Electronics > Projects, Designs, and Technical Stuff
OK to use 1N4148 100V-max diode to suppress relay coil transients?
JDW:
For those of you asking about the COIL CURRENT, it varies by relay. Some are only 5mA, other coils 8.5mA. The largest coil current is 40mA for the G7L-2A-P-CB DC48 relay because it is 48V across the coil and a 1220Ω coil resistance, with between 6H~15H of inductance, varying by armature ON/OFF state.
uer166:
In my design it was about 200mA at 48V (big coil, ~9W), at about 200mH (although note armature inductance changes depending on contact position as they open).
It had plenty of margin, your 48V at 40mA will be perfectly fine with a 1n4148, even with infinite armature inductance. Before suggesting to use random diodes, I would draw the circuit out, and fully understand the potential problems, only then come up with a solution.
Edit: you already know about inductance change ;D
uer166:
--- Quote from: JDW on April 17, 2020, 02:12:57 am ---I would expect to see more Zeners used across relay coils, which I do not.
--- End quote ---
I would expect relay manufacturers to never use zeners, because they don't know your drive circuit voltage withstand capability. When selecting a zener clamp (which btw has more variables than a recirc diode), it's a tradeoff between how fast it collapses the coil current, and how much voltage the FET/BJT/whatever need to survive.
Zeners are used routinely, especially in bigger contactors with economizers.
srb1954:
The peak unsuppressed voltage doesn't appear across the diode since the the diode goes into forward conduction when the relay driver is switched off clamping the voltage across the diode to about 0.7V.
The diode is in reverse mode when the relay is on so the total supply voltage is then applied across the diode. This supply voltage, plus some safety margin, should be what determines the diode voltage rating.
A slow diode like a 1N4007 has an issue with clamping voltage overshooting a little (maybe 5V) when the relay driver is switched off as it has a transient higher forward conduction voltage during the forward recovery time before it settles to its normal forward conduction voltage of 0.7V. This transient overvoltage puts a little more stress on the driver transistor or FET so it is an additional consideration when choosing your driver device. A faster diode like the 1N4148 diode has an advantage that it has a much quicker forward recovery time and therefore clamps the peak voltage more effectively.
JDW:
--- Quote from: srb1954 on April 17, 2020, 03:26:01 am ---A faster diode like the 1N4148 diode has an advantage that it has a much quicker forward recovery time and therefore clamps the peak voltage more effectively.
--- End quote ---
A Schottky would be faster still, yet we don't often see them used as relay coil suppression diodes. Would you say that is due primarily to reasons of cost and leakage, or something else?
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