Electronics > Projects, Designs, and Technical Stuff
OK to use 1N4148 100V-max diode to suppress relay coil transients?
srb1954:
Cost and leakage would be a factor but also the more limited availability of higher reverse voltage ratings for Schottky diodes.
There is really no need for the much greater speed of a Schottky diode in this application as the rise time of voltage across the relay coil is slowed by the turn-off time of the driver device and stray capacitance associated with the relay coil and a reasonably fast Si diode can cope perfectly OK.
Messtechniker:
To be on the safe side I'd do two things here:
1.) See if you can find any specs for the diode integrated with relays where such a diode is present.
2.) Exercise say 10 relays per operating voltage with your envisaged diodes over a few days and review the results. :phew:
tggzzz:
The simple requirements are that:
* a diode should not conduct when reverse biassed, i.e. the relay coil is not energised. That means a 100V PIV spec should be compared with the PSU voltage powering the relay coil
* a diode should be able to conduct the same current flowing through the relay coil. Thus if the coil current is 100mA then the diode's forward current rating should be >=100mA
* when the coil is de-energised, the coil current Icoil will continue to circulate through the diode until energy has been dissipated in the diode and coil resistance, P[diode]=Vf*Icoil, where Vf is the diode forward voltage. That can be related to the diode's pulse current rating how often the coil is de-energised
If you want to shorten the time the diode conducts, then have a series snubber resistor to dissipate some of the energy.
Depending on the application, you may need to take account of spikes on the PSU rails.
You can confirm your understanding by using a spice simulation.
trobbins:
--- Quote from: tggzzz on April 17, 2020, 10:16:47 am ---
* a diode should be able to conduct the same current flowing through the relay coil. Thus if the coil current is 100mA then the diode's forward current rating should be >=100mA
* when the coil is de-energised, the coil current Icoil will continue to circulate through the diode until energy has been dissipated in the diode and coil resistance, P[diode]=Vf*Icoil, where Vf is the diode forward voltage. That can be related to the diode's pulse current rating how often the coil is de-energised
--- End quote ---
In effect, the list point for continuous rating is not a requirement - it is the pulse rating that is a requirement. How long a duration the diode will conduct the decreasing inductor current as it dissipates its energy through its own winding resistance will depend entirely on the relay, and that may need some testing to determine if the diode's continuous current rating is exceeded. The 1N4148 has a 200mA continuous rating, but has a higher peak rating, which can be assessed as to whether the application is imposing a repetitive action or non-repetitive action. The application appears to fit easily under both the continuous and peak rating limits, so there is no need to assess whether the peak current rating is exceeded.
tggzzz:
--- Quote from: trobbins on April 17, 2020, 11:38:47 pm ---
--- Quote from: tggzzz on April 17, 2020, 10:16:47 am ---
* a diode should be able to conduct the same current flowing through the relay coil. Thus if the coil current is 100mA then the diode's forward current rating should be >=100mA
* when the coil is de-energised, the coil current Icoil will continue to circulate through the diode until energy has been dissipated in the diode and coil resistance, P[diode]=Vf*Icoil, where Vf is the diode forward voltage. That can be related to the diode's pulse current rating how often the coil is de-energised
--- End quote ---
In effect, the list point for continuous rating is not a requirement - it is the pulse rating that is a requirement. How long a duration the diode will conduct the decreasing inductor current as it dissipates its energy through its own winding resistance will depend entirely on the relay, and that may need some testing to determine if the diode's continuous current rating is exceeded. The 1N4148 has a 200mA continuous rating, but has a higher peak rating, which can be assessed as to whether the application is imposing a repetitive action or non-repetitive action. The application appears to fit easily under both the continuous and peak rating limits, so there is no need to assess whether the peak current rating is exceeded.
--- End quote ---
Whether or not the pulse rating is relevant depends on the time constants and energy.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version