Inductor question?

[smanches]

I have yet to try this, but am curious...

Say you have a 12v 10A inductive load. If you were to use a 60V zener as the protection diode, would that reduce the back EMF current to ~2A? I've seen this used once, and didn't quite understand it at first. But I think this is the reason it was done.

 

[Sceadwian]

Depends on the inductance smanches, and the circuit impedance when the zener is conducting, The Zener diode doesn't limit the current it limits the voltage, obviously they didn't want it going above 60 volts, probably to avoid avalanching some transistor or other diodes.

 

[crutschow]

I have yet to try this, but am curious...

Say you have a 12v 10A inductive load. If you were to use a 60V zener as the protection diode, would that reduce the back EMF current to ~2A? I've seen this used once, and didn't quite understand it at first. But I think this is the reason it was done.

The current at the moment the switch opens is 10A @ 60V (or whatever the zener voltage is at 10A), neglecting stray impedances. As has been stated numerous times, you can not instantly change the inductor current.

 

[smanches]

I understand that, but the back EMF is an amount of energy, isn't it? If the current cannot flow, then voltage must build up until the current can flow. Since voltage has ramped up to something higher than it was, doesn't the current have to drop?

 

[Sceadwian]

No, the current rises with the voltage, but the time it can be sustained goes down. The voltage goes up until conductance occurs, the energy has to go somewhere. The total power dissipated is always the same though.

 

[crutschow]

I understand that, but the back EMF is an amount of energy, isn't it? If the current cannot flow, then voltage must build up until the current can flow. Since voltage has ramped up to something higher than it was, doesn't the current have to drop?

No. The back EMF is voltage, not energy (volts times current equals power, volts times current times time equals energy).

The stored inductor energy is determined by the inductor current and inductance (equal to ½LI² in joules). This energy will generate whatever voltage is necessary to maintain the instantaneous inductor current into whatever impedance the inductor is connected too. As the energy is dissipated, the current will drop.