Andy1845c

Will a rectifier diode work as a protection diode in a transistor/relay circuit?

I don't have a very good selection of diodes on hand. I have 1N5402 rectifier diodes, BYV26C Fast soft-recovery controlled avalanche rectifiers, and some Zener Voltage Regulator Diodes. Would any of these work?

I don't yet fully understand how the diode works in this application dispite doing some research on protection diodes.

The relay is being switched with around 12 volts. What criteria must a diode meet for this application?

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erosennin

Your 1N's will do the job fine, assuming you're not going to be switching at some crazy rate, not that the relay would be able to keep up anyway.

When the relay current is stopped by the transistor, in the small time the current is dropping to zero, the coil in the relay being an inductor sends the voltage through the roof (in the opposite direction) in an attempt to keep the current going.
So instead of sending this back through your transistor and most likely frying it, it gets diverted through the diode and your transistor is happy.

More experienced people - feel free to correct/add anything.

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What is a joule per second?

Sceadwian

The protection diode is pretty simple to understand. 99+% of the time the diode will be reverse biased and basically out of the circuit, when power is cut, like when an inductor is switched (relay coil) it will actually produce voltage to maintain the current going through it as the magnetic field sustained by the current collapses. If the relay is switched off hard the voltage it produces can be very high. Dozens to hundreds of volts, and beyond. You need fast recovery diodes to meet this requirement, as a generic soft-recovery diode will allow virtually the entire high voltage pulse to pass before it actually electrically switches back to a passing state. (The diode is supposed to reroute the voltage/current from the coil back onto itself so the magnetic field is dissipated from the coils own electrical resistance) Schotky's can be good for this, but it really depends on how fast the coils current is actually cut. If you dissipate some of the power in the transistor powering the coil itself by slowly switch the transistor into an off state and flyback diode might not even be required. I've seen capacitors used to 'snub' relays in slow switching instances, no diode required.

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mneary

The diode must have a reverse voltage rating of more than 12V, and a forward repetitive pulse current rating that's more than your relay coil current.

Either the 1N5402 or BYV26C will work. Zener voltage regulator diodes are not generally suitable. In your case, if the coil current is less than about 200mA (coil resistance more than 60 ohms), you could use the popular 1N4148 (1N914). For coil currents up to 1A, the 1N400x (1N4004 etc) is popular.

Diver300

I don't think that you need fast recovery diodes for a freewheel diode.

Fast recovery diodes will switch off quickly. That has little to do with switching on quickly, which is what is needed for a freewheel diode.

Without a freewheel diode, you can get hundreds of volts, which will damage things. A perfect freewheel diode would remove the surge completely, but perfect diodes don't exist. There is always the forward voltage drop, and how fast the diodes start to conduct, but the main consideration here is how much surge can be tolerated.

All the diode has to do is keep the surge down to what the transistor can survive, which is certainly 20 - 30 Volts, so the diode doesn't need to be anywhere near perfect.

The combination of the time the transistor takes to turn off, and stray capacitances means that the current rise isn't instantaneous, and just about any diode will turn on fast enough to limit the voltage rise to a few volts.

ericgibbs

hi Andy,
The post by 'diver' sums up the requirements for the protection diode.

There is one other point, when using a diode across a relay/solenoid coil.
While the diode is conducting due to the back emf from coil the relay/solenoid will not release.
The diode in fact slows down the release time for a few millisec, for most applications this is not a problem.

If its essential that you have to minimise the 'release/hold on time' you can use a zener diode or a diode/zener series combination.

Choose the zener breakdown voltage a few volts less than the transistors breakdown voltage.

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Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

picbits

I've used zener diodes before and they are much better than normal rectifier diodes for fast relay switching.

eblc1388

Be very careful about cause and effect.

With the back emf protection diode fitted, the relay coil current will not be cut off suddenly and is allow to flow into the diode, until the stored coil energy is dissipated via the resistance of the coil. Therefore the magnetic field decay slowly and NO High back emf voltage will be generated in this case.

As such, a diode that has the correct voltage rating of the power supply and sufficient current capacity to allow the normal coil current to flow will be good.

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L.Chung

Roff

Just to clarify, with this method the zener needs to be connected across the transistor rather than across the coil (cathode to collector in the case of an NPN). An alternative is to put the zener across the inductor, but then the zener voltage needs to be reduced by the value of the supply voltage, and a diode needs to be in series with the zener.

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Ron

ericgibbs

hi Roff,
This is the method I used to use on the old 'paper tape punch' solenoids, else the pins used to foul the paper.

EDIT:
A diagram explains it better.
The value of the VZ is chosen knowing the Vce.Vcb breakdown of the transistor.
On a 24Vdc rail, with a transistor rated at say, 50Vbce I would choose a 12V/16V zener.

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Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

Andy1845c

Hey guys, thanks for all the replys. I have a much better understanding of what I am doing now.

The relay is controling a motor on a dust collection system, so there is no exact timing or rapid switching.

What Sceadwian said about using a capacitor to snub the relay has perked my curiosity. Would the cap be on the base of the transistor to slow its switching down? Or would it be across the coil of the relay, before the diode in the circuit? I guess google might yeild a circuit like this for me if I look

I don't think I will use this method, but i'm curious

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Diver300

Your original post made me curious, so I put a scope on a small relay that has a 1N4001 freewheel diode.

The relay takes 80 mA at 5V

I found that the coil voltage rose at 6v / microsecond when the transistor turned off. I think that this means that the capacitance is about 13 nF, which I guess is from the interwinding capacitance of the relay

The forward voltage on the diode peaked at about 1.8V for 500 ns before settling down to the forward drop of about 0.7V which it continued for about 10ms until the coil current had stopped.

I think that the peak voltage is because the diode takes time to turn on, but the diode is certainly keeping the coil voltage far below what would damage the transistor.

As I understand the capacitor method of keeping the voltage withing limits, a capacitor is in parallel with the coil. There is no diode with this method. When the current is turned off , the energy from the coil ends up in the capacitor, so the bigger the capacitor, the smaller the voltage surge. The circuit will oscillate, but decay quickly and the peak voltage is well defined.

A resistor in series with the capacitor will only increase the voltage a bit, and it will stop there being a very large current pulse as the capacitor is charged when the current it turned on again.

Roff

I would be surprised if a 1N4148 has that sort of forward recovery. How long is the ground lead on your scope probe?

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Ron