AC power limiting circuit

[Heeran]

Hello,
I would like some help on building an extremely simple current limiting circuit. I would like for the circuit to operate as follows. Input 115-230Vac from mains. The output to the load will be 115-230Vac. The maximum load that can be used is 115Vac - 230Vac at 10watts. If the watts exceeds 10watts, the current limiting circuit will cut off power supply to the load, when the load is removed and the correct load is connected, the power supply will supply the correct load.

Please assist me with this circuit as well as what changes will be requires should I want to change the maximum limit form 10watts to above or bellow.

I have provided a circuit that I found on the net but don't know how to use it as I don't know the component required, also if this circuit will work as per my requirements.

Kind regards.

 

[Gryd3]

Hello,
I would like some help on building an extremely simple current limiting circuit. I would like for the circuit to operate as follows. Input 115-230Vac from mains. The output to the load will be 115-230Vac. The maximum load that can be used is 115Vac - 230Vac at 10watts. If the watts exceeds 10watts, the current limiting circuit will cut off power supply to the load, when the load is removed and the correct load is connected, the power supply will supply the correct load.

Please assist me with this circuit as well as what changes will be requires should I want to change the maximum limit form 10watts to above or bellow.

I have provided a circuit that I found on the net but don't know how to use it as I don't know the component required, also if this circuit will work as per my requirements.

Kind regards.

The image you posted is for DC, this will not work on 100-240V AC.
To make things a little more difficult, you have stated 10Watts as your limit... which means you need to monitor Current AND Voltage. Simply monitoring Current like the above circuit does would allow for a wide range of Watts because you are allowing the voltage to vary.
This would be easier for you to accomplish if you picked a specific voltage and used a resettable fuse.

 

[Heeran]

So if I use the common voltage which is 230Vac in my country, I can use a fuse that will allow the system to shut of above that limit. Is the resettable fuse the same as a circuit breaker?

 

[Gryd3]

So if I use the common voltage which is 230Vac in my country, I can use a fuse that will allow the system to shut of above that limit. Is the resettable fuse the same as a circuit breaker?

It could be looked at with a similar function... but it does not cause an 'open circuit' when it is tripped. It simply acts like a huge resistor which vastly reduced the current flow which essentially leaves it in an 'off' state. It also resets itself when the load is removed.
A circuit breaker will actually disconnect a portion of the circuit to stop current flow.

You just need to ensure the fuse/breaker is tolerant of the voltage you want to use. (or higher)
You then pick an amperage limit based on the Watts you want to limit the load to.

10W @ 230V AC = 43mA so pick something close to this.
There is a draw back with this method. If you use the same fuse/breaker on a 110V system, you can only achieve 5W. (Because Watts = Voltage * Current)
You need to pick a different amperage limit based on the voltage you're working with.

 

[Heeran]

This is a huge help, I am trying to fine the correct rating for the fuse but no one seems to have it, can you possibly advise me or direct me to a company that will keep a range of these fuses for AC application such as my application.

 

[Gryd3]

This is a huge help, I am trying to fine the correct rating for the fuse but no one seems to have it, can you possibly advise me or direct me to a company that will keep a range of these fuses for AC application such as my application.

Therein lies a problem. The supplier I checked only go as low a the 200mA range which will be in the ballpark of 40W-50W for your application.
(The current limit for 10W is pretty low... we may need to build something)

 

[Heeran]

Is it possible to build something for this application that is small. Also can the design that will be built be able to be modified for the different AC input voltages to accomplish the same end results be means of changing some components.

 

[Arouse1973]

What is the load you are driving?
Adam

 

[Gryd3]

What is the load you are driving?
Adam

That should have been the first question, but due to the low current, only thing I can think of is a shunt resistor and a comparator to drive a latch...

 

[Heeran]

In my country, we operate with 230Vac, the load I am using is 230Vac at 10watts maximum, anything above 10 watts,, I want the power supply to or to drop so the load does not operate.

 

[Gryd3]

In my country, we operate with 230Vac, the load I am using is 230Vac at 10watts maximum, anything above 10 watts,, I want the power supply to or to drop so the load does not operate.

Is this an inductive load like a small motor, or is it something simple like a light?
Or is this something that will be used with multiple different devices?

 

[Heeran]

It is a simple light.

 

[(*steve*)]

Any reason you can't use a normal light dimmer?

 

[Heeran]

Yes, I am building a lamp housing with this overload protection for a university project. But I would like the design of this circuit to be able to adapt to the different AC currents that are in different parts of Africa just by changing component ratings to achieve the same end result.

 

[(*steve*)]

Why does a light bulb need overload protection?

Surely you chose the correctly rated light bulb?

Or is this for some sort of transient voltage protection?

We need to know a lot more about the WHY and a lot less about your ideas of HOW.

If you need a light to run from a wide variety of voltages, one option is to use a SMPS (that can cope with a wide input voltage range) to create a lower voltage rail and run the lamp from that (possibly a LED for better efficiency)

 

[Heeran]

Okay, the reason behind this is that this unit serves as an energy saver unit. So by installing this circuit, it prevent high wattage bulbs from being used so by doing this, a low wattage bulb is the forced option thus reducing electricity consumption.

The two lights that can be used is the LED or CFL light bulb long as the 10 watts and bellow because both these options come in much higher wattages.

 

[(*steve*)]

Ok that puts a different light on it.

Let me think about this.

 

[davenn]

nice pun Steve

 

[Heeran]

Okay

 

[(*steve*)]

I have been thinking about this. It seems that some sort of constant current source would work, except that a load with a power factor significantly different from 1 would not receive the entire 10W.

Similarly any device with peaky power requirements (say a higher start-up current) may not start.

I think you're also wanting to use this to limit current so (say) a lower voltage rated bulb could be used. Unfortunately this would not be effective because a 110V 10W bulb requires twice the current of a 220V 10W bulb.

You might be able to design some sort of slow acting constant current source, and place it "inside" a bridge rectifier placed in series with the load, but it would be exposed to the maximum peak input voltage (say 400V or so) which would require high voltage transistors in the constant current device. This device could be called on to dissipate 10W or so, so it would need to be heatsinked, and the heatsink would need to be isolated so as not to present a shock hazard.