LED Indicator Lamps - Voltage Drop

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 2191U5-24V2191U5-24V 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?

0 Catwell over 12 years ago

Don't draw power from the circuit being tested to check for continuity. Use an external power supply for it.

Alternatively, you could make a microcontroller based circuit monitor. This might be the cheapest solution, with some PICs being less than 50 cents.
All it would do is sample each wire and report back all it ok, or which is broken.

C
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0 Former Member over 12 years ago

I agree with Cabe that a Microcontroller is by far the easyiest way to go. you might could try a change in your thought process. Instead of having the led turn off for a broken wire have it come on via a parrallel wire feed through a current regulating diode for each wire. you would have to do the math and make sure that you couldnt drive the diode unless the wire is broken via a voltage control resistor on the input side of the wire. a plus to having the light come on is that you dont have to pay attention you just glance for a light. hope that this helps
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0 Former Member over 12 years ago in reply to Catwell

I am using an external power supply for testing the wires. The device being tested has nothing but the "through wires" in it that can be connected by user discretion. Oh, how I so wanted to use a microcontroller for the process. But a PLC that's less than 50 cents? Did I read that right? I have started using a "smart relay" microcontroller that is less that $100.00, but I've never seen or heard of one that cost less than fifty cents. Could you elaborate on this?

Also, I've gotten some info from other sources about the use of "comparators" in the circuit.

I'll attach a drawing of the circuit (as originally connected and not working) for reference:
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0 Former Member over 12 years ago in reply to Former Member

is not the whole plc that is inexpensive its the microconroller chip that is cheap u you would still need to buile a board of some configuration for it.

I grabbed these specs off of mouser.com I have worked with them for years and they are usually pretty good this was $1.39

Manufacturer:

Freescale Semiconductor

Core:

ARM Cortex M0+

Processor Series:

Kinetis L

Data Bus Width:

32 bit

Maximum Clock Frequency:

48 MHz

Program Memory Size:

8 KB

Data RAM Size:

1 KB

On-Chip ADC:

Yes

Operating Supply Voltage:

1.71 V to 3.6 V

Operating Temperature Range:

- 40 C to + 105 C

Package / Case:

QFN-24

Mounting Style:

SMD/SMT

Packaging:

Tray

Series:

Kinetis L