detecting break in current flow

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aliyesami

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hi
I am running bunch of LEDs and I want to detect if the LEDs are out.
I first thought of using LDRs but that's not efficient way , I know DC current flow can be detected but I don't know how.
can someone tell me how to detect break in DC current so I can detect and take actions ?

thanks
 

Hi,

Just monitor the voltage accros the LED, or more precise, the voltage accross a small shunt resistance in each branch. The voltage will be an image of the current.
You will need a comparator for each led.
 

Being all them arranged in series would make easiest the control, but the factor of redundance become null.
 

gag2000 said:
Hi,

Just monitor the voltage accros the LED, or more precise, the voltage accross a small shunt resistance in each branch. The voltage will be an image of the current.
You will need a comparator for each led.
Click to expand...

and how can I measure this voltage and get a digital signal out of the reading ? ADC comes to my mind but any other simpler ways?
 

aliyesami said:
and how can I measure this voltage and get a digital signal out of the reading ? ADC comes to my mind but any other simpler ways?
Click to expand...
Perhaps using an analog comparator, but I fear that depending on the LED bias current, its drop voltage variation with temperature could be not negligible.
 

You must have a current shunt to regulate it to a known drop voltage. If the voltage exceeds the expected range, it is open circuit. An SCR with voltage divider to set trigger voltage is a convenient but expensive way to bypass an open LED and maintain the current regulation loop. Depending on the current level, a zener is another solution but also expensive for each LED.

There is no cheap way.

Normally LED failures can be prevented by good current regulation, thermal design and solder/ESD control, as reliable source of LED failures is very low.

Why is this important?
 

I came across this on web , hall effect ICs for detecting current , does it seem a nice solution ?
**broken link removed**
 

SunnySkyguy said:
Normally LED failures can be prevented by good current regulation, thermal design and solder/ESD control, as reliable source of LED failures is very low.

Why is this important?
Click to expand...

I am fully aware that most likely this is not the case of the OP, but in some industrial environments, even the failure of the inner light of a button of the equipment have to be monitored as part of the self-test feature available.
 

For LEDs in series you can monitor the voltage across the LED closest to the ground connection.
You can use a comparator with a trip point of about 1V. If it's above 1V the LED is conducting (ON) and if it's below 1V the LED is OFF.
 

appreciate all your suggestions but iam really liking this chip ACS711 , very small package and can handle currents upto 15 Amp and 100Volts.
what do you think?
 

I tested this chip but unfortunately it will not work for me as the LDD driver I am using has extremely low current at the input when there is no load .. as low as 1mAmp (see the attached specs) .
so actually what I need is to find out either total absence of current or voltage , so based on my new changed requirements can you suggest a solution


 

1 mA input quiescent current is irrelevant for the problem. If the module is set to "off", you'll hardly detect if the LED chain is open or closed. Obviously, a certain minimal output current must be set to enable the detection.

But I agree that using a high ampere hall sensor in inappropriate. Load disconnection can be most easily detected in the output circuit.
 

so whats the best way of detecting load at the output circuit ? I saw some earlier posting of using 'shunt' resistor and measuring the voltage across it , is there an example about it ?
also wont the shunt resistor reduce the total power delivered by the LDD driver?
 

By nature of a constant current switcher, an additional shunt won't reduce the output, just increase the input power by a small amount.

But you don't necessarily need a shunt, monitoring the LED voltage could be sufficient under circumstances. As a first step, I would determine the internal circuit of the switcher to understand the expectable voltage range of the output terminals relative to the input and control pins.
 

If you gave us a clue as to how much current the LED chain is taking it would be helpful. All you have said is that a 15 A chip is no use. Also you have not indicated hat the current detector output is going to do
If you are just detecting a current in the order of 1mA, earth the emitter of a small NPN transistor. Connect its base in series with the negative end of the LED chain. Now the collector will look like a short to earth when the current flows and will be able to switch up to 20mA.
Frank
 

sorry I thought I gave it , the LDD output is max 700mAmp
the problem is that the current is not fixed .. .on the dimming end there is < 1AMp current and on the full brightness the current is 700mAmp .
how can I handle this type of situation ?
instead of current , can I measure the voltage at the LDD input for the break in power ?

- - - Updated - - -

with switcher you mea LDD driver that iam using? Its specs are giving in the datasheet .

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ahUKEwj40ZvsnOLLAhXK2B4KHdXqDlEQFggcMAA&url=http%3A%2F%2Fwww.meanwell.com%2Fmw_search%2FLDD-H%2FLDD-H-spec.pdf&usg=AFQjCNGmDq5nNmjl6NrY9aS_-XuRhcZWog&bvm=bv.117868183,d.dmo
 

Why are you repeating the < 1 mA point? It should be clear that you can't detect output disconnection without LED current.

Its specs are giving in the datasheet.
Click to expand...
Specs yes, but not the switcher topology (internal circuit). It would be useful to find it out to decide about feasible detection circuits.
 

ok so detecting break in current is not possible .
what about detecting absence of voltage?
company would not give me internal circuit.

many electronic gadgets hawe this feature of power failure detection how do they do it ?
 

Is the purpose to detect one bad led? How about incorporating redundancy? Put 2 led's in parallel, inside the same plastic lens (each led with its load-balancing resistor). It should be easy to tell if one led is blown.
 

ok so detecting break in current is not possible .
what about detecting absence of voltage?
Click to expand...
Reviewing your posts, I feel that a clear specification is still missing. You want to detect a) failure of the power supply b) LED failure

company would not give me internal circuit.
Click to expand...
I said find out, not ask Meanwell.

many electronic gadgets hawe this feature of power failure detection how do they do it ?
Click to expand...
Any example?
 

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