# Wiring two leds

I have two questions.

If I have one led 2.2v@20mA and other one 2.2v@15mA, can I use them in series connection? Or do I have to use parallel wiring?

And if I have 2.2v@20mA and 3.2v@20mA leds, can I use them with parallel wiring?

I think answer to both of them is no, but am I right?

The easiest way to use two LEDs in series with different maximum currents is simply to constrain the current to the smaller of the two maximums. If you really want to run the LEDs at different, but similar, currents, it's possible as follows:

simulate this circuit – Schematic created using CircuitLab

D1 sees 20mA nominal D2 sees 15mA nominal

Connecting them in parallel is easy, but you need two resistors, each calculated for the respective Vf, supply voltage, and desired current.

simulate this circuit

Generally for best LED life it's better to run LEDs at less than their maximum rated current, and note that the rated current may be less at the actual operating temperature- precise details will be in the data sheet and application notes for any reputable manufacturer.

Modern indicator LEDs actually tend to be excessively bright when run anywhere near full rated current. 0.5mA to 5mA is often quite sufficient- it saves power and is less of a distraction. The 'power on' LED in my computer case casts a blue-lit shadow several meters across the room even in somewhat subdued light- there is really no need for that.

If I have one led 2.2v@20mA and other one 2.2v@15mA, can I use them in series connection? Or do I have to use parallel wiring?

You can use them in series, as long as the current is below the lower max; that is, keep the current through them below 15mA. Also, you'll need a voltage greater than (at least) 4.4V, but you'll need some headroom for a current limiting resistor.

And if I have 2.2v@20mA and 3.2v@20mA leds, can I use them with parallel wiring?

You can't wire them directly in parallel (only the 2.2V one will turn on if you do). However, if each one has a series resistor, then that will be fine, then those resistor/LED pairs can be wired in parallel.

If you wire the LEDs in series, you only need one resistor in series with them, and both LEDs will have the same current. The resistor calculation must use the total forward voltages of all the series-connected LEDs.

LEDs are not fussy about the current, as long as you keep it somewhat below their Absolute Maximum rating. Lower current will produce less light, but modern LEDs are very efficient. I find that 5 - 10 mA is adequate for most use.

If you connect LEDs in parallel, you must include a current-limiting resistor in series with each LED.