the most reliable low-end microcontrollers

[Zero999]

A suggestion for the high side you can do with the catch diode is to place a zener in reverse in series with it, this allows the zener to do the dissipation without exceeding the transistor ratings. Zener needs a rating equal to coil energy, the mov is cheaper than a 5W zener, or you can use a TVS diode at a cost in between.

Why not just put a plain old resistor in series with the diode?

It's much cheaper than a MOV/zener and the peak voltage can be calculated using Ohm's law. I generally recommend making the resistor's value equal to the coil so the pulse is no more than the power supply voltage plus a diode drop and the turn off time is halved.

[Simon]

I still don't understand why just the diode is not the best option. No extra components to get hot or breakdown and the bulk of Dissipation is done in the coil that would normally dissipate that energy anyway. I know there may be some exceptions, but for general use I can't see an issue with just a diode.

[Rufus]

I still don't understand why just the diode is not the best option. No extra components to get hot or breakdown and the bulk of Dissipation is done in the coil that would normally dissipate that energy anyway. I know there may be some exceptions, but for general use I can't see an issue with just a diode.

If a clutch disengages slowly there is a period where the clutch is slipping and wearing out the friction material. That may or may not be significant depending on the load, its momentum, and how slowly. I can't imagine how disengaging quickly would produce more wear.

[Simon]

well from memory a back EMF spike is pretty fast. So wouldn't that energy dissipate quite quickly and suddenly stop causing a quick disengagement of the clutch? Yes the clutch may stay engaged for a bit longer than intended, but we are talking milli seconds which is irelevant to something that is turned on or off for minutes at a time.

[Rufus]

well from memory a back EMF spike is pretty fast. So wouldn't that energy dissipate quite quickly and suddenly stop causing a quick disengagement of the clutch?

With a diode there is no spike. The current in the coil continues to flow and decays like any other LR circuit except with the complication of a 0.7v diode forward voltage in series and the coil inductance changing as the armature moves.

If these clutches are a significant part of your product you ought to know how they behave and wear.

[Simon]

If these clutches are a significant part of your product you ought to know how they behave and wear.

well presumably they will be teaching me that of course there are other things like fans. and I'm not sure how we ended up talking about clutches it seems to have started because I gave the example that I had already had to help them on back EMF. As much as they forgot about back EMF on clutches they also didn't do much on fans either.

[SeanB]

And don't forget that with fans they generate voltage as well when they are not powered, but have air blowing through them. As well you will have to remember the clutch and fan store considerable amounts of energy in the inductor, as well as having high currents. Best is to use relays to drive them, as you will have to do high side switching in almost all cases. The relays can be driven by mosfets, and you probably will want to use logic level mosfets that are rated for 60v drain, with a 47V zener across the mosfet, no other catch diode needed. Faster turn off, and the mosfet is protected. Faster turn off on a relay switching DC current means faster moving contact motion, and less arcing and wear on the contact. Funny thing about a relay is that mechanically it has a life of generally over a million cycles ( and I have seen some do tens of millions of cycles before they wear to the point of failing) while at full load it has a life of only thousands of cycles.

[Simon]

yes I warn them about fan motors as well. The fans we use take 24 V and 11 A. So when they are turned off they generate quite some power from the fan that is still spinning with momentum.

[Zero999]

All right I'll start from the beginning.

Why it's important to engage/disengage a clutch as quickly as possible.

Clutches dissipate more energy when they engage.disengage slowly, which is when slipping occurs and higher dissipation means more heat is dissipated.

The more quickly the current through an inductor is interrupted, the higher the voltage across its terminals and vice versa. If you took a perfect superconducting inductor, placed an idea diode in reverse parallel with it, connected it to a constant current source and allowed the current to reach the steady state, then disconnected the inductor from the current source, the current would flow indefinitely. This is because there are no losses. If the diode were replaced by a perfect open circuit, the current would stop instantaneously and the peak voltage would be infinite. In reality, the situation is always in-between the two extremes. It's not a perfect inductor and there's never a true open circuit, an arc will occur at some voltage.

To deactivate a coil as quickly as possible, you need to trade off maximum peak voltage for off time, the lower the peak voltage, the longer the off time will be.

[jerry507]

What is the lifespan of a MOV used in that fashion? My understanding of a MOV is that absorbing spikes reduces it's lifespan.

[poorchava]

This varies greatly with type, technology and application. MOVs are generally meant for surges which are a fault condition, not which are expected. This is to say that if you expect regular surges/transients/pulses you should rather use Transil diode. MOVs can fail as sither short-circuit, open-circuit or constant resistance. On the top of that they they are not recommended by NFPA because they are recognized as a potential source of fire.

[jerry507]

Yea, that is what I have always done as well. The question I have then is what is the best way to quickly dissipate the energy stored in an electromagnetic field then? This can apply to either a electromagnet, motor or whatever. It seems like some form of diode with a beefy resistor actually dissipating the energy is the best solution then?

[Simon]

by the sounds of it that is not a bad idea. But the resistor needs to be a small enough value so that there is not a large voltage drop across it that will do damage. I assume on the other hand it must not be too low otherwise there will be a considerable voltage drop across the coil instead.

[SeanB]

I have seen lift motor brakes that used a varistor to absorb the switch off spikes. They worked well enough, and the Tech was kind enough to go and get 130V varistors from stores after i pointed out that a 400V varistor did pretty much nothing on a brake coil powered from 100VDC, and why the 130V units were not exactly good when used across a 230V rail.

That was an expensive controller upgrade, the quote was controller only and installation, but the inspector insisted on a new trailing cable ( old one was gutta percha dating back to 1939 and was in good order, provided you did not mind an insulation resistance that precluded more then 48VAC across it before it leaked a little) and, 30m of 40 core flat cable later, it was replaced. 2 weeks later the cable was replaced again, along with the controller, as it turns out the 24VAC outputs did not work well with 230VAC applied to them as a result of the car cutting the cable after it settled in a lay that met with a sharp edge on the car frame. The inspector also insisted on repainting the room, new door locks, new power isolator ( I have the old 1939 made unit in my collection, complete with built in meter) and some other shaft and car upgrades  along with new door switches, that brought the install price up to 5 times the quote, we only paid the quote price. Rest was because the lift had 30 years with them.

[mc]

Why it's important to engage/disengage a clutch as quickly as possible.
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To deactivate a coil as quickly as possible, you need to trade off maximum peak voltage for off time, the lower the peak voltage, the longer the off time will be.

In theory, that's all true, however the type of clutches that Simon is on about won't be killed by slow switching of, unless you somehow manage to make them take several seconds to fully drop out. Provided the power to them is cut smoothly, they'll survive, as they're designed to be controlled by a simple switch/relay. Only reason for back emf protection, is to protect what's controlling them.