# 9v battery LED drive circuit



## Dave_H (Jul 7, 2019)

Could someone recommend a simple, low-cost buck converter circuit to drive a single white (or other) LED at up to 20mA, from one 9v battery whose voltage could drop to less then 4v?

I've looked at a number of LED drivers but most are boost for driving multiple LEDs in series, for display backlighting. For long run-time I just want one LED driven.
NCP3066 is not bad but has fairly high internal consumption. Others don't take up to 9v (say 10v max.) on the inputs, or go below 4-5v on inputs.

I use small 9v snap-on caps with LED and switch which work OK but are still wasting most of the battery most of the time.

I have an nearly endless supply of 9v alkalines recovered from battery recycle which are still good, some appear virtually unused (topic for another discussion).

Dave


----------



## archimedes (Jul 7, 2019)

Not sure what Pak-Lite is using, nor what you would consider "low-cost" but you might be able to salvage one from that ?

When I went to check their website just now, it seems they are still on holiday sale too


----------



## Dave_H (Jul 8, 2019)

Thanks, was looking more for a circuit which can be duplicated and modified as necessary for LED current and number of series or parallel LEDs.

Product looks like it may use simple resistor current limiting. Awhile back I bought a 3-pack of similar devices for $5, which have single LED resistor and switch, so $20 each is steep.

Typical 9v alkaline uses 6 AAAA cells (actually LR61 which are similar) and typical smoke alarm runs down to 7-8v on low battery, so lots of capacity remains.

I've opened up high-power 12v automotive lights and found a variety of switching buck drivers like PT4115 which would be good except they don't spec operation below 6v. They might work in degraded mode below that, with one series LED driven.

Using a backlight boost converter with a string of LEDs (at least 4-5) driven at low current might provide suitable brightness and good run-time; another option.

I also experimented with manual switching of series LEDs to compensate for brightness as battery runs down. Light had two parallel strings of three series yellow LEDs. Using a DPDT switch I was able to selectively short out one or two LEDs as the brightness decreased, keeping the same series resistance. I also worked on electronic version using comparators but did not quite get it working; and switching conversion with constant current would give more uniform brightness.

Dave


----------



## DIWdiver (Jul 8, 2019)

The Linear Technology LT3592 should fit the bill at a reasonable price point. 

At a much lower price point would be the Semtech SC4541. At only 5 components plus the LED, switch, and battery, it's pretty close to the simplest buck regulator you'll find. It runs at around 2 MHz though, so you wouldn't want to be sloppy with the wiring. At 400 kHz the LT3592 would be a bit more forgiving.


----------



## Dave_H (Jul 11, 2019)

DIWdiver said:


> The Linear Technology LT3592 should fit the bill at a reasonable price point.
> 
> At a much lower price point would be the Semtech SC4541. At only 5 components plus the LED, switch, and battery, it's pretty close to the simplest buck regulator you'll find. It runs at around 2 MHz though, so you wouldn't want to be sloppy with the wiring. At 400 kHz the LT3592 would be a bit more forgiving.



Semtech part is not bad, although older device using bipolar switching transistor. Efficiency 65-70% suffers a bit at low current which the target (~10mA). Can be used in buck or boost mode.

Some parts have fairly "high" operating current e.g. up to 3mA, does not matter much when driving high-power LEDs. But in low load case, for (example) 3v LED driven driven at 8mA, and part input 8v drawing 3mA, the part is consuming as much as the load itself; on top of that is the conversion efficiency.

I also prefer leaded package such as SOT which is easy to work with, compared to leadless devices.

Monolithic Power MP2410 also looks good: works down to 4.2v, operating current ~1mA, SOT package, high efficiency, Digikey carries it and price not bad.

Someone suggested putting two (reasonably matched) 9v batteries in series, to get around the 6v minimum of some drivers such as PT4115. Each could run down to 3v which is pretty much exhausted.

Dave


----------



## snakebite (Aug 28, 2019)

try 2 whites in series with 100 ohms.
not as efficient as a buck that will suck them dry but works well.
will dim glow weeks after the initial brighter phase.
when it hits "moon mode" for all practical purposes the battery is done.
not much energy left.
i have built hundreds of these for the prepper crowd.
they are a breeder.
a spent 9v gives up its clip to make another light.


----------

