# How many LEDs from fixed supply

1. Nov 4, 2015

### John1397

How many led's 20ma at 3 volt can one run from a fixed supply voltage of 110 volt dc and a maximum supply of 20ma? I come up with 36.

2. Nov 4, 2015

### Staff: Mentor

You can power more, but then they'll be dimmer.

3. Nov 4, 2015

### vk6kro

You would have to include the voltage drop across a series resistor.

A rule of thumb is to allow 25% of the supply for a resistor.
So 25% of 110 volts is 27.5 volts.
A resistor dropping 27.5 volts with 20 mA flowing in it would have a resistance of 1375 ohms.
You can't buy these, so you could use 1500 ohms.

Assuming 1375 ohms, this leaves 82.5 volts for the LEDs.
Divide by 3 to get 27.5 LEDs.

Call it 27 LEDs in series with a 1500 ohm resistor.

If you decided to try this, beware of high voltages. You can use them, but obviously the wires and connections must be covered before you apply power.
Also 110 volts DC is not the same as 110 volts AC which comes from wall outlets in some countries.

4. Nov 4, 2015

### meBigGuy

1. saying 110VDC makes me suspicious. What is the source of your 110VDC, and have you measured it. (just checking)
WHy 110VDC and a single series string rather than a lower voltage and multiple strings? (safer, etc)

2. The voltage drop across each diode can be very different. That is, not all devices are identical. And not all will be the same brightness.
The spec sheet may have typical and worst case numbers which illustrates the issue. Across 30 LEDs this can be significant.

3. A series resistor (as vk6kro said) or other current limiting device is important because of the device variation. 20ma * 27.5V = 0.55W, so get a 1 watt resistor.
Here is a current source example: http://www.instructables.com/id/Power-LED-s---simplest-light-with-constant-current/

4. There are switching LED string driver IC's. http://cds.linear.com/docs/en/design-note/dn461f.pdf, for example

5. Nov 5, 2015

### CWatters

+1 to that.

Are you sure the power supply isn't really a 20mA current source that will work up to 100V ?