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[Gaffers]

I am working on a project where I need to step down 12v to 7.4v to power this:

https://www.7dayshop.com/products/7dayshop-bike-light-mega-bright-t6-cree-led-front-bicycle-light-with-headlamp-strap-kit-DS-009

I am using a 12V DC 12680 battery (or a couple in parallel if the 6.8Ah is not sufficient)  Of course I would like something that is as efficient as possible, thus no big resistor burning wasted energy.  There are other devices that will be powered from the same power source, some 12V, some 5V, etc.

I know of lots of step downs to 5V but nothing for the 7V mark.  I don't mind building something bespoke if I have to but I am stuck

[tigers_gonads]

At the end of the day, if you have a 12 volt dry cell power supply and you want to drop it to a lower voltage you have no real choice but to slap a resistor in the circuit 

[Marks DTM Calib]

I am working on a project where I need to step down 12v to 7.4v to power this:

https://www.7dayshop.com/products/7dayshop-bike-light-mega-bright-t6-cree-led-front-bicycle-light-with-headlamp-strap-kit-DS-009

I am using a 12V DC 12680 battery (or a couple in parallel if the 6.8Ah is not sufficient)  Of course I would like something that is as efficient as possible, thus no big resistor burning wasted energy.  There are other devices that will be powered from the same power source, some 12V, some 5V, etc.

I know of lots of step downs to 5V but nothing for the 7V mark.  I don't mind building something bespoke if I have to but I am stuck

Switchmode, LT do some good ones and had demo circuit boards with all the bits on, worth a look

[tigers_gonads]

I am working on a project where I need to step down 12v to 7.4v to power this:

https://www.7dayshop.com/products/7dayshop-bike-light-mega-bright-t6-cree-led-front-bicycle-light-with-headlamp-strap-kit-DS-009

I am using a 12V DC 12680 battery (or a couple in parallel if the 6.8Ah is not sufficient)  Of course I would like something that is as efficient as possible, thus no big resistor burning wasted energy.  There are other devices that will be powered from the same power source, some 12V, some 5V, etc.

I know of lots of step downs to 5V but nothing for the 7V mark.  I don't mind building something bespoke if I have to but I am stuck

Switchmode, LT do some good ones and had demo circuit boards with all the bits on, worth a look

Tell me more 

[Gaffers]

Having a look now, and very promising.  The output voltage is a bit vague.  the LTC3633A-1 for example has 1.5-12v output.  How do I specify 7.4v or does it magically detect what is required? 

[zirk]

Having a look now, and very promising.  The output voltage is a bit vague.  the LTC3633A-1 for example has 1.5-12v output.  How do I specify 7.4v or does it magically detect what is required? 

Normally done with specific components on the feed back rail.

What sort of current are you looking at your required voltages?

[Gaffers]

Tough to say as the product spec does not state the current.  The battery supplied is 6Ah and lasts 2 hours in full beam thus suggested peak current is 3A.

[Gaffers]

I have just found a 12v to 7.5v step down.  Would being 0.1v over the working voltage cause issues?

[Kevin Wood]

Having a look now, and very promising.  The output voltage is a bit vague.  the LTC3633A-1 for example has 1.5-12v output.  How do I specify 7.4v or does it magically detect what is required? 

That is a dual regulator chip, so maybe not what you want ideally, but you'll see the pins Vfb1 and Vfb2 are fed from potential dividers from the output. The chip drives the output to keep these at a reference voltage level, so you calculate the divider to give you that reference voltage when the output voltage is correct.

I'd second Mark's suggestion to get an eval board for a regulator, as component choices and layout can be important.

[zirk]

I have just found a 12v to 7.5v step down.  Would being 0.1v over the working voltage cause issues?

No, should be fine, this is for a led bulb, correct.

[Gaffers]

I have just found a 12v to 7.5v step down.  Would being 0.1v over the working voltage cause issues?

No, should be fine, this is for a led bulb, correct.

For a curcuit which drives a LED bulb.  I am guessing that the battery supplied, when fully charged, probably supplies over 7.4v anyway and reduces to below 7v as it depletes.

[Kevin Wood]

If you want to roll your own, the LM2576 is cheap, gives 3A output and needs minimal additional components. IIRC, there's an adjustable version you could configure to give the voltage you need. If you've found something off the shelf, then probably not worth the bother, though.

[zirk]

I have just found a 12v to 7.5v step down.  Would being 0.1v over the working voltage cause issues?

No, should be fine, this is for a led bulb, correct.

For a curcuit which drives a LED bulb.  I am guessing that the battery supplied, when fully charged, probably supplies over 7.4v anyway and reduces to below 7v as it depletes.

Yep.

Good thing about using a regulated  output though is as your voltage source (12v battery) drops in voltage after time, the regulated output voltage will be constant, up to the point where the battery voltage drops below the regulators requirement, if that makes sense.