# Resistor help

Discussion in 'General Electronics Discussion' started by Craigsr83, Jul 16, 2014.

1. ### Craigsr83

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Jul 16, 2014
I have a two series circuit with two leds rated 3.20-3.5 with max current 20ma on a 9v power supply. so a total of four leds in two series with a 100Ω resistor on each, which should bring current down to 20ma but when I test forward voltage across each led I'm getting between 4.2-4.5 volts on all leds. I've tested the resistors and both are showing about 100-102Ω. What would cause this other than a bad resistor. I've also tried swapping out the 100Ω resistors with 220Ω resistors with same result.

2. ### Gryd3

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875
Jun 25, 2014
You have 2 parallel LED circuits that consist of 2 LEDS and a single 100Ω resistor in series.
These are then connected to a 9V source.

If this is the case, it does not appear that your resistor is doing anything which is odd if you have measured it to confirm. Can you please post a picture of your circuit?

3. ### BobK

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Jan 5, 2010
What do you get when you measure your 9V supply? Across the resistor?

Bob

4. ### Craigsr83

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Jul 16, 2014
Bob I get somewhere around 8.6 across the resistor. when my 9v supply is not under load it reads about 14v.( using 9v ac/dc adaptor)

5. ### Gryd3

4,098
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Jun 25, 2014
Something is very wrong here.
Can you please supply a picture?
Is the supply one of the 'multi-select' wall warts? Is is set to the wrong voltage?
What reading do you get from the power supply while it is under load?

6. ### Craigsr83

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Jul 16, 2014
about 8.6v I'll supply a diagram. Also I'm only telling half the story. the same 9v supply is ran to a SPDT switch that toggles between the circuit described above and to a circuit that has 6 leds all rated max 3.5v 20ma two series both containing two leds each both on a separate 100Ω resistor and two parallel each having one led both having separate 220Ω resistors. All leds on latter circuit read 3.4v across the leds.

7. ### Craigsr83

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Jul 16, 2014
R1,R2,R3,R4= 100Ω R5,R6=220Ω

8. ### Gryd3

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Jun 25, 2014
From the sounds of things, R3 - R6 function just fine and provide roughly 20mA to the LEDs.

The issue is the R1-R2 side... You had stated you read 8.4V across the Resistor, and 4.2V across the LED which is much higher than the 9V supply you are using, This also means you are pushing 80 mA through the LEDs and pulling 160mA for that side of the circuit...
The opposite side however is only drawing half that...

You have tested both resistors and confirmed 100Ω, and have doublechecked all of your connections? Do the LEDs on the faulty side still function? You had stated you get the same result with 200Ω values on R1 and R2?

9. ### Craigsr83

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Jul 16, 2014
Yes double checked everything and yes they light up but if left on they'll burn out after about 15 minutes.

10. ### BobK

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Jan 5, 2010
If you get 4.2-4.5V on each LED and 8.2V across the resistor that is a total of at least 16.6V. Are you sure you are measuring correctly? Are these LEDs lit up WAY bright? It is possible your "9V" wall wart is putting out much more than 9V when not loaded very highly, but 16.6V in one case and 9V in the other makes no sense.

Bob

11. ### BobK

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Jan 5, 2010
Okay, I think this is explained by the fact that your "9V" wall wart is one of those that puts out 9V at a specific current, relying on it's internal resistance to drop from a higher voltage. What current is it rated for?

I would suggest you use a voltage regulator after the wall wart, or get another one that is regulated.

In fact, it looks like the wall wart is acting more like a constant current source with around 80mA. (8.2V at 100 Ohms is 80mA). The side with 4 parallel strings works correctly because it needs 80mA.

Bob

Last edited: Jul 16, 2014
12. ### Gryd3

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Jun 25, 2014
That's what I am trying to understand... as don't supplies dip under load?
The supply was stated to put out 14V with no load, and apparently 8.6V under load (undetermined amount of load)
Edit: My reply sounded stupid when I read it back... but realized that the bigger working circuit has a different (lower) total resistance, which explains why the supply would dip lower to accommodate it.

13. ### Craigsr83

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Jul 16, 2014
its rated at 9v 200ma. I see what you are saying and will try adding a voltage regulator on that side of the circuit.

14. ### Craigsr83

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Jul 16, 2014
I guess I may have it wrong I figured I was reading 4.2v-4.4v across each led because its two series circuits ran parallel that both sets of leds would both get the 8.6 and that its not divided between all of the leds.

15. ### BobK

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Jan 5, 2010
Next time, when you say "measured", measure it first!

16. ### Craigsr83

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Jul 16, 2014
I did.

(Mod Note: don't be nasty)

17. ### Gryd3

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Jun 25, 2014
Does not mean it was measured correctly, or as expected. Sometimes the little details can through us all off.
When you state you measure the voltage across an individual part, the probes should literally be on each side of the part with nothing else between. No assumptions should be made about what is is based on whether or not it's in series or parallel.

This would have been easier to troubleshoot if the voltage was measured across the whole circuit. (Supply voltage during operation) and the voltage drop across each component by itself.

Alternatively, the comment above was in good humour considering the amount of oddball posts and requests that get brought up here. I've made a couple really stupid assumptions and comments and have always been treated with respect in the replies.