Limiting AC current (230V)

Hello all,

I am designing a 230V dimmer using a 12A output triac which I control via a cpu (through optocoupler etc). Is there any way to limit the output current to some 4A (max load about 900W)? Using a fuse before the load would just blow the fuse in case the load needs more current, so it is not an option.

The same is for the output of an AC relay. The capacity of the relay output contact is 250V/10A. I would like to limit this to 7A (about 1600W or so). I need the same solution for this, too.


Why do I need this? Here is an example:
Lets say that the user is supposed to drive a load up to 1000W with this dimmer. In case he connects a load of 1500W, ideally I would like to have a current limiter before the load, so as not to cause trouble to the unit.

Any ideas would be really appreciated

Thank you

Hi,

The problem with triacs is, that once they are fired, they stay conductive until zero cross.
So if the low ohmic load gets connected...there will be a lot of overcurrent for the next few milliseconds.

The next problem is how you want to measure the current.
Lets say you have a 23 ohms load. With 230V this means 10A and 2300W.

Now imagine you use a triac and do a phase delay of 90°..this means half time ON and half time OFF.
Power is half also, 1150W.
RMS current is 7.07A, RMS voltage is 1626V.

For calculating power, current or voltage from a phase controlled sine signal is not easy. You need a true RMS calculation.
Did you decide how to do this? A simple solution won't work....at least not if you want some kind of precision.


Klaus

Easiest way might be to just fit a thermal cut out to the triac heat sink.
That is practical, because it will be death by overheating that would normally kill the triac under steady state overload conditions.

But as Klaus has already pointed out, because triacs cannot be actively switched off, there is no way to protect against sudden massive overloads.

Typically, when a light bulb goes "plink" and dies at turn on, it very often causes the triac to fail short circuit.
A very fast fuse would be about the only option.

Thank you Klaus and Tony for your feedback and the time you spent for it.

Klaus you are correct with the calculations and I agree with you as regards the precision. But lets say, I do not need precision, lets say I need a "mechanism" that will not allow more than 4-5Amps to pass to the load, no matter what. Of course you will say "you need to measure the current first!!!" and you will be correct again.

I was thinking to add to the system an "AC Current transformer module" something like a 5A/5mA, in other words a mini transformer (a coil) that produces a small current proportional to the current that is passing through its core and via an op-amp to add some kind of feedback back to my controller, cutting the driving of the optocoupler if this current exceeds a threshold for some time....

Or maybe I could do something similar with a ACS712 based circuit which I have already ordered for some testing, following the same principle as above.

The point is, "is that the correct way to go or there is some other way to deal with such issues?"

The above principle would possibly work with a relay, too. If the current produced by the coil is for some time more than the maximum allowed, the microcontroller will stop driving the relay, and the dimmer will stop working.

Tony, using a thermal cut is a solution for the triac but not for the relay, unless I add some kind of series resistance in the output to use the thermal cut on it, in both cases (either the output is driven via relay or triac). Again the solution that comes in my mind is to send that feedback signal back to my controller and let it decide for the rest. ONE of the reasons why I want to "play" very safely (passing up to 7Amps from the contacts of the 10A relay OR 4Amps from a 12A triac) is to prevent failures due to unstable conditions. OK, if the overload is massive then you will damage the device. For exmple, If the specifications are for 4Amps load and you connect a 25A load, then a fuse (at least) should be added to prevent unpleasant results!

In practise I would like to have a system that will cut off for its own protection before it gets damaged.


Maybe I am talking nonsense here...please feel free to make your comments. I appreciate your input!


Nick

Hi,

you need a concept.

you have to decide what you want to.
* Do you want rectified limit peak current? (What peak value timing)
* Do you want rectified averaged/filtered current?
* Do you want RMS curent?

Using a current transformer does not say what measurement method you want to use.

Klaus

Hello again,

I would say RMS current but you see, since I do not have the experience on the design of such systems, I am not sure which method would be the easiest to follow. An idea is to place a resistor across the output of the transformer, measure the voltage, multiply with 0.707, get the figure via Ohms law. Placing a load of some 900W would produce a figure which I could set later as max in my controller for the case of the triac.


The device will be a dimmer that will be capable of driving up to 900W resistive loads (dimmable) or up to 1600W loads (ON/OFF state via the relay).

Does it makes sense to you?

Thank you again for your comments


Nick

Hi,

what you describe is far away from true RMS measurement.

****

place a resistor across the output of the transformer, measure the voltage

Click to expand...

