### Network

#### skydiverMN

Jul 28, 2010
11
I've read the sticky about LED circuits but I still have a few questions.

I'm simply trying to create this garage door open indicator, with an additional LED (2 total)
Garage Door Open Indicator

I'm a little embarassed about this one: Is my transformer Health Zenith 125c-a outputting AC? I'd have to say YES from this info: Input: 120VAC - Output: 8VAC-10VA, 16VAC-10VA, 24VAC-20VA.

Assuming that this is supposed to be outputting AC I actually get 9.4v AC when wired up for 8v. No big deal, I just use this number in my calculations.

Okay, so here's what I've done so far:
Using one of the easy-to-find LED and resistor calculators I've wired up a few. For the most part they're lighting up fine, but I'm confused about a few things...

When I measure the voltage at the LED I get 9.4v when measure in one direction, and when I switch the test leads I get something like 1.6v. Not all LEDs have different voltages when I reverse the test leads. Weird, right? Using the calculator, shouldn't my voltage be fairly close to what I've used in the calculator? If the LED is 2.5v, 20mA, shouldn't I just need a 360ohm resistor? (Link for this example) If this example is correct, I would think that I'd have approximately 2.5v at the LED, right? I've tested all my resistors (3 or 4 in series, depending on which LED I am using) and they're all 100 Ohm (98.8 - 99.3 Ohm when tested). I thought that the LED might be turned around, but this didn't visibly change anything (due to the way AC works?).

What should I be expecting with what I'm doing? Is the 9.4v vs. 1.6v depending on how it's installed expected or unusual? Do I need to do something different when using AC output (include a diode)? Am I doing something wrong or interpreting the data that I'm getting incorrectly? Help this newbie out! Thanks for your time.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,081
An LED is conducting only in one direction. the "D" in LED stands for Diode.
If you want to use an LED on AC, add a second diode (plain switzching diode like 1N4148) or a second LED antiparallel to the LED antiparallel meaning Anode of diode1 to Kathode of diode2 and vice versa.
From your measurements I assume you use the Dc measuring range of the Voltmeter. otherwise reversing the connections should not result in different readings. But since the voltage across the LED is AC, your measurements are not very usable.
What voltage you'll see across the LED depends on the type of LED and the current through the LED. Every LED color has a different characteristic. Plus: the voltage across teh LED depends in an exponential fashion from the current. Best thing is to consult the LED's datasheet. There you will find recommended operating conditions including recommended current and the resulting voltage drop.

Regards,
Harald

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
In most cases, 20mA is safe.

If you need to determine which way around to connect a LED, a 4.5V battery (3 AA cells) and a 1K resistor should allow you to determine this with little risk to the LED.

(Note that some flashing LEDs can be destroyed by even a low reverse voltage.)

The best way is to look at the datasheet to determine how to tell the polarity. If you don't have a datasheet (common situation), most LED's polarities can be determined by just looking at them.

#### skydiverMN

Jul 28, 2010
11
still Qs. And this isn't complicated. Maybe it's just me?

So, I've followed the voltage and current recommendations when using the resistor calculator. I've been using the max voltage for a given LED and subtracted about 10% to take into consideration my +-5% resistors. I'm not sure if I need to do this, but it makes sense to me.

Seeing I'm not getting the expected voltage (across the LED) as I think I'd find in a DC circuit, my real question is, am I running the LED within spec (using my resistor calculator and the values from the package)?

I've been working with this LED : here
Current is 28mA (max) (working with 25mA)
Typical Voltage is 2.25, with a maximum voltage of 2.6V (working with 2.5v)
For my 9.4v AC supply, I'm using a 330 Ohm resistor.
Is this calculation correct, regardless of what my meter is telling me?

My entire circuit involves my transformer, a normally-closed pushbutton switch (3A @ 120V), resistors and one LED. I'd like to have 2 eventually, wired in series, but I'm trying to understand what's going on with one.

I might have destroyed my other LED (blinking) as I can't get that one to work at all. Zero resistance when I check the Ohms. I'm not sure if that's the expected outcome on a destroyed LED (open circuit?).

Thanks for the hand-holding!

#### BobK

Jan 5, 2010
7,682
Did you read Harald Kapp's post? If you do not have an antiparallel diode you are most likely exceeding the peak reverse voltage on the LED.

Bob

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
I need to add a section on running LEDs from AC it seems...

#### skydiverMN

Jul 28, 2010
11

I need to add a section on running LEDs from AC it seems...

Yep, I'd agree....

Some questions that I've wanted answering:
1) When designing an AC circuit do you have to worry about + and - like a DC? Or in my case do you simply pick one to be your + and design the LED (and other components like a switching diode 1N4148) based on this selection?
2) What voltage should you expect when measuring at the LED? Supply voltage or the calculated voltage as used in the LED calculations
3) Is adding a diode like a 1N4148 or just wiring up another LED antiparallel enough, or will both cause an issue?
4) How do you determine volts AC at a specific spot in a circuit? Does it work exactly like using a DC voltmeter? My voltmeter has both an AC and DC selection. Or do I just use the DC selection?

These are definitely newbie questions, and I appreciate all the help.
I'm sure that I'll have more, but this'll get me started...

Thanks!

Last edited:

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
Y1) When designing an AC circuit do you have to worry about + and - like a DC? Or in my case do you simply pick one to be your + and design the LED (and other components like a switching diode 1N4148) based on this selection?

Neither. There is no +ve and -ve and you can't just "pick one".

The use of a rectifier diode (I would generally not recommend a 1N4148) will remove half of the AC giving you a voltage which periodically (50 to 60 times per second) varies from 0 to 1.414 times the "rated" AC voltage.

2) What voltage should you expect when measuring at the LED? Supply voltage or the calculated voltage as used in the LED calculations

This depends on the LED.

For a simple LED, common voltages are around 1.8V 2.2V 3.4V. If the LED is actually an array of LEDs (i.e. more than one) then the voltage across it would be a multiple of this. If the LED incorporates a series resistor (a very small number do) then the voltage across it will be one of these voltages plus the voltage drop across the resistor (which in turn varies with current).

The easiest way to determine it -- if you don't have the datasheet -- is to place the LED in series with a battery and a resistor and measure it.

3) Is adding a diode like a 1N4148 or just wiring up another LED antiparallel enough, or will both cause an issue?

In either case you will also need at least a series resistor to limit current. Another option is to wire 2 LEDs in anti-parallel and both will light (for DC only one will light -- this can be the basis of a simple tool to detect polarity.

4) How do you determine volts AC at a specific spot in a circuit? Does it work exactly like using a DC voltmeter? My voltmeter has both an AC and DC selection. Or do I just use the DC selection?

Measuring AC volts works the same as measuring DC volts.

If the voltage is AC, use an AC range. If it's DC, use a DC range.

These are definitely newbie questions, and I appreciate all the help.
I'm sure that I'll have more, but this'll get me started...

No problems.

#### skydiverMN

Jul 28, 2010
11
So, I just built it...

Got the indicator done differently from the link that was included in the beginning. I wanted to make sure it was fully closed, not fully open. I used a micro switch with a roller end, and bent some aluminum bar stock. It took some bending and unbending to get it right, but it works nicely now.

I wired up my transformer using the online calculators and let it run for awhile. No fire, and no burned out LEDs, and that's good enough for me....

Thanks for all the help. Still, AC powered electronics are confusing for me....

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