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.
As Roff noted, a series resistor will slow the relay turn on. If you want to limit the operating current but still get fast turn-on, you can put a capacitor in parallel with the resistor between the relay coil and the transistor collector. Pick a capacitor large enough to supply full current through the relay coil during its operate time after the transistor turns on. Then as the capacitor charges, the current will slowly reduce to whatever the resistor and relay coil resistance allows.
When the transistor turns off, the capacitor will discharge through the resistor, ready for the next relay turn-on.
Wrong. Sorry, you are not visualizing what I explained (schematic would have been good). The resistor is in series with the coil. The diode is in parallel with the coil-resistor-in-series. See? The circulation current must go through the resistor on collapse and also the drive current goes through the resistor.It's a hard sell because a series resistor has no effect on the collapsing field. It would have to be in parallel with the coil for that. The transistor still has to absorb the inductive voltage spike (although the spike has less energy since the coil current is less than rated). That's why a diode or resistor-capacitor is typically placed across a diode coil to absorb the inductive energy.
Remember, I am not in favor of a capacitor and point out it doesn't help. I further point out you could be chasing your tail with more components like more diodes.You can't place a diode in series with the cap because once it charges up, the diode will prevent its discharge for the next cycle.
See above. Anode on collector. Cathode on 5V. Collector also to coil through resistor but resistor could be on either coil lead. In fact, it might be better to use a SIP resistor with a common pin to 5V and individual resistor pins to each coil+, from a components count standpoint if you have multiple relays.How do you put a diode on the collector to absorb the collapsing energy? You could use a zener to ground, but a 1N4148 will only help if it's placed across the relay coil. To see this, just trace where the current flow when the transistor turns off (the inductive current will keep trying to flow in the same direction).
It is a fairly universal characteristic of relays that they don't need as much power to stay closed as to close them in the first place.A relay is designed for a certain current and voltage so that it will operate reliably under all conditions. If it would reliably operate with a lower current, then the manufacturer would have designed it that way. It's not to a manufacturer's advantage to overdesign a relay, since that increases it's cost. If you limit the startup current with a resistor you reduce the margin and it may not close reliably all the time, say for example, at a high temperature where the coil resistance is higher. That's why it's good practice to hit the relay with full voltage and current to turn it on. After that you can reduce the current to the holding value if you want to save power.
Why did you quote me and then say "people have this backwards"? Are you saying I was wrong? Seems that your post agrees with me as did Ericgibbs nice simulation of the zener circuit I suggested (on page1).
I was well aware of the much faster release times using a zener+diode, and I thought that was very clear in my post. My comment was re the circuit blueroom asked about, which showed JUST ONE diode, the zener, and it was BACKWARDS.
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 analysis is really simple: If there is 80 ma flowing in the relay, the flyback current cannot be higher than that.
How can the diode current be greater than the relay coil current? All the diode does is continue to conduct the coil current when the switch shuts off. The coil inductance can generate a voltage spike but it can't generate a current spike.Sure it can. It only flows for 10s of msec each time the coil is switched off.
Sure it can. It only flows for 10s of msec each time the coil is switched off.
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