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Need help stepping voltage down.

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danielarmstrong

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I work for a company called Shockwatch and I am working on a simple indicator for my test rig. Our units interrupt to the key switches on forklifts and monitor impacts and other variables on the trucks. In order to operate a truck containing one of our units you must log-in with a key on the unit which closes a driver enable relay allowing the lift to start. Currently the only way to test the driver enable relay in-house is with a meter. I am wanting to put an LED on my test rig that will light with the driver enable relay. However, we power our test units at 12 volt dc which is too much for the LED to handle. All I have at my disposal is tons of resistors at various sizes and a handful of caps (25v 100uF). I tried using 10m resistors and the LED wont light when I apply power. When I used the resistors I get a voltage drop from 12 to 2.5 which seems ideal but why wont the LED light? My guess is it is dropping the current too much. I am new to designing circuits and any suggestions would be great.
 

danielarmstrong

New Member
Thank you for your prompt reply. I used a 1k resistor and got the circuit working. Now I'm worried that, since this is part of my permanent test rig, in the event that we need to test a unit at a higher voltage (up to 48 volts) that it will burn my LED out. I am thinking that I need to use relays to circumvent this possibility. I know the basics of relays but I have no clue how to implement one in this situation. The only relays I have found in our bins have the following info on them:

0.6A 125V AC
0.6A 110V DC
2A 30V DC

Would these be ideal for such an application? If not, what should I look for?

I apologize for all of the ignorant questions but I have always been a software engineer and I am quite new to electronics beyond simple repairs.
 

Mr RB

Well-Known Member
One solution would be to use a resistor that gives 20mA through your led, when you attach your HIGHEST input voltage (say 48v). Then the led will still light with lower voltages like 24v and 12v but will not be as bright. You should still be able to see it though.

Ohterwise you might try google for "simple constant current source for LED".
 
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danielarmstrong

New Member
Okay, I got the driver enable relay indicator working. Now I am trying to add an indicator for the key reader. This time I need to up the voltage. I get around 1.9 volts coming in but the LED is very dim on this indicator. I tried adding a transistor but all I managed to do was kill two transistors. How do you properly implement a transistor? I have tried looking at schematics but I just don't get it.
 

Willbe

New Member
You can get constant current JFET diodes that don't much care whether it's 12v or 48v.
You can also wire a xsistor or an IC voltage reg. to supply constant current.
 
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Mr RB

Well-Known Member
You can get constant current JFET diodes that don't much care whether it's 12v or 48v.
...
Can you post the part number please? Or more info on these devices?
 
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