In this case I would be tempted to replace the 10 LEDs with a single 1 or 3W LED, it will fit easier and can be driven from the 3.7V LIPO with a simple constant current generator with minimal power loss, anything else will probably dissipate as much heat in the resistors as the LEDs them selves.

something like this should last close to 7Hrs on that battery http://www.newark.com/cree/mx6awt-a1-r250-000d51/led-high-brightness-cool-white/dp/25R3684?ost=MX6AWT-A1-R250-000D51

the trick is to not let the battery get too flat or it will destroy it (The battery), so some kind of cut off circuit will be needed

I assume your making some kind of night light ?

peter

In this case I would be tempted to replace the 10 LEDs with a single 1 or 3W LED, it will fit easier and can be driven from the 3.7V LIPO with a simple constant current generator with minimal power loss, anything else will probably dissipate as much heat in the resistors as the LEDs them selves.

something like this should last close to 7Hrs on that battery http://www.newark.com/cree/mx6awt-a1-r250-000d51/led-high-brightness-cool-white/dp/25R3684?ost=MX6AWT-A1-R250-000D51

the trick is to not let the battery get too flat or it will destroy it (The battery), so some kind of cut off circuit will be needed

I assume your making some kind of night light ?

peter

Hi Peter - Whenever I try to answer one of these questions I am always pleased when one of the fellows, like yourself, with more knowledge and experience add their insight. I have come back to your recommendation for a constant current generator to power the LEDs to try to learn something new. I have attempted to build a constant current generator using a MOSFET and a 741 OP amp. While I can get it to work just fine at higher voltages I have not been able to get it to work under the circumstances that Dave is encountering. He has a 3.7 volt battery and the White LEDs have a forward junction voltage of 2.8 volts. Did you have a circuit in mind for this application or were you thinking of using a larger supply voltage than 3.7 volts? I am guessing that my circuit is not working as there is only 0.9 volts to work with between the LED's and the battery's 3.7 volts. I know you are currently busy with your project so there is no rush in answering this.

Thanks John

I have been thinking about this as a side project, but I'm having trouble understanding it and i have to admit I'm getting back into electronics that i haven't seen since college and your help would be appreciated.

thanks Dave

Hello John,

Although I don't have your circuit (you could post it) I suspect that your constant current source won't work at low supply voltages because you are using  a 741 op amp. This device was first designed and sold by Fairchild in 1968 so the design is 46 years old. Things have moved on.

Looking at the TI data sheet (SNOSC25C –MAY 1998–REVISED MARCH 2013):

The part is only specified for operation with +/- 15V supplies - it'll work with less but they don't guarantee anything.

The input voltage range is worst case +/- 12V - this means that if either input gets within 3V of the supply rails then the amplifier won't work properly, so the least supply you could ever use is 6V.

The output voltage swing is worst case +/- 12V so once again it can't drive near the supply rails.

For your current source application pick a rail to rail input and output op amp (often described as RRIO in data sheets) so that you can work with inputs and outputs near the supply rails. Pick one which is specified to work with a 3V or less supply, (but OK with maximum).

You could consider the Microchip MCP6281 but watch out for the maximum 5.5V supply.

It is getting hard to find the spec you need in dual in line packages - there is much more choice in surface mount.

Make sure the MOSFET in the current source will turn on enough with the gate voltage you have available.

Image is for illustrative purposes only.
Please refer to product description

 

Manufacturer:

MICROCHIP

Order Code:

1439456

Manufacturer Part No:

MCP6281-E/PMCP6281-E/P

MK

Hi Michael -Thanks for your insight. You are surely correct. I took the circuit esign from an example in "Practical Electronics for Inventors" and I am sure it wasn't intended for a 3.7 volt source. Your comment is exactly what I like about this site. I learn new things and am starting to progress from the 46 year old technology that has been the basis of my electronics for too long. I lament the loss of the through the hole components as they were easy to work with but I am finding ways to adapt to the surface mount. Hopefully I can learn to use the surface mount before they come out with the next big paradigm shift in design. I will research the MCP 6821.

John

Hi Michael,

It has been two weeks since you told me about the MCP6281 Rail to Rail Op Amp. I just want to tell you that I got some from Newark and have wired one into the constant current circuit that I was trying to make work with the 741. It worked beautifully. I have no present application but I learned a lot from the exercise of trying to help Dave Bailey with his LED question and from the follow up that you and Peter gave me. Here are two pictures of my test setup. One shows the MosFET gate voltage and drain current with one LED and the second picture is of the same setup with a second LED in parallel with the first. The drain current remained rock solid at my preset 4.7 mA.

Thanks Again for your expert insights.

John

I'm happy that it's working for you. Do you have an oscilloscope and  pulse generator. If so it can be very interesting to look at the current (by looking across the current sense resistor)  while applying step changes to the current demand. Sometimes you get a lot of ringing and even oscillation - then you can experiment by adding a capacitor in the right place to control it.

MK

Just noticed something John

The diagram on your text book and my diagrams on the PSU project pages are fundamentally the same, I have not included the OP feedback circuit yet, that's for a bit later in the tutorials but the driver I'm showing (Up to 10A vs  20mA for an LED)

Power Stages - The Modular Bench Power Supply ++

Hi Peter, It just goes to show that great minds think alike. I am following your PSU tutorial with interest.

John