Wiring LEDs in series

[mapexmbirch]

I have 100 Ultra bright LEDs and I wanted to get mains voltage (240V) and rectify it and then apply the ~340V (rectified mains) to about 80-90 of the LEDs connected in series. I thought this would be simple.  But I just connected one LED to 5V supply and it drew 33mA (no Resistor). I thought putting another in series with it and connecting it to a 9V battery would keep the current the same, (or maybe a bit less) since voltage divides in series and current divides in parallel.   But the current went up to 55mA.  How? Was it because there was no resistor?

P.S the voltage drop across the LED is 4V and it is rated at 20mA.

Also tell me if powering it off rectified mains is a stupid idea.  I feel like it is. I would obviously put a fuse and insulate all the connections.

[Daniyal]

Normally you don't want to connect it directly to the mains because the LEDs are normally driven by fixed current to have certain brightness and it's quite dangerous by doing so, most of the time you would want a dedicated LED driver circuit, and when you connect two LEDs in series they will obviously need more current than just driving one LED there is nothing in your circuit monitoring or limiting the current, and please don't just go and connect them to the mains like you described make sure you know what you doing, this could be dangerous if you don't have adequate knowledge about this. There are lots ICs on the market will do the job, for example you can have a look at the "tinyswitch" from power integrations, have a look at their application notes to get some design ideas.

[Dave]

It is a stupid idea. Your tests weren't a smart idea either. LEDs, like all diodes, are meant to be current driven.
The current in your tests was probably determined by the internal resistance of your power sources (5V supply and 9V battery) and the resistance of your ammeter.

I suggest using a high voltage buck switcher for driving them off rectified mains voltage.
Before you go designing one of these circuits, are you sure you have enough knowledge and experience to attempt such a thing?
Based on your tests, I would assume you are fairly fresh in electronics. Mains voltage can be deadly, if you don't know what you are doing.

[mapexmbirch]

I don't have a lot of experience with LEDs, I assumed that they had some internal resistance which would cause the 4V drop.

[Dave]

If you have a voltage source you want to use to power an LED, use a current limiting resistor in series.
You can calculate the required resistance using this formula:
R = (U_Source - U_LED) / I_LED

As for the power rating of the resistor, you have a very similar formula, you just multiply instead of dividing:
P = (U_Source - U_LED) * I_LED

And please, if you have just started experimenting with LEDs, stay away from mains voltages. Get a decent power supply that goes up to 30V or so and experiment with that. You don't want to end up in a coffin for no good reason.

[IanB]

I don't have a lot of experience with LEDs, I assumed that they had some internal resistance which would cause the 4V drop.

No, LEDs do not behave like resistors and they do not have any kind of fixed internal "resistance". An LED like that could fail to light up at 3 V and could burn out at 5 V.

The important specification for LEDs is the supply current. If the LED is designed to run at 20 mA you need to regulate the current to 20 mA.

If you make a small mistake when connecting them to the mains they could all explode like a bunch of firecrackers. Trying to power LEDs directly from the mains is a Bad Idea.

[mapexmbirch]

Thanks guys

[madires]

There a some special high voltage LED driver ICs, mostly buck converters which are limiting the current.