

That's possible but can only be done at the expense of a higher peak current, which could mean a much bigger SCR.

i'm not using an SCR. i made an extremely fast acting switch. only because:
1) i don't have an air compressor for air injected projectile
2) i don't have an SCR, nor the money to buy one :)
so far, with a full discharge, there is very little welding in the switch. so its good for now.

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