Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

50 x 3w + 2 x 50w with one power source - c'est possible like?

Status
Not open for further replies.

mattjames1980

New Member
Hi, newb to electronics, i was wondering if it is at all possible to wire

25 3w white leds (3.5Vf, 750mA)
25 3w blue leds (3 - 3.5Vf, 750mA)
2 50w white leds (16-18v,3000mA - apparently?!??!!)

with say one of these?

**broken link removed**?

have been to the led series/parallel wizard but can only do LEDs of one type...???
 
The white LEDs have a Vf of 3.5 V so you can put 13 in series. However that only leaves a tiny voltage so I would suggest 8 or 9 in series. That leaves 48 - (3.5 * 8) = 20 V across the resistor (16.5 V for 9 LEDs). You need a resistor of 20/0.75 = 26.6666 Ω, so use 27 Ω (22 Ω for the string with 9 LEDs). They need to be rated at 25 W or so. The power dissipated will be 15 W but it never hurts to have a safety margin.

You need 3 strings to get to 25 LEDs (two 8s and a 9) so the total current is 3 * 0.75 A or 2.25 A.

The same calculation applies for the blue LEDs, so another 2.25 A

The 50 W LEDs can be in series, and the voltage will be 32 - 36 V, so 12 to 16 V will be left. You need a resistor of about 5 Ω, rated at 75 or 100 W. That current is 3 A.

The total current from the supply is 3 + 2.25 + 2.25 = 7.5 A, so the power is 360 W.

You could cut the current down a tiny bit by increasing the resistors, or put the small LEDs into 5 strings not 6, with smaller resistors.

On big LED currents like that, more sophisticated systems that control the current are common. They may be more efficient and the current it better controlled. Resistors are not controlling the current, they are just restricting it to about the right amount. A lot of power is wasted in the resistors.

You might get away with 4 strings of the smaller LEDs if you use a proper current control circuit. if you have 13 LEDs the voltage is 45.5, only leaving 2.5 V. That would be enough if you are controlling the current. If you try with a resistor, you will get far too much current if the LED voltage turns out to be 3.4 V not 3.5 V

That power supply on ebay doesn't say what the input voltage is. It might not match the mains voltage where you are. It is probably not very well made.
 
Simpler to use an old computer PSU, already has 3.3V and 5V at about 25 A and also 2 x 12V at about 15A

The raw 3 watt LED may be better of being powered of the 5V with individual resistors (2 Ohm, 2 Watt) or as a string of 3 with a resistor (2 Ohm 2 Watt) from the 12 volt supply.

The 50 Watt 12 volt panels will not need any series resistors from the 12V supply as they usually have a small switchmode regulator on board and will happily run from about 8 through to 28 volts without issue
 
Last edited:
hi damika, thanks for that. work for a company that supplies PC's and deposes of WEEE items so should be able to find one or two PSU's sitting about...

will no doubt be pestering people over the next few weeks???
 
The raw 3 watt LED may be better of being powered of the 5V with individual resistors (2 Ohm, 2 Watt) or as a string of 3 with a resistor (2 Ohm 2 Watt) from the 12 volt supply.

You should be aware of the tolerances on the 3.5 V drop. The range quoted for the blue LEDs gives you an idea of how variable the voltage drop is.

You should check the current to make sure that it is correct. If you measure the voltage across the resistor, you can work out the current.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top