# Measuring resistance of components to adjust resistors ?

Discussion in 'Homework Help' started by Eledris, Mar 14, 2013.

1. ### Eledris Thread Starter New Member

Mar 14, 2013
2
0
Hello folks,

I'm kinda new to electronics and still trying to figure out how everything is linked together.
I plan to build a small PCB with connectors to attach another circuit and I want to realize a measurement of the connected components in order to adjust either the applied voltage from my power generator (output DC) or to use a switch logic which brings in different resistors to control the current.
I imagine it'll be easier to keep the power supply on a fixed voltage.

Currently I stuck with my scope to measure the resistance of the connected parts. I want to use LEDs which will be connected in series. The quantity of LEDs can differ and therefore the resistors needed to adjust the current should differ accordingly. Instead of having the end user choose the amount of LED's (and if you think forward it also depends on type of LED, manufacturer, etc..) I want my circuit to measure and calculate the correct resistor(s) and then start the switching logic to put these into the circuit.
I read about different methods like using an ADC for doing a so called "dual slope" or the "wheatstone" method. But - frankly - I have no idea how I could realize such a method and which parts I would need.

So - my main question is if it's possible to measure the resistance of a specific group of components without having a "running" voltage/current, but only a measurement voltage/current.

Any hint or ressource I could read to gather further informationwould be highly appreciated.

Thanks,

Ele

2. ### WBahn Moderator

Mar 31, 2012
23,848
7,379
It's very hard to get a feel for what it is you are trying to do.

You want to choose a different amount of LEDs for what purpose?

How would YOU determine that 5 was too few, 6 was ust right, and 7 was too many?

You might take a step back and describe what this circuit is supposed to accomplish. Forget about how it might go about accomplishing it, first be clear one why this circuit should exist in the first place.

3. ### Eledris Thread Starter New Member

Mar 14, 2013
2
0
EDIT: After reading myself more into the topic I understood LEDs are current depended components and they regulate the current themselves from the forward voltage they're provided.
Furthermore I cannot imagine how the correct "value" should be measured without checking the "state" (on/off,brightness lvl) of the connected LEDs.

So therefore I believe my "design idea" just died.

Q: Can you confirm that installing LEDs will always require you to check the forward voltage of the LEDs, check the amount of installed LEDs and calculate the appropriate resistor based on the given DC output of your power supply?

Tks,

Eledris

Hi WBahn,

I want to have different setups of LED's. E.g.: different amount, different color. Therefore the resistor I will need to apply the correct amount of current based on a 48V DC power supply will be a different resistor depending on the type and amount of LEDs connected.
To keep it easy I do not want to use different power supplies for the different setups. Actually I do not want to have different electronics if you want to drive only 1 LED or drive 5 of them.
The purpose is easy: You may decide you want to use 3 LED modules for your installation but your neighbour may need 5 of them.

So my idea was to "measure" the resistance of the LED's when they are connected and before the system is powered up. I could also get an additional "adjust button" block, which will enable this routine and takes care of carrying the right resistors into my schematic.

Hope this made a little clearer what my idea is.

Ele

Last edited: Mar 15, 2013
4. ### MrChips Moderator

Oct 2, 2009
18,165
5,702
It is not appropriate to measure the resistance of an LED because it is not constant and it varies non-linearly with applied voltage. In other words, it does not obey Ohm's Law.

LEDs operate on a constant current where 20mA is a typical value. Most LEDs will work fine with a current of 2 to 10mA.

What you need is a constant current source of about 5mA and this will work with any number of LEDs wired in series so long as the supply voltage is high enough to maintain that current.

5. ### WBahn Moderator

Mar 31, 2012
23,848
7,379
If you choose LEDs that give the desired brightness at the same current, then you can use a constant current source as MrChips said. But you will probably find that the amoung of current you want for this LED is different than you want for that LED. But it's a pretty easy matter to make an adjustable current source so that you can adjust the current to any (reasonable) desired level. Then the only thing you will not be able to do is deal with the case where they want two LEDs in series but they want to change the brightness in one but not the other.

6. ### WBahn Moderator

Mar 31, 2012
23,848
7,379
This is a flawed understanding. Like normal diodes, and LED will let essentially any amount of current flow through it once the voltage across it gets above the knee voltage. That is why you need current limiting resistors or some other external means of controlling the current in them. Without that, you will very quickly have a dark-emitting altered diode (also known as a DEAD component).