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DC LED Indicator Lamps - Voltage Drop

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msnead

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I'm trying to design a "tester" for checking continuity of "through wires" in an electric hinge we make. The hinge will be mounted to a swing test stand and cycled repeatedly to test the integrity of the wires. The wires in the hinge are about 24 or 26 AWG. I am using a 2191U5-24V LED indicator lamp (Chicago Miniautre Lighting - Newark PN# 88M9621) to see that the wire is "good". On one side of the hinge (wire) I am connecting to +24VDC through the use of a power supply. The other side of the hinge (wire) connects to the RED lead on the LED, and finally the WHITE lead is connected to -24VDC. No problem.



However, I need to connect as many as 12 wires for a total of 12 LED's and I need to power up a 24VDC Ice Cube relay (for breaking the "enable" on the motor for the hinge swinger). It's been a long time since I was in electronics school, but it didn't take me long to remember VOLTAGE DROP. With only 1 (one) LED connected in series to the positve for the coil on the relay, I don't have enough to pull the relay in. I have only 6VDC. I've tried connecting the lights in series and in parallel, but still don't end up with enough voltage to pull in the relay. A PLC would solve the problem easily enough, but I was hoping to save cost. Does anyone have any suggestions?
 
Provide a sketch showing how it is wired. I cant figure out what you are trying to do from your description.

This a LED in series with a built-in resistor. Based on this, the operating current is E/R = 1/30 =33mA. Wiring a relay coil in series with these obviously wont work (if that is what you are trying to do) because it would take a very sensitive relay to pull in under 30mA.
 
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Yes connecting a led in series with a relay even a small relay isnt good, you'll probably fry the led and the relay wont pull in.

If you want the led to come on when the relay closes then put the led in parallel with the relay, a suitable resistor to put in series with the led directly off 24v would be a 2k2.
 
Here's an image of the Autocad file for the circuit. What I was hoping to accomplish is that each "terminal" corresponding to a particular "through wire" will light the LED. If a wire fails, the LED will obviously go out. The reason for the relay was to power a relay, of which I could use a set of N.O. contacts to control the "enable" circuit for the swinger. When the light fails, the motor stops, the count the swinger achieved prior to failure could be observed. Drawing.PNGEnclosure.PNG

One thing I did notice while waiting for replies: I can take every one of the RED leads for the LED's and connect them to positive 24V. Then take all the WHITE leads and connect them to one side of the coil of the relay (i.e. terminal 2). Then come out of the relay (terminal 6) and go to the negative 24V. All the LED's will come on and the relay will pull in. The relay doesn't get exactly 24V, but does have enough to pull in. The more LED's I connect, the more voltage I have across the coil. However, this arrangement doesn't work as it's intended - if I remove a wire (or LED), the coil doesn't drop out ... until I get down to less than 4 LED's.

Considering what I want to do, is there any alternative method(s)?
 
You are starting with 12 parallel circuits (33mA per LED, for a total of 12*33 = 396mA), and you are trying to detect that the first wire broke, meaning the current drops from 396 to 363mA.

This is not the way I would do it. Why do you even care which wire broke? Just wire all of the leads in the ribbon cable in series, use that to hold in the relay. When the first wire breaks, regardless of which one it is, then the relay drops, and stops the test. If you gotta have an LED, wire it across the relay coil.
 
Why do you even care which wire broke? Just wire all of the leads in the ribbon cable in series, use that to hold in the relay. When the first wire breaks, regardless of which one it is, then the relay drops, and stops the test. If you gotta have an LED, wire it across the relay coil.

Well, actually it wouldn't bother me at all to know which wire broke, however, those that I am designing it for do care. Maybe they are just lazy? LOL. I suppose they are wanting to know which wire fails so they wouldn't have to actually perform a continuity test when one does. I'm sure your methodology would work, but not sure if it's going to "satisfy" those I'm building it for. Ohhhh well, I can't really see any other way of doing it.
 
Thanks, Mike odom. You're the second person I've heard of the mention of "comparators". I had no familiarity with these at all and doing a little research over the weekend, agree that this would be the best approach. However, since I have never worked with them, your drawing is especially useful. Will the comparators have to be actually soldered into a circuit board, or can I just solder wire leads to them? Thanks
 
well, you can wire anything, but circuit boards make it a lot easier and cleaner... doesn't have to be etched, you can just buy simple perf board for prototyping and wire it together, but the board will hold all the parts in place. Note: not shown on the schematic are the power pins for the comparators... +24V = pin 3 and ground = pin 12.
 
Thanks very much Mike. I figured there was a way to do it without a PLC. Not everyone, however, is willing to do the research to figure out how to do it "old school". I appreciate you helping me "figure it out".
 
There are many people on this site with very good ideas about the best way to do things, I just happened to be first is all...
basically, the more info you give up front about what it is you are trying to do, the more they can help you with it instead of going off on a tangent (which I tend to do quite often myself)...
 
yikes... I was just reviewing the schematic, and I got a decimal wrong... R1-R4 shouldn't be 1.0k, they should be 10 ohm. 33mA across 1k will try to drop 33v, not 33mV like I was trying to do. Across 10 ohm they will drop 330mV. That means R13 can be 1k instead of 100... this will set the reference input to 238mV. If the LED is on, the (+) input will be at 330mV, and since the (+) > (-), the output will be (+), turning the transistor and relay off. If the LED is off (open circuit), then the (+) < (-), and the output of that comparator will be (-). Since the comparators are open collector, you can tie all outputs together, and when one turns on the output goes low and turns on the transistor/relay.
 
...I figured there was a way to do it without a PLC. Not everyone, however, is willing to do the research to figure out how to do it "old school". ....

Here is a solution which is even "older school" (diode OR-gate), with way fewer parts.

Each wire under test ( you can have as many as you want, even hundreds) requires one resistor and one LED. Do not use LEDs that have an integral resistor; buy just bare-assed LEDs.

The wire shunts the LED while it is intact, and if it breaks, the LED current flows into the base of a single switching transistor, indicating which wire(s) broke in the process. For the simulation, I modeled one of the wires as a switch, starting out as a short, and simulating a wire break at one second. The relay pulls-in when the first wire breaks, so you can do what you want with the relay contacts.
 

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It's good to see that people still "tinker" with electronic circuits. I wish I had continued doing so after I left electronics school. My "exposure" to it there was just a taste. Now, I've grown accustomed to simply buying something "pre-packaged" for my application and have forgotten about what it actually takes to design such a thing. I wish to thank everyone for their input and rest assured, I will be back once I start trying to implement some of these practices.
 
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