What voltage?
* The actual voltage will vary from +peak to -peak and back within one mians cycle. Take five measurements and all will show totaly different values.
(with the same load and the same phase angle...the values could be: +0.2V, -3.1V, -1.5V, +0.8V, +1.3V.... doesn´t this look like random values?
* The average voltage should show 0.0V... independent of load current
* The RMS voltage is hard to calculate.. either you use an RMS_to_DC circuit = expensive, or you use a lot of samples with a fixed sample rate and calculate it on your own
* The rectified average voltage has some errors caused by the voltage drop of the rectifier diodes. And - because you use phase control - the output has low information.
* The peak_and_multiply_with_0.707 value is only true with pure sine signal, but not with phase controlled signals.

Please study some circuits. Choose a circuit you thik it works. Make yur modifications on the circuit. Show use your circuit with all necessary informations.



Klaus

Thank you Klaus

I agree with your comments and that is why the above mentioned way (with the tranformer and the resistor) would work with the relay output (since my loads are resistive), this is where the idea came from. That is not the case with the triac, the reason why I place the initial question. Maybe there is some other way to approach this. I think it is rather impossible for dimmer manufacturers not to include overload protection in their designs, unless it is a cheap design.

I will try some modiffications and measurements and come back


Regards
Nick

Nick;
the problem is; once that you have detected an overcurrent condition (by whatever the means), the Triac will nevertheless remain in conduction for the remaining of the half cycle. Worst case, we are talking a 10 ms delay. There is nothing you can do -easily- to prevent that.

Relays don't work. The very fastest power relays I've measured take about 5 ms to operate. That is in top of the time the sensing circuit requires to detect reliably the overcurrent conditions.

So the only protection available to you is to obtain the fastest fuse possible, carefully study its I2t curves, and then select a large enough Triac which will withstand that overload.
I've successfully protected Triacs that way, but the resulting Triac will have to be rated several times the steady state current.

The above post pretty much sums it up...

Only other thing I can really add to the discussion might be a polyfuse.
If you are unfamiliar with these, they are well worth knowing about:

https://www.littelfuse.com/~/media/...fuse_ptc_selection_guide_application_note.pdf

Really more applicable for thermal protection of loudspeakers, motors and transformers, but may be a simple practical way to prevent slowly cooking your triac from steady long term overload.

I really cannot see any solution to sudden transient overload.

Perhaps you want an instantaneous cut-off when the set current limit is exceeded; it is not meaningful to involve the power or wattage at this point.

A conventional triac cannot be turned-off for the remaining half-cycle. We obviously need another fast switch.

Mechanical switches (like relays) are too slow to act in quarter cycle; 5 ms is too fast for big relays (forget reed relays but you can use solid state relays that can be faster).

Incorporating a SSR in a light dimmer is too much of an overkill. But the idea is appealing.

One way used in industry is a large, slow, low on drop (1V) IGBT (TO-247) controlled so that any time the peak current exceeds "x" the IGBT is gated off for at least one second, in conjunction with a thermal (bi-metallic strip) switch on the heatsink of the triac/SCR, this provides pretty comprehensive protection, excepting that you need a rated TVS across the IGBT so that it is never exposed to higher than rated Vce. Of course 2 such IGBT's provides the complete solution for dimming and self protection...

Thank you all for your comments. I got some nice ideas.

Perhaps I was not clear enough, possibly due to my skills in English and I got misunderstood.

Triac and Relay are two different things in my design, two different routes in the PCB, two separate circuits. I will drive the load either with the Triac or the Relay. Not both of them simultaneously! So, what I am telling to the user is "If you want, you can plug a dimmable load up to 4Amps and you can fully control it without any issue....if your load is above 4Amps, you can only control the ON/OFF by choosing to use the "Relay-Option", but obviously you cannot dim it"

In both cases, I would like to protect the design without blowing a fuse.
1. In the case of the Triac, if the user tries to dim a higher than 4Amps load (lets say a load that is 6Amps) I would like to cut the output so as not to damage the triac (yes in the worst case I need to wait for 10ms for the next halfe cycle).
2. In the case of the Relay (in that case the user can only power-ON or power-OFF the load), if the user connect a higher than 8Amps load, lets say 10Amps, I would like to cut off the output, too.


The polyfuse that Tony proposed maybe a solution, I am studying it. I thing it may do the job. The solution with the IGBT sounds more complicated for a start, but more or less in theory does the job. Obviously cost and size are important issues and I am not sure if such an approach will be acceptable. I will give it a try!

Thank you indeed for your time

Nick