best way to drive an AC inductive load

Hi all,

Sorry if this question may seem basic to you guys or if it was already discussed. I searched for quite a while and could not find a clear answer.

In very simple terms, I would like to build a DC controlled power outlet so that I can control home appliances from a microcontroller, or from some other digital circuit.
The problem is that at least one of the appliances I need to control is inductive (a pet water fountain) as it contains a small AC motor. In addition, in the future, I would like to use the same circuit or an expanded version to control devices such as my washing machine or drier.

If the load was purely resistive, I would have happily built my circuit already. The problem is that inductive loads can and do generate overcurrents/voltages when the relay opens and closes the circuit.
However my knowledge stops here...
How big should my relay be to manage these transitory currents?
Some application notes suggest that a triac or an SCR would be better in this case as it could switch during zero crossing, but then they list other issues such as heating problems and a non-infinite open-circuit resistance.

My issues is that I am not sure whether these would be problems for my circuit.

I would also be equally happy if I could find a pre-assembled product that I could drive from a microcontroller. However, I haven't been able to find anything specifying what to do in the case of inductive loads.

Some premade devices list maximum current values for resistive loads, but how about my little motor? Will it be large enough to destroy the circuit?

Finally, are there any online resources I can access to learn more about this? As a computer engineer I have very little knowledge of power electronics, but I'd be happy to learn more!

Thanks a lot

Davide

Probably the most robust way is to use a relay to close the AC circuit. It won't care if the load is inductive, capacitive, or resistive. You can get pretty small 15A with 5V or 12V coil drive. It provides the mains isolation as well. You'll want to make sure to include a reversed diode across the coil to catch the inductive kick from the coil when the driver opens up.

For instance like this one from Digikey G5RL-1A-E-HR DC5. Of course there are many other styles available.

Ray

Thanks Ray!
I am familiar with putting a reversed diode on the DC side to protect the driving circuit. Do you suggest I put one on the AC side too? What does it mean for it to be reversed in this case? No compensation capacitors or inductances on the AC side?

And how about a failing relay? Does it heat up? Catch fire? or simply stay open/closed?

thanks again!

davide

rhaynes said:

Probably the most robust way is to use a relay to close the AC circuit. It won't care if the load is inductive, capacitive, or resistive. You can get pretty small 15A with 5V or 12V coil drive. It provides the mains isolation as well. You'll want to make sure to include a reversed diode across the coil to catch the inductive kick from the coil when the driver opens up.

For instance like this one from Digikey G5RL-1A-E-HR DC5. Of course there are many other styles available.

Ray

Click to expand...

Here's a nice little write-up to expand on rhaynes' concept (which is what I'd do, too).
Controllable Power Outlet - SparkFun Electronics

Their circuit uses an additional BJT transistor to control the current going through the relay's coil. Depending on how much current your control circuit can produce, this may or may not be needed. I'd probably keep it in your final design, so that any high voltage/current from the inductive kick-back of the coil won't be directly connected to (I'm assuming) your microcontroller's pins (they don't handle big votlage spikes at all... things go *poof*). I know that's what the diode is for, but two layers of safety is better than one... especially when you are expirementing.

No, nothing on the AC side, just switch the line voltage. The circuit pointed to by enjunear is exactly what I had in mind though there are physically smaller relays available if space is an issue.

Ray

---------- Post added at 10:09 ---------- Previous post was at 10:03 ----------

If you wanted to stay with a solid state, no moving parts type of design, you might also look at using a zero crossing opto coupler like the MOC3161 (or other in the series) with a TRAIC. Because it would switch when the AC crosses zero inductive loads are not an issue.

Just a thought.

Ray

yep I had seen that writeup, but it being my first post I wasn't allowed to post a link .
If you read through the comments there's someone saying that the circuit is good only for resistive loads and that for inductive ones in DC (which puzzled me because the writeup is about AC) you're better off with alternative solutions. That's why I started wondering whether I need something more in AC too.

csdave said:

And how about a failing relay? Does it heat up? Catch fire? or simply stay open/closed?

Click to expand...

The most likely failure mode for the relay is mechanical in nature. Either it fails to close or sticks closed. There are safety requirements for line voltage rated relays that they don't do something catastrophic when they fail.

---------- Post added at 10:53 ---------- Previous post was at 10:23 ----------

csdave said:

yep I had seen that writeup, but it being my first post I wasn't allowed to post a link .
If you read through the comments there's someone saying that the circuit is good only for resistive loads and that for inductive ones in DC (which puzzled me because the writeup is about AC) you're better off with alternative solutions. That's why I started wondering whether I need something more in AC too.

Click to expand...

