Hi All,

I'm new to this forum, I'm doing a personal project which is a strobe light with a blink rate of 100ms every second. I have 2 options for the leds
1. 3x10W Leds at 12v
2. 8x3W Leds at 3.4v
Going for the first one I'll need a step up converter from 3.7V to 12V and capable of providing around 3amps, on the other hand, going for the 3.4V I'll need a voltage regulator capable of providing around 5.6amps.

Currently, I have a working prototype using the 10W leds with the following step up pololu U3V12F12 which is rated for 1.4A max, it works but the question is whether is OK given that it is only getting bursts of 100ms. If there is a better way to achieve this, which I'm sure there is, I'd like to hear it.

Thank you in advance, I appreciate all the help I can get.

below is the schematic of the current prototype using an attiny85 that runs the signaling to the mosfet for the blinking.

 

Hello,

What will be the supply voltage of the attiny85?
As far as I know it can handle 5 Volts.
Your schematic shows 12 Volts connected to the attiny.

The U3V12F12 is not upto the current you need.


Looking at this graph, it is only capable of 300 mA.

Bertus

 

I missed that, I'm using a digispark board which can handle 12v. I put the attiny in the circuit for simplicity.

 

Hello,

Your step-up converter is very weak.
Even the most powerfull version of the step-up converters is to weak for the needed 30 Watts:
https://www.pololu.com/category/243/u3v70x-step-up-voltage-regulators

bertus

 

Hello,

What will be the supply voltage of the attiny85?
As far as I know it can handle 5 Volts.
Your schematic shows 12 Volts connected to the attiny.

The U3V12F12 is not upto the current you need.

View attachment 222549
Looking at this graph, it is only capable of 300 mA.

Bertus

Do you know any alternative that will be more suitable for this. The main issue I have is size, I need it to fit in 70x27mm with all the components.

edited
I just saw your reply above, what other options do I have if I'm limited in space? or otherwise what options do I have for the other options of the 3.4v?

 

So let me ask this newbie question, as I mentioned in the main post, I have that circuit working, and I see every blink is drawing around 1.5A for the 3 led which is half of what they should be drawing. What does that mean? Does that mean that is the max current the converter can provide? what can happen running that setup for a long period of time (100ms blink every second)?

 

Another point is that you seem to be treating the LEDs as voltage driven devices. You need some means of limiting the current. Also for you 3.4 volt parallel configuration you would need a matched set of LEDs so they shared the current between them.

Les.

 

Another point is that you seem to be treating the LEDs as voltage driven devices. You need some means of limiting the current. Also for you 3.4 volt parallel configuration you would need a matched set of LEDs so they shared the current between them.

Les.

Right but current is derived from the voltage right, so If I want to drive a led that has a forward current of 700mah and voltage of 3.4v then I need a regulator that can provide a constant 3.4V and the required current right?
So if I go with the 3.4v even though the battery can provide the required current, I need a regulator that can work in that current and maintain a 3.4v, otherwise, if I say use a resistor to limit the current to the led, as the battery drains the current will as well right? so in that case what is the best approach?


Also, what do you say about my question in comment #6?

 

I need it to fit in 70x27mm with all the components.

What space are you allowing for a heatsink?

 

What space are you allowing for a heatsink?

Given the leds will be on for 100ms every second, I don't think I need a heatsink, in the circuit I have currently working it doesn't' even get warm.

 

Right but current is derived from the voltage right, so If I want to drive a led that has a forward current of 700mah and voltage of 3.4v then I need a regulator that can provide a constant 3.4V and the required current right?
So if I go with the 3.4v even though the battery can provide the required current, I need a regulator that can work in that current and maintain a 3.4v, otherwise, if I say use a resistor to limit the current to the led, as the battery drains the current will as well right? so in that case what is the best approach?


Also, what do you say about my question in comment #6?

And as the current drops, the LED will dim to out. BUT, with a regulator, as the voltage drops, the regulator will stop working. I don’t see much of a difference.

 

The current is NOT derived from the voltage with LEDs. They do not behave like resistors. They behave more like like zener diode. This is why they need to be driven from a constant current source or at least from a slightly higher voltage with a limiting resistor to make the current less dependent on the actual forward voltage of the LED that you are using. ( There is a random variation of forward voltage of samples of LEDs of the same part number.) There is another fault with your design. You step up regulator is only rated at MAXIMUM INPUT current of 1.4 amps. so with 3.7 volts input that is only 5.18 watts The output will be less than this as the converter will not be 100% efficient. So if it was 90% efficient the maximum output would be 4.66 watts.

