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1N4148 as a flyback diode on a small 5v relay?

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blueroomelectronics

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The 1N4148 has a 200ma version. Would this be enough for use as a flyback diode on a 5v 80ma relay? What is the general rule of thumb reguarding flyback diode ratings?
 

ericgibbs

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The 1N4148 has a 200ma version. Would this be enough for use as a flyback diode on a 5v 80ma relay? What is the general rule of thumb reguarding flyback diode ratings?
hi Bill,
The 1N4148 is rated at 1amp for 1sec , non re-current.
Unless the relay is being operated on/off very fast it should be OK
 

SPDCHK

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I've never had a problem with 1N4148, not even with 12V relay coils.
 

MikeMl

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The 1N4148 has a 200ma version. Would this be enough for use as a flyback diode on a 5v 80ma relay? What is the general rule of thumb reguarding flyback diode ratings?
The current though an inductor cannot change instantaneously. The peak current that flows through the snubber diode is equal to the current that was flowing in the relay coil at the instant that the series switch opened.

Look at this simulation:
 

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ccurtis

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Recovery time is the primary diode parameter because the purpose of the diode is to protect the series switch from excessive counter-EMF. Counter EMF develops instantaneously (with fast rise-time) when the switch is opened, but the diode prevents it from reaching damaging levels to the extent it turns on quickly enough. The 1N4148 is a fast diode and therefore a good choice for the application.

The diode inverse voltage and forward current rating must be at least as great as the relay coil voltage and current.
 

ericgibbs

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ericgibbs

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Bill,
Must admit I've no idea why.:confused

Did you see the other links on that link.?
 
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blueroomelectronics

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Well 3.3V relays are scarce compared to the common 5V ones. I would figure it'll work fine with 5V or 12V for that matter.

Where can I find the 2N equivalent of the BC109B?

The supply will be unregulated for the relays although 5V 1.5A SMS wall warts are cheap these days.
 

ericgibbs

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Mr RB

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Thanks Eric, wonder why a 5.1 Zener is used as a flyback?
Because they didn't know what they were doing.

The diode current at turnoff is the same as the current when on, as Mike said above. This current through the diode has the effect of continuing the relay magnetic field until the current finally drops to zero. The effect is a slow release of the relay.

In some circumstances this can cause arcing of the contacts as they are pulled apart too slowly. For some high-end apps they use a zener across the relay coil, but in series with a diode and the zener is inversed to what is shown above. This provided the fastest relay release possible while still allowing the flyback voltage to be "clipped" by the zener at whatever voltage is needed to ensure the Vce safety fo the switching device.

So zeners are sometimes used, just NOT as shown above. :)

Also in some old high quality (MIL spec) equipment I salvaged relays from they just used a series snubber of cap+resistor on the relay coil. Presumably this is more reliable than a diode+zener or at least the zeners of the time. You only need to clip the peak of the flyback voltage, and relay coils don't have a lot of stored energy generally.
 

ccurtis

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That zener doesn't make any sense (used the way it is). Zeners have relatively poor recovery time from reverse bias to forward bias due to their higher junction capacitance. The are also relatively expensive compared to a diode. It may improve the drop-out time of the relay, over a junction diode, but not by much. If a zener is going to be used it should be across the transistor, where it goes into zener avalanche very quickly when the transistor turns off.
 

mvs sarma

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Instead of 1N4148, better to prefer FR series diodes or BA15xseries.

I fear that Initial surge even before the current starts flowing, might damage 1N4148.
 

MikeMl

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Instead of 1N4148, better to prefer FR series diodes or BA15xseries.

I fear that Initial surge even before the current starts flowing, might damage 1N4148.
What initial surge?
 

mneary

New Member
The 2N2222 takes forever to turn off using this simple circuit. There's no worries about the 1N4148's speed.

The 2N2222 base has 25 pF (not even counting Miller capacitance), and with 2k2 drive resistor, the time constant at the base will be no faster than 50 ns. Turning it off faster would be a lot of work.

The 1N4148 needs 4 ns to fully conduct.
 
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