Well, relays are used all the time to switch inductive loads. If the load is known, a snubber circuit can be used to reduce the arcing and improve the relay life but there are other issue. But for your application pick a relay that handles a higher current than your load like a 30A relay for switching a 10A inductive load and I don't believe you'll ever have any problems.

Ray

rhaynes said:

No, nothing on the AC side, just switch the line voltage. The circuit pointed to by enjunear is exactly what I had in mind though there are physically smaller relays available if space is an issue.

Ray

---------- Post added at 10:09 ---------- Previous post was at 10:03 ----------

If you wanted to stay with a solid state, no moving parts type of design, you might also look at using a zero crossing opto coupler like the MOC3161 (or other in the series) with a TRAIC. Because it would switch when the AC crosses zero inductive loads are not an issue.

Just a thought.

Ray

Click to expand...

The zero-crossing TRIAC is a good idea. I used one once upon a time for an AC motor switch... worked quite well, but you need to be careful with heatsinking it properly. A relay is a nice simple circuit to start with. The problem with really big inductors (read: motors) on AC is that if you open the relay at the wrong time, you could arc over the contacts from the AC motor's own back-EMF ("kickback").

enjunear, Except in very specialized cases you can't really control when a relay opens a circuit relative to the AC cycle. There will be times when it opens at the worst time and there will be some arcing. If the contacts are sized correctly the arcing is actually beneficial as it cleans contaminates off the contacts. If the contacts are too small the arcing can sputter away material and cause the relay to prematurely fail.

As for a zero crossing TRIAC design, a bigger problem than the heatsink is the circuit needs to be protected not from normal shutoff, after all TRIACs remain conductive until the AC falls to zero and then open, but from someone unplugging the load at the peak of the AC cycle where the back EMF from a motor or transformer could easily overvoltage the TRIAC. If you can disconnect the load manually you really need to use a snubber or MOV part to protect the TRIAC.

But I agree with you that a simple correctly sized relay is very simple and robust. And the OP was talking about controlling a pet water fountain which would be a pretty small load for a relay. I imagine a 5A rated relay could switch the fountain all day without a problem.

Ray

rhaynes said:

enjunear, Except in very specialized cases you can't really control when a relay opens a circuit relative to the AC cycle. There will be times when it opens at the worst time and there will be some arcing. If the contacts are sized correctly the arcing is actually beneficial as it cleans contaminates off the contacts. If the contacts are too small the arcing can sputter away material and cause the relay to prematurely fail.

As for a zero crossing TRIAC design, a bigger problem than the heatsink is the circuit needs to be protected not from normal shutoff, after all TRIACs remain conductive until the AC falls to zero and then open, but from someone unplugging the load at the peak of the AC cycle where the back EMF from a motor or transformer could easily overvoltage the TRIAC. If you can disconnect the load manually you really need to use a snubber or MOV part to protect the TRIAC.

But I agree with you that a simple correctly sized relay is very simple and robust. And the OP was talking about controlling a pet water fountain which would be a pretty small load for a relay. I imagine a 5A rated relay could switch the fountain all day without a problem.

Ray

Click to expand...

Ray, thanks for the explanation. I'm sure the OP would find much of it useful. As for my original comment, I didn't intend it to mean that one would want to carefully control the relay turn-off time, just that opening the circuit during high peak-current conditions could cause adverse affects on the components.

In general, I think the OP would be well-served by using an oversized relay (30A), since he sounded intent to build a switchable AC outlet (per the article he/I referenced). If he decided to reuse the outlet on a later project (with a load larger load), then he may as well have a design that is robust enough to handle switching a nominal 10A load, that he could find in a residential environment.

rhaynes said:

The most likely failure mode for the relay is mechanical in nature. Either it fails to close or sticks closed. There are safety requirements for line voltage rated relays that they don't do something catastrophic when they fail.

Click to expand...

If it adds any reassurance, we've been switching AC loads of between 100Watts and 8kWatts with both mechanical relays and SCR based solidstate relays from microprocessors for over 20 years. I vaguely recall one relay failure (contact jammed closed) and there was one spectacular SCR failure (overheated and partially melted). These have switched a variety of loads, more resistive than inductive. Otherwise these have both been remarkably sucessful.

To ensure safety isolation between PC and the load circuits, and also having distances of up to 50 meters between PC and mains distribution panel, we've used a double isolation method. This echoes enjunear's recommendation of 'two layers of safety'. The PC's ports have open collector TTL outputs which are wired to little PCB mounted reed-relays which switch 12 volt DC control lines. These 12v lines illuminate panel LEDs and are wired through to the remote mains distribution panel where they switch the relays or SCRs. These devices have survived some rather nasty accidents, severe mains fluctuations and abuse, all without problems.

rhaynes said:

Well, relays are used all the time to switch inductive loads. If the load is known, a snubber circuit can be used to reduce the arcing and improve the relay life but there are other issue. But for your application pick a relay that handles a higher current than your load like a 30A relay for switching a 10A inductive load and I don't believe you'll ever have any problems.

