Which Diode?

You would do better simulating the circuit with spice. The inductance of the coil should be a big part of the current pulse shaping, with the wire resistance and the capacitor internal inductance and resistance minor players. If not, there will be a lot of energy dumped into places you do not want it to go. Of course, you will also have to get hold of the formulas that allow you to predict the inductance of various coil forms. E.G.

You can download a free spice simulator from Linear Technology:

--
John Popelish

Mike,

You can determine which diode to use by multiplying the cap size times the voltage stored, this gives you Jules. Jules = C*V = I * T so if 100 volts *

200uf = 0.02jules ( V*C ) then a diode with a rating of 30 amps surge for 60hz ( really only 8.33ms sees current ) ( I * T ) = 0.25 Jules is more than enough. You should not go beyond 25% of the surge rating for continuous applications. The surge ratings are for fuse considerations and these surges are for less than 10 times for the life of the diode The one amp diode with a 30 amp surge rating should not be used beyond 7 amps for 8.33ms or 0.058 Jules.

Ray

I'm making a very simple coilgun. This basically works by charging up a capacitor and discharging it through a coil. I understand that I should place a diode across the coil to stop any current going back into the capacitor. Which properties of a diode should I be looking at when I make my selection? In this example let's say that the capacitor has a voltage rating of 100V and a capacitance of 200uF. When I look at the available diodes the properties of each is meaningless to me.

TIA

make

voltage

Okay, that helps a bit, thanks. How do I calculate the peak forward current? Do I calculate the resistance of the coil (plus the ESR of the capacitor) and then just apply Ohm's law (using the maximum voltage of the capacitor)?

Thanks again.

Before answering the question, I would like to know how he intends to connect the diode.....

My first impression was that he was told to put a quench diode in parallel with the coil to clamp any oscillatory transients. In this case, I believe the diode need not be very heavily rated.

Most of the responses I've seen, though, appear to assume that the diode will be in series with the coil, in which case the coil current will rise slowly according to the time constant formula to a maximum of approximately Ec/Rcoil (depending on the relative values of C and L.)

Shouldn't be much more complicated than that, should it?

Neil

the

*

Errr... Better double check that energy storage formula. The energy stored in a capacitor is E=0.5*C*(V^2), where E is energy is Joules, C is capacitance in Farads, and V is the voltage on the capacitor in Volts.

So for a 200uF capacitor fully charged to 100V, it should store one Joule of energy.

I apologize for disagreeing with you, but one must nip misinformation in the bud before it spreads and gets out of hand.

than

surges

with

I'm not sure I agree with these ideas either. Limiting yourself to a peak current of 7A (for 8.33ms) on a typical 1A rated diode such as the 1N400X series devices is probably more conservative than really necessary in many cases (emphasis on the words "many cases", not "all cases"). Do you have some references that you can point us to that support your ideas and methods?