Led indication for a coil

[Murugesh_89]

Hi,
I want to show led indication regarding the solenoid status.

Is it good way to do it as per the attached image..


My doubt is even though if i am having free wheeling diode, will the back emf destroys the LED by providing high reverse voltage?

Thanks,
Murugesh

 

[KlausST]

Hi,

with free wheeling diode you don´t have reverse voltage (only the diode forward voltage: < 1V)

Klaus

 

[Audioguru]

Your freewheeling diode is a 50Hz, 60Hz rectifier diode that switches very slowly, so the circuit will probably produce a high voltage spike before it conducts. Then the LED might be destroyed. Simply use a much faster diode.

 

[Tunelabguy]

Your freewheeling diode is a 50Hz, 60Hz rectifier diode that switches very slowly, so the circuit will probably produce a high voltage spike before it conducts. Then the LED might be destroyed. Simply use a much faster diode.

...or put a small capacitor in parallel with the diode. But if your diode is that slow, it probably isn't protecting the FET switch either. It also matters how fast your FET is turning off. The FET can't turn off instantaneously either. So the back EMF cannot rise any faster than that. If you are driving the gate of the FET with an extremely fast and low-impedance source, then you might have a problem with too-fast turnoff. If you don't need that ultra-fast turnoff, then just add a resistor (maybe 560 ohms) in series with the gate of the FET. This resistor, in combination with the very large gate capacitance will form a low-pass filter and limit the turnoff speed. So you have many solutions. And you may not even have a problem.

 

[KlausST]

Hi,

A capacitor causes high switch ON current peaks. This increases EMI.

There are faster diodes: UF4007 for example.

I really don´t know if the diodes is slow at turning ON. (This would cause high voltage spike).
But i know it is slow with recovery..(this causes high current spike [EMI] when diode is still conductive while FET is turning ON. This is typical for high frequency PWM)

Klaus

 

[Tunelabguy]

Hi,

A capacitor causes high switch ON current peaks. This increases EMI.

There are faster diodes: UF4007 for example.

I really don´t know if the diodes is slow at turning ON. (This would cause high voltage spike).
But i know it is slow with recovery..(this causes high current spike [EMI] when diode is still conductive while FET is turning ON. This is typical for high frequency PWM)

Klaus

A capacitor around the LED would not be "switched" on through the LED, so the LED would not experience any current spike. And any other current through the capacitor will would be current-limited by the 4.7K resistor. So I see no downside to having a capacitor in parallel with the LED.

 

[KlausST]

Hi,

...or put a small capacitor in parallel with the diode. But if your diode is that slow, it probably isn't protecting the FET

... i thought of the free wheeling diode, not the LED.

Klaus