Click to expand...

One of the features of switching inductive loads with zero-crossing SCRs is that the highest currents may pass during times of the lowest voltage (ie during zero-crossing), so you really ought to design for worst-case currents, including short-circuited loads during the time taken to trip a MCB or fuse, I'd suggest at least double the expected maximum current rating.

**broken link removed**
Just remember, when a solid state relay is 'OFF', the leakage will ensure that the 'live' line still carries mains voltages! High impedance warning indicators should be used to display the hazards.

Hi all,
I looked around a little for relays and SCRs and indeed both seem fine. To simplify building the thing I was thinking of building the driving circuit by soldering components onto a prototyping board (no printed circuit) and of using a flanged-mount relay such as this: **broken link removed**.

The question I have now is whether I can safely flange mount the relay on the prototyping board and running wires from the driving circuit onto the relay and then from the relay onto the AC lines. This should keep the AC off the board, or is isolation an issue here?

These relays should be fine, provided you are happy that they have adequate current carrying capacity for your intended loads.
If you are able to terminate the wires with spade connectors firmly and securely, with insulated boots (and are sensible with colour coding the lives and neutrals), then you should be okay to mount these on the pcbs, though thre will be a very dense cluster of wires on each relay! Would it not be possible to mount them on the metal chassis/enclosure and take the low voltage coil wires to the pcb and the switched mains wires to the inlets and outlets which will also be on the chassis/enclosure?

Just 2 extra thoughts, don't forget to earth the chassis/enclosure properly and don't forget to put an appropriate fuse in the relay's live mains circuit.
Good luck!

Thanks DX
You're probably right regarding the enclosure. I am still looking for a suitable one actually. I haven't seen any GFCI boxes like the one on Controllable Power Outlet - SparkFun Electronics here in France. But all apartments are required to have proper grounding and a circuit breaker (hope it's the right word) installed (and mine does). Do you suggest I put an extra fuse in the outlet on the AC side or just that there be one in the line leading to the outlet?

For the enclosure, you could use an aluminium box, its easy to drill holes in these for the wires!
RS | Accueil | Boîtiers et ventilateurs | Métal / Plastique | Boîtes en aluminium non peintes
You should use grommets (passe-fils) to protect your wires from the shape edges of holes.

The Circuit Breakers in your appartment (disjoncteurs) are good for protecting the wiring in the appartment, but you will need additional protection for the relay circuit. If you have chosen a 20Amp relay but (for example) you actually intend switching loads of 12Amps, then please fit a 12Amp fuse (fusible) to the live connection before it gets to the relay contacts.

DXNewcastle said:

For the enclosure, you could use an aluminium box, its easy to drill holes in these for the wires!

Click to expand...

Make sure if you use an aluminium box that you use a 3 prong power connection and ground the ground wire to the box. If you use a plastic box the 3 prong connection is not so important.

Ray

---------- Post added at 10:39 ---------- Previous post was at 10:25 ----------

csdave said:

The question I have now is whether I can safely flange mount the relay on the prototyping board and running wires from the driving circuit onto the relay and then from the relay onto the AC lines. This should keep the AC off the board, or is isolation an issue here?

Click to expand...

When mixing line and logic signals always make sure you layout the circuit, be it perfboard, PCB or just flying wires, so there's a clear visual / physical seperation between line and logic circuits. You want to be able to quickly spot violations of this keep out zone, stray wires, pieces of whatever, etc. A person tends to glance over a circuit and call it good without throughly looking it over. With a specific keep out zone you can easily and very throughly say that at least that zone is clear.

Lastly it would be a good idea to ohm out between the logic power/ground and both lines to make sure it's completely open before powering it up.

Ray

DXNewcastle said:

For the enclosure, you could use an aluminium box, its easy to drill holes in these for the wires!
RS | Accueil | Boîtiers et ventilateurs | Métal / Plastique | Boîtes en aluminium non peintes
You should use grommets (passe-fils) to protect your wires from the shape edges of holes.

The Circuit Breakers in your appartment (disjoncteurs) are good for protecting the wiring in the appartment, but you will need additional protection for the relay circuit. If you have chosen a 20Amp relay but (for example) you actually intend switching loads of 12Amps, then please fit a 12Amp fuse (fusible) to the live connection before it gets to the relay contacts.

Click to expand...

Hi DX,
Actually I think a plastic box would be better, why should I use a metal one? Now that I think of it, rather than building an outlet it's probably more convenient if I build a box that can be plugged into the wall and onto which I can plug my device.
In this case where do you suggest to mount the relay? On the plastic frame? On the same board as the driving circuit?

