Need Advice on a 10x1W LED Strobe Project

[natejp]

I would like to make a 1-20Hz strobe light using ten 1W 80 Lumen LEDs. They have a typical Vf of 3.2V when driven at 300mA.

I will be using a 10v source, and 5x LM317/Resistor regulators, each driving 2 of the LEDs in series. The regulators take the 10v source and match it to the required Vf for the LEDs (~6.4V), while providing the required 300mA. The regulators/Led units will be in parallel, each drawing 300mA, for a total of 1.5A at 10v.

Thus, the total amount of power I need the strobe to control is 10v at 1.5A or 15W.

All the strobe circuits I can find relating to LEDs have to do with lower power applications, and a simple 555 strobe cannot handle the current (1.5A) that it will require to drive my LEDs.

Is there a simple way to use a 1-20Hz variable 555 timer circuit to control a transistor, such that the transistor is the gate for the 1.5A at 10V, and the 555 simply applies the small current to open the gate?

Thanks,

Nate

 

[NickS]

Sure. Are you comfortable using MOSFET's? They will not require a continuous current to turn on and can be found with low series resistances.

 

[natejp]

Thank you for the quick reply. I feel like I can certainly get my project going with just a few helpful tips like yours.

I have never built anything with a MOSFET, but I have played around with the NPN and PNP transistors in my snap electronics kit, so I have a fundamental understanding of how they function.

Having just done a bit of research, I gather that MOSFETs act in an identical way, and come in the same two flavors, "n" and "p."

So, using an n-MOSFET in my project:

The 555 timer circuit would be connected in parallel with the "Source" pin of the MOSFET on the +10V side of my power supply, so that both the timer and the source were +10V. Then, the output of the timer would be attached to the "Gate" of the MOSFET. The + side of my LED/Regulator array would be connected to the "Drain" pin of the MOSFET, and the - end to ground.

Looking radioshack's MOSFET's specs, does a V(GS) of +/- 20V, mean that I can apply any voltage between -20V and 20V to the Gate to open it, allowing my 10V to flow from Source to Drain?

http://www.radioshack.com/product/index.jsp?productId=2062618

Thanks again,

Nate

 

[NickS]

Having just done a bit of research, I gather that MOSFETs act in an identical way, and come in the same two flavors, "n" and "p."

Similar but not the same

So, using an n-MOSFET in my project:

The 555 timer circuit would be connected in parallel with the "Source" pin of the MOSFET on the +10V side of my power supply, so that both the timer and the source were +10V. Then, the output of the timer would be attached to the "Gate" of the MOSFET. The + side of my LED/Regulator array would be connected to the "Drain" pin of the MOSFET, and the - end to ground.

Actually that is backward for an ntype MOSFET and it would operate as ON all the time due to the body diode.

The other problem with using a ntype device above the diodes is that the source voltage (+ side of diode) would be rather high because you want all the voltage to drop across the diodes so if you apply 10V to the gate and you expect almost that much at the source then your Vgs will be almost 0 which will not even begin to turn the FET on continuously.

When using FET's as switches only operate in saturation Vgs > Vt & Vds > (Vgs - Vt) otherwise their on resistance is much higher. And at 1.5A of current it won't take much to burn up parts. In contrast when un saturation mode many FETs can have resistances lower than .05 ohms (112.5mW power dissipation @ 1.5A)
Vds = Drain Voltage - Source Voltage
Vgs = Source Voltage - Gate Voltage
Vt = threshold voltage

So a better option is to use a p type MOSFET with source tied to 10V and the drain tied to the load(diodes/resitor). Then tie the gate to 10V through a 10k resistor and to your timer. When the timer is low(0v) it will pull down the gate and turn on the FET when the timer is high or disconnected the 10k resistor will pull the gate high and turn off the FET.

However this is not enough to finish your project. What are you using to limit the current? Because it sounds like you plan to drop ~10V across 2 diodes that only need 4.8 combined. so either you need to limit your current or you will be cremating semiconductors.

Finally I am not a high power LED designer but I can tell you that you really need to spend some time figuring out how you intend to pull all that heat out of the diodes.

 

[(*steve*)]

I will be using a 10v source, and 5x LM317/Resistor regulators, each driving 2 of the LEDs in series. The regulators take the 10v source and match it to the required Vf for the LEDs (~6.4V), while providing the required 300mA. The regulators/Led units will be in parallel, each drawing 300mA, for a total of 1.5A at 10v.

Major issue is that you will want to set the LM317s up as current regulators, not voltage regulators.

LEDs are a current driven device, not a voltage driven device. Driving them at a constant voltage will cause them to die very quickly (especially quickly if they are higher powered devices)