Zener diodes getting hot

Thread Starter

Karl2022

Joined Jan 8, 2024
2
Good Day
Im running zener diodes in a circuit to protect cooling fans from EMF on an inductive load. The pumps 24Vdc come from the supplier with a cooling fan wired in parallel to the supply power to the pump. Periodically during the life cycle of this wiring, the fans tend to fail due to back feed when shutting the motor/pump down. By adding the Zener diode in across the power supply only to the fans then these fans tend not to fail, as it offers another route for the feedback current to bypass the fan. The fans are 24Vdc 4.8w fans.
However when I use a 1.3w zener diode the system is stable, the fans don't fail and the diodes don't get hot, however, when I use 5w zener diodes, the diode is so hot I am not able to touch it. Please could someone explain this to me or suggest an alternative.
Thank you
 

Ian0

Joined Aug 7, 2020
9,792
Good Day
Im running zener diodes in a circuit to protect cooling fans from EMF on an inductive load. The pumps 24Vdc come from the supplier with a cooling fan wired in parallel to the supply power to the pump. Periodically during the life cycle of this wiring, the fans tend to fail due to back feed when shutting the motor/pump down. By adding the Zener diode in across the power supply only to the fans then these fans tend not to fail, as it offers another route for the feedback current to bypass the fan. The fans are 24Vdc 4.8w fans.
However when I use a 1.3w zener diode the system is stable, the fans don't fail and the diodes don't get hot, however, when I use 5w zener diodes, the diode is so hot I am not able to touch it. Please could someone explain this to me or suggest an alternative.
Thank you
What voltage zeners are you using?
 

MrChips

Joined Oct 2, 2009
30,790
Zener diodes, like all fixed voltage loads need a current limiting resistor to protect it from too much current.

For example, if the device clamps the voltage at 24.0V and the supply voltage is 24.1V, then the current through the device as calculated using Ohm's Law is,

I = V / R
I = (24.1V - 24.0V) / 0Ω = ∞A

If you want to limit the power dissipation to 5W @ 24V,
I = 5W / 24V = 0.2A

If you anticipate that the supply voltage could be as high as 25V, then the series resistor is
R = V / I = (25V - 24V) / 0.2A = 1V / 0.2A = 5Ω
Wattage = 1V x 0.2A = 0.2W
Use 5Ω 1/2W resistor.
 

WBahn

Joined Mar 31, 2012
30,034
Could you show a schematic of how things are connected? If you are just snubbing the inductive kick current, you can probably do that using a regular diode. What current does the pump draw?
 

boostbuck

Joined Oct 5, 2017
509
If the zeners are conducting and getting hot in normal operation there is something wired incorrectly. Shouldn't the zener diodes only conduct when 'back feed' occurs?

As a guess I should think the 5W zeners are too low voltage, compared to the smaller ones.
 

Ian0

Joined Aug 7, 2020
9,792
Is this not a job for TVS diodes? Ones specified for protecting 24V supplies will not start to conduct below 28V, so there will be no standing dissipation, and a TVS diode will outperform an ordinary zener of dissipating transients.
24V fans will withstand 28V quite easily.
 
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