Hi, I have been looking around for ways to limit the current flowing through a circuit to a maximum value. I have only been able to find ways of doing this for DC circuits, however this is an AC circuit. I would like to limit a 25V 4A AC transformer to only have an output of 250mA (give or take). I would appreciate any advice you have on how I could accomplish this.

 

The simplest way is probably to use a second 1:1 transformer, which can only supply .25 amps.

 

Sorry I forgot to mention that I did find a solution using a secondary transformer, but that would not be suitable for the needs of this project. However thanks for the help so far!

 

A resistor?
A fuse?
An ohmicly biased fet in parallel with the load that sinks a proportionate amount of current, so the load itself never sees more then 250mA?

 

The simplest way is probably to use a second 1:1 transformer, which can only supply .25 amps.

And then what, it burns up it's windings to maintain the limit? That is not a well thought out 'solution'.

A true automatic current limit, where the unit starts to drop it's output voltage by the amount nessesary to maintain the .25amp maximum current, would need to work just like current limiting circuits for DC, but would required to measure and control both the positive and negitive portion of the sine wave. Some kind of direct coupled AB amp stage with current sensing/foldback or possibly a modified H-drive circuit with current limiting could be made to work.

Lefty

 

And then what, it burns up it's windings to maintain the limit? That is not a well thought out 'solution'

In my bathroom there is a shaver socket, protected by just such a transformer. It has never burnt out or burnt anything else yet.

 

It's not practical because the technology is obsolete, but a saturable reactor would be ideal for AC current limiting. Here's the Wikipedia article - http://en.wikipedia.org/wiki/Saturable_reactor

 

A resistor?
A fuse?
An ohmicly biased fet in parallel with the load that sinks a proportionate amount of current, so the load itself never sees more then 250mA?

Ouch! What are you thinking?

 

Connect a 115V light bulb in series to the primary of the transformer; choosing its wattage until the current does not exceed 250mA.
Or,
Connect a 24v light bulb in series with the 24VAC secondary;
choosing its wattage until the current does not exceed 250mA.
A 12V automotive bulb may work too.

Miguel

 

A true automatic current limit, where the unit starts to drop it's output voltage by the amount nessesary to maintain the .25amp maximum current, would need to work just like current limiting circuits for DC,.....
Lefty

I was thinking along the same lines and was going to spice it. I quickly aborted because of the OP's transformer limitations. If his load requires 25VAC and the transformer is 25VAC then any solid state solution would not deliver the full 25VAC.

StarFleet, how much voltage drop can you tolerate?

 

Maybe putting a MOSFET in series with the load can do the job. The resistance of the Fet is controlled by an op amp which gets its inverted input signal through a rectified voltage from the load current sampling resistor.

 

Sorry I forgot to mention that I did find a solution using a secondary transformer, but that would not be suitable for the needs of this project. However thanks for the help so far!

If another transformer is disallowed then you will need to use your 25 volt transformer to power (after rectification etc) an oscillator which can only supply .25 amps.

Any attempt to place an impedance, resistive or inductive in the path of the 25v supply will reduce that supply.

 

Well I guess I was thinking about this as a solution to a problem, however I should probably just tell you what the problem is, and let you suggest a solution. Basically I have a sensor that is either on or off. It is a rain sensor the is designed to be used either to directly handle the load or to be analyzed as either on or off. Because of this it can handle upward of 5A at 24V. Since I want to analyze if it is on or off, it can not use up a lot of current (same as a short). What would you suggest as a solution to determining if the sensor is active or not while not using a lot of current (which would be needed for the load).

 

Well I guess I was thinking about this as a solution to a problem, however I should probably just tell you what the problem is, and let you suggest a solution. Basically I have a sensor that is either on or off. It is a rain sensor the is designed to be used either to directly handle the load or to be analyzed as either on or off. Because of this it can handle upward of 5A at 24V. Since I want to analyze if it is on or off, it can not use up a lot of current (same as a short). What would you suggest as a solution to determining if the sensor is active or not while not using a lot of current (which would be needed for the load).

How is this related to your original post? If you wanted an ON/OFF or current indicator then that's what you should have posted! I'm certainly glad that I didn't spend any design time as per your original question and requirements.

 

Well I would prefer to have an current limiter, as that would be the best solution (as I could use it down the road for another project). That is why I posted the original question, but since it appears to be harder than I initially thought, I figured I might make it easier to satisfy my current delema instead of the next one that is down the road (if that makes any sense?)

I didn't think that an on/off current indicator would even work, since there would be 4A flowing (and I can only allow .25A or so) , unless it was regulated (which was another reason I was asking this question). The current must be large enough to actually trigger the sensor but not so large that it would direct all of the current to ground. I believe that this is what would happen w/o a current limiter of some sort?

So this pretty much brings me back to my original question (but with clarification of why its needed)

I believe that the max current for the sensor is around 5A

 

Well I would prefer to have an current limiter, as that would be the best solution (as I could use it down the road for another project). That is why I posted the original question, but since it appears to be harder than I initially thought, I figured I might make it easier to satisfy my current delema instead of the next one that is down the road (if that makes any sense?)

I didn't think that an on/off current indicator would even work, since there would be 4A flowing (and I can only allow .25A or so) , unless it was regulated (which was another reason I was asking this question). The current must be large enough to actually trigger the sensor but not so large that it would direct all of the current to ground. I believe that this is what would happen w/o a current limiter of some sort?

So this pretty much brings me back to my original question (but with clarification of why its needed)

Edit: I believe that the activation current for the sensor is around 0.1A upto 5A

It would appear that you don't understand the ratings of transformers. 4 Amps is the MAX current that should be drawn from that transformer. It doesn't mean that it delivers 4 Amps regardless of the load connected to it. We need to apply Ohms Law here.

 

What is mean is that the sensor will draw all of the current available to it, because it has close to zero ohm resistance once active. This is unacceptable because then the load (not sensor) would receive less much less than 3A of current. I could use ohms law but then the resistor would need to handle a huge amount of heat dissipation.
I also believe that if the sensor is unregulated and connected to ground it would far exceed the maximum rating of current from the transformer.

Please correct me if im wrong.

 

A rain sensor is generally a tipping bucket that swings a magnet past a reed relay with each tip. The conventional usage is to apply DC to the reed with a resistor. When the relay closes, it shorts to circuit ground, but that current is limited by the resistor. It's usually enough to make a logic level signal to be counted by a microprocessor.

 

Since I want to analyze if it is on or off, it can not use up a lot of current (same as a short). What would you suggest as a solution to determining if the sensor is active or not while not using a lot of current (which would be needed for the load).

Just stick a suitably sized resistor in series with the thing. You just want to know if the switch is closed or opened, right? You want a voltage output of 0V or 24Vac? What are you using to "analyze" the sensor? A counter? A microprocessor? A solenoid controlling air to a balloon which inflates more as the rain continues?

 

Folks, don't y'all think it's long past the point that StarFleet should be asked to post a schematic of this circuit? Hell, we're on the 20th post and we are shoveling sh-t against an incoming tide!

StarFleet, please post it.