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Flyback diode ?

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tron87

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Im using a pic out pin to control a 24v relay which runs a motor in a project how do I choose what fly back diode to use to protect the pic from back emf ?
 
1N4004 or similar
 
A PIC has an output of only 5V maximum at 25mA maximum. It will not drive a relay coil that needs 24V and you didn't say how much current the relay coil uses.
 
Sorry the relay drives a 24v load the coil side of the relay is 5v.A little confused about actually choosing a flyback diode do i pick one with the highest reverse voltage breakdown rating i mean how is the back emf voltage calculated is it likely to run into 100 of volts ?
 
Hello again,

I thought i gave you a part number already. The max voltage is 24v so the diode has to be rated for higher than that. A 50v diode would work too.
The reverse voltage is simply the applied voltage when the relay is energized.
 
The diode reverse breakdown voltage should be more than 2X the supply voltage. The current rating should be more than about 1/5 of the steady inductive load current, i.e. a 1A diode is good enough for a 5A inductor...
 
Hello there Mike,

How did you arrive at those numbers? The applied voltage across the diode is 24v and when the driver turns off the diode conducts at around 0.7v or so. Also, the current through an inductor does not decrease but tends to stay constant over short periods of time so when the circuit opens the inductance pumps the same current through the diode as it had just before the circuit opened.
The above tells me that the diode has to at least be rated for the supply voltage (plus a little margin of safety of course) and the diode should be rated for the coils active current (ie a 100ma coil needs a 100ma or better diode).
 
The reverse voltage breakdown being 2X the supply voltage is a safety factor.

The forward current rating of a rectifier type diode , say a 1N4001 is continuous 1A Average, but it is rated for 30A for one 60Hz Half-Cycle (8.33mS), or is rated for a Peak Forward Surge Current of 14A lasting 20 Half-cycles (166ms), which would be an average current of ~5A.

When you connect a snubber (catch) diode around a relay or solenoid, how long does it take for the current to decay? Most inductive loads I have measured have a decay time-constant of less than 100ms. Even though the 1N4001 data sheet doesn't have a graph specifically for Snubber rating, the graph below suggests that a 1A diode will suppress an inductive load carrying 5A just before being turned off. I have done it routinely, and not had any problems...
 

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Hello again,


Oh you mean you tested these diodes at 5 times rated current for 100 cycles? That's interesting.

I guess i feel comfortable with the 2x voltage rating but i just dont feel comfortable with the 5x current rating. 100 cycles is more than one second, but that's for half cycles not full cycles. The transient thermal response suggests a little lower than that, like 3 amps or something like that. But even that would be stretching it a little as we dont even know how fast the relay might be turned back on again. It takes much longer to cool off than to heat up. I'd be a little worried using a diode not rated for the full current. Buck regulators require at least rated current or higher in most cases for example.
I only suggested the 1N4004 because most relay coils only draw 500ma or less, but that would have to be verified too i guess.
 
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... Buck regulators require at least rated current or higher in most cases for example....

Buck Regulators are turned ON/OFF 50,000 to 1,000,000 times per second. Relays and Solenoids are turned ON/OFF like ONCE per second.
 
Hello,

Yes, that was just an example. Once per second still doesnt allow the diode to cool down after a heavy heating period. That's what i was talking about.
 
I agree with Mike. At the typical operating frequency of solenoids or relays, a diode rated for less than the coil current should be fine. Certainly it doesn't hurt to use a diode that's rated for the coil current but that's not necessary.
 
250,000 plus Cessna Aircraft use 1A rectifier diodes to suppress the master battery contactor (Continuous duty coil, about 1A coil current), a similar contactor used when the aircraft is connected to an external starting cart, and the starter relay (Intermittent duty coil, about 4A during cranking). I am not aware of any failures of of these diodes...
 
Hello again,

The question isnt whether or not it will work in one application or even several applications. The question is will it work in an application in which the frequency is not known. It's not really possible to answer that question. If the frequency is very low it may work due to the ample diode cooling time period. If the frequency is higher the diode could very well burn up.
 
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