OT Clamping diode on inductor - Page 2
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  1. #21
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    To the OP.
    The answer is in post # 17.
    The 1N5408 is a 1000 Volt unit and will hold up when the clutch shuts.

    There is a forward voltage spike that is generated when the magnetic loop is closed at power on.
    It can be high enough to take out a 400 Volt diode used to suppress the stored energy during denergize.

    I worked on a project years ago where thousands of 1n4004 were changed to 1000 Volt versions. The measured forward voltage spike was 600 - 700 Volts above the voltage on the relay coil when the armature made contact and closed the magnetic loop. This is a very narrow fairly low energy pulse that is very hard to catch on a scope reliably, but the voltage is high enough to cause problems exceeding the reverse breakdown voltage rating of lower voltage units.

    Bill

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  3. #22
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    The diode takes up the current in the coil so that the switch only has to break the current up to the forward voltage of the diode. If your clutch draws 3 amps, the diode will pass 3 amps until the magnetic field is depleted. The duration it has to pass that current for depends on the magnetic energy stored in the coil, forward voltage of the diode and resistance of the coil, so whether you can rely on the surge rating of the diode depends.The reverse voltage of the diode only has to be as high (plus a safety margin) as the driving voltage.

    The coil will build voltage until the diode begins conducting. Chances are rectifier diodes intended for 60hz operation will be fine, but a fast acting diode may suppress spikes better. The change in arcing will be minimal but the electrical interference reduction may be substantial, so consider what else is nearby.

    Since the diode allows current to circulate for much longer it will take longer for the clutch/relay/solenoid or whatever to release. This is particularly bad for high inductance, low resistance loads with a low forward voltage diode. In your case, it probably won't matter. But in cases where it does you can add a resistor in series with the diode (an additional capacitor can smooth out interference, if that's an issue) such that the voltage spike is equal to supply voltage but in the opposite direction.

    Example: Say you have a solenoid valve that draws 3 amps at 24V. You select a 3A 50V fast acting diode (fast acting hardly adds much cost, if any) but notice it causes a 100ms delay in the off time of the valve. If that doesn't matter, then you are done. If it does, then you calculate the resistance of the coil to be 8 ohms. You add 8 ohms in series with the diode, which drops the delay down to 50ms, but raises the voltage at the switch to -24V during the spike. You can go further with this but it is a tradeoff between arcing and switching speed.

    Zener diodes in series with a regular diode is also a fair choice for the fastest switching you can manage, usually used when the coil is driven by a transistor. It is a more expensive option though. (Say you have a 24V coil driven by a 60V transistor, you could put in a 50V zener diode and have a faster turn off time than turn on time)

    A final note: consider the operating temperature of the diode. A diode already warmed to 75C by the coil will have a lower amperage rating, and much lower surge rating than one at room temperature. Frequent on/off cycles will also reduce the diodes surge rating. Most people never see any issues with these diodes so don't overspec them much. The reason for that is because everything still works fine after the diode dies but for some weird reason you have to keep replacing a relay or contactor more often than you would expect.

  4. #23
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    Quote Originally Posted by SAF View Post
    Just went through this problem on a buddy's older model 25HP Scag zero turn mower. The electronic ignition timing module, safety interlock module and clutch switch were having longevity issues.
    This is exactly what I went through with the 25hp Kohler Command on my Bobcat ZTR... it ate clutch switches and regulator/rectifier modules.

    I installed a 1N4007 across the clutch, and solved those problems. A 1N5408 would probably have been a better choice, but I had a pile of '007's in my ham-shack.


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