Flywheel Diodes

Does anyone know roughly how to estimate the voltage and current capacities of a fly wheel diode being used across an inductive load? Like the size of the voltage spike involved during a worst case switching as well as the fly wheel current (if I use a rough V = L*di/dt calculation, using a conjured up maximum current and switching time to guesstimate di/dt I get rough voltages, but I have no idea how accurate they are and I don't know how the current is calculated either).

Even very rough ratios just to get an idea would be nice. I just have no idea how high the voltage spike could be other than it could be higher than the supply voltage, and for the flywheel currents...I have no idea how big those are at all. For example, a 6V motor with a current draw of 6A might need a flyback diode rated X times the supply voltage and Y times the current draw. Or something like that.

Even something as rough as the flywheel current is less/more than the load current draw would help.

I believe it is only the peak ratings that matter in this case, rather than the continous ratings, right? Since freewheel diodes are dealing with spike-like events.

If the diode is in place, there is no flyback voltage above the forward drop of the diode. The initial diode current is equal to the current that was flowing in the inductor before the current was interrupted, e.g., if 1 Amp was flowing in the inductor, 1 Amp will initially flow through the diode before the current decays. This fact causes diode-snubbed relays to hold on for milliseconds after they are switched off.
Without a diode, the peak flyback voltage is mostly a function of the inductance and parallel capacitance (generally interwinding capacitance), unless breakdown occurs. Keep in mind that v=L*di/dt, where di/dt is the rate of change of the current. If the current goes to zero in essentially zero time, v can be huge. Shunt capacitance will absorb the current which wants to keep flowing. The larger the capacitance, the lower the peak voltage. Of course, if the inductor has fairly high Q (low effective series resistance), prolonged decaying ringing will occur at the resonant frequency of the LC circuit.

I see. So the breakdown voltage rating of the diode doesn't really matter as long as it can withstand the supply voltage? Other than that, just make sure the current rating is equal to the maximum current draw of the load? Makes sense. Seems simple enough, I thought there was more to it.

EDIT: Oops...I just noticed I had C instead of L in my equation. I kept wondering why Ron was repeating something I was saying,

dknguyen said:

I see. So the breakdown voltage rating of the diode doesn't really matter as long as it can withstand the supply voltage? Other than that, just make sure the current rating is equal to the maximum current draw of the load? Makes sense. Seems simple enough, I thought there was more to it.

Click to expand...

Yep, you got it.

Ron H said:

This fact causes diode-snubbed relays to hold on for milliseconds after they are switched off.

Click to expand...

Which can be problematic if it's being used to switch currents near the maximum rating of the relay. The contacts open at a slower speed so they're more prone to arcing which can shorten the life of the contact. To get round this problem I normally recommend adding a resistor of the same value as the relay coil (or higher if the switching circuit can take the voltage) to speed up the decay and reduce arcing.

Hero999 said:

Which can be problematic if it's being used to switch currents near the maximum rating of the relay. The contacts open at a slower speed so they're more prone to arcing which can shorten the life of the contact. To get round this problem I normally recommend adding a resistor of the same value as the relay coil (or higher if the switching circuit can take the voltage) to speed up the decay and reduce arcing.

Click to expand...

Good point.
Clarification: In series with the diode.

contact breakers

how a transistor, a flywheel diode and an optical switch were used to eliminate the contact breakers and capacitor? please help me!