Les.

 

Your requirements are designed for failure.

What are the pulsed specs, for the 3 watt LEDs?

You may be able to drive them directly from the battery, with that pulse rate. (I would still place a small dropping resistor in series with each one)

Then live with the fact that they will dim as the battery drains.

 

It is a commonly overlooked situation with DC-DC converters:

Power out is ALWAYS less than power in. Sometimes it is much less.​

​

In practical terms it means you must do some calculations which you have so far failed to do and certainly don't appreciate, Let us begin with your output power.

1. 30 Watts @ 12 volts is a current of 2.5 amperes
2. Any DC-DC converter will require more power in than 30 Watts. Let us say it is 80% efficient
3. The required input power is 30 / 0.8 = 37.5 Watts.
4. 37.5 Watts of input power at 3.7 volts 10.135 amperes.

I can tell you this. If you try to take 10 amperes out of a Lithium battery you WILL discharge and destroy it in short order. The 80% efficient figure is a first approximation to what is possible. The closer you get to 100%, the better. On the other hand, at less than 80%, it gets worse very quickly. In order to have a DC-DC converter perform at this level it must be on a specially designed and fabricated PC board. A breadbord just will not do the trick at these power levels. This project is a complete waste of time and cannot succeed. If you want to make a strobe light I suggest you consider a deep cycle marine battery.

 

Thanks for your replies, I do want to say that I do appreciate the calculations, that is the main reason I started this thread, because even though the test circuit works I know it is not right.
You are saying the project is set up for failure, but there are existing commercial strobes similar to what I'm describing being powered by a single lithium ion cell, which means it is possible.
My intentions creating this thread and this project is to learn and make it as good as it possibly be.
Thank you all and I hope to get more information that will help me complete this project the right way.

 

Sure...similar, but 30 watts?

I don't think Papa's calculation accounted for the pulse, which would reduce that 10 amps to 1 average.

100ms per second...right?

I think the 5.6 needed for the 3 watt LEDs divided by 10 is reasonable, that's about 560mA average.

I have no idea how long the charge would last.

 

Thanks for your replies, I do want to say that I do appreciate the calculations, that is the main reason I started this thread, because even though the test circuit works I know it is not right.
You are saying the project is set up for failure, but there are existing commercial strobes similar to what I'm describing being powered by a single lithium ion cell, which means it is possible.
My intentions creating this thread and this project is to learn and make it as good as it possibly be.
Thank you all and I hope to get more information that will help me complete this project the right way.

The reason you see thing on the market is that they have the knowledge and resources to design and make things profitably. In order to learn how these thing are done you have to scale back your expectations. In my youth I was an amateur astronomer. In order to see dim thing far away you need a mirror with a large diameter. You don't just wake up one day and say I want to grin a mirror that is 12 feet in diameter. You start by making a mirror that is 3 inches in diameter and work your way up. I never did have the resources or the knowledge to make that 12 foot mirror. I did make a few smaller ones however.

 

Sure...similar, but 30 watts?

I don't think Papa's calculation accounted for the pulse, which would reduce that 10 amps to 1 average.

100ms per second...right?

I think the 5.6 needed for the 3 watt LEDs divided by 10 is reasonable, that's about 560mA average.

I have no idea how long the charge would last.

With Lithium cells, you have to be mindful of the instantaneous current demand, not just the average.

 

Correct, that's for the OP to determine, I can only assume he knows that.

But, then again...

 

The reason you see thing on the market is that they have the knowledge and resources to design and make things profitably. In order to learn how these thing are done you have to scale back your expectations. In my youth I was an amateur astronomer. In order to see dim thing far away you need a mirror with a large diameter. You don't just wake up one day and say I want to grin a mirror that is 12 feet in diameter. You start by making a mirror that is 3 inches in diameter and work your way up. I never did have the resources or the knowledge to make that 12 foot mirror. I did make a few smaller ones however.

Again thanks for your comment, I must say that I didn't wake up one day and say I want to build a rocket ship. This seems a rather simpler project, yes it is over my current knowledge and that is why I'm posting this.
Thank you any way.