Regarding the fuse. If I understand it is supposed to protect the circuit from a short circuit coming from something connected to it, right?
Now would this also protect someone from electrocution if they were the means through which the circuit is closed? For the fuse to be useful for the circuit it needs to blow before the circuit breaker does, doesn't it? I am missing some knowledge here. I thought a fuse alone would not be enough otherwise why would we have circuit breakers? Does anyone have a pointer to some information on this?

---------- Post added at 20:51 ---------- Previous post was at 20:44 ----------

rhaynes said:

Make sure if you use an aluminium box that you use a 3 prong power connection and ground the ground wire to the box. If you use a plastic box the 3 prong connection is not so important.

Ray

---------- Post added at 10:39 ---------- Previous post was at 10:25 ----------



When mixing line and logic signals always make sure you layout the circuit, be it perfboard, PCB or just flying wires, so there's a clear visual / physical seperation between line and logic circuits. You want to be able to quickly spot violations of this keep out zone, stray wires, pieces of whatever, etc. A person tends to glance over a circuit and call it good without throughly looking it over. With a specific keep out zone you can easily and very throughly say that at least that zone is clear.

Lastly it would be a good idea to ohm out between the logic power/ground and both lines to make sure it's completely open before powering it up.

Ray

Click to expand...

very good points. If I use a plastic box what should I ground the ground wire to? Or should I just bridge it over from the input to the output sockets?

csdave said:

Regarding the fuse. If I understand it is supposed to protect the circuit from a short circuit coming from something connected to it, right?
Now would this also protect someone from electrocution if they were the means through which the circuit is closed? For the fuse to be useful for the circuit it needs to blow before the circuit breaker does, doesn't it? I am missing some knowledge here. I thought a fuse alone would not be enough otherwise why would we have circuit breakers? Does anyone have a pointer to some information on this?

---------- Post added at 20:51 ---------- Previous post was at 20:44 ----------



very good points. If I use a plastic box what should I ground the ground wire to? Or should I just bridge it over from the input to the output sockets?

Click to expand...

1. Comments on fusing. The wall outlet is protected at 15-20A with breaker (or perhaps a fuse) to mainly protect the wiring to the outlet and cord to the product plugged in. Products have internal fuses to protect them and those will usually blow if something goes wrong internally. If you get a short in your project the line protection (breaker) will allow for much more current than the 15 or 20A rating and may produce some very exciting fireworks before the breaker opens. If the breaker is a GFI type then you won't get fireworks if the short is to ground but you could get them if a line to line short develops.

On the other hand if your project only requires an amp or two adding a 5A fuse can substantially reduce the fireworks and add local protection. But these protections are mainly for fire and do not protect from electrocution.

2. Comments on the ground wire. If you have a plastic box then just jumpering the input ground to the output ground is fine for the minimum. Personally I usually connect the line ground to my circuit ground. The idea there is if some line isolation fails the line current will be returned to ground, albeit through my low voltage circuitry. That's the safest setup from the human prospective. There are lots of products without 3 prong plugs but they follow strict safety guidelines in their design, something we are not as likely to do.


Ray

rhaynes said:

1. Comments on fusing. The wall outlet is protected at 15-20A with breaker (or perhaps a fuse) to mainly protect the wiring to the outlet and cord to the product plugged in. Products have internal fuses to protect them and those will usually blow if something goes wrong internally. If you get a short in your project the line protection (breaker) will allow for much more current than the 15 or 20A rating and may produce some very exciting fireworks before the breaker opens. If the breaker is a GFI type then you won't get fireworks if the short is to ground but you could get them if a line to line short develops.

On the other hand if your project only requires an amp or two adding a 5A fuse can substantially reduce the fireworks and add local protection. But these protections are mainly for fire and do not protect from electrocution.

2. Comments on the ground wire. If you have a plastic box then just jumpering the input ground to the output ground is fine for the minimum. Personally I usually connect the line ground to my circuit ground. The idea there is if some line isolation fails the line current will be returned to ground, albeit through my low voltage circuitry. That's the safest setup from the human prospective. There are lots of products without 3 prong plugs but they follow strict safety guidelines in their design, something we are not as likely to do.


Ray

Click to expand...

Hi Ray,
I did some reading on circuit breakers and understood the difference Thanks .

Now thinking of the device to be plugged into the wall, depending on the socket there may be no way to tell which is the phase and which is the neutral. Is there something smart to do here that I am missing?

hi again,
just a question about the flyback diode. How should I choose one? The sparkfun tutorial mentioned above Controllable Power Outlet - SparkFun Electronics uses a 1N4148. This is rated for 200mA... Isn't this too little to withstand the current resulting from the coil's discharge?
In addition, I have opted for a slightly bigger relay:

**broken link removed**

How do I determine the specs of the diode?