Boost Buck Converters

[asbo]

If I was to buy one of these could I feed in any voltage, within its range, and always get out a precise voltage?
Even if it was fluctuating?

It pretty much says so in the listing but I thought I'd ask the helpful people on here just to be sure.

[xman]

Yep, it's a buck inverter and that is exactly what the're for doing. Just be careful not to draw to much from it because the current coming out won't be as high as that being drawn into it, (the higher voltage has to come from somewhere). I have one for charging 12volt batteries in the quads from a 6 volt solar panel. It could just as easily be a 5volt plug pak for charging the 12volt batteries.

[asbo]

Thanks xman

[xman]

Yer just be well aware Asbo, if you were to feed in 6v and wanted 12v volts out at 1/2, you would need to be using over about a 1 1/4 amp 6 volt supply to get the required 1/2 amp out. You can also get one chip that does 3.3vdc to 5vDC buck inverters that are set or adjustable ones via one external pot but these chips are limited to a couple of hundred mA output maximum. If you use these buck inverters to work as a normal step down inverter rather than as a step up meaning say drop 24vDC down to 12vDC which they can do, you are then restricted by the voltage drop creating heat as with a normal regulator and as with normal regulators, the more voltage required to drop, the less current they can output also. I'd keep my finger on the chip on the buck inverter when you 1st test it and just make sure it doesn't overheat. You may require a simple heatsink mounted on the chip on the board you linked to depending on exactly what your trying to power from it. The data supplied with that buck inverter claims it to be good for outputting up to over 1 amp. I personally think heatsinking would be required to achieve this level.

[professor_jonny]

just note the -ve's are tied together and not isolated also some of those cheaper devices do have a fair bit of ripple voltage on the output.

they are cheap for a reason.

We use dc to dc switchmode power supplys for running electronics on solar setups as the terminal voltage of the batterys at 28v can kill things.
the draw back is that they do introduce about 20% power losses.

if you only want to boost up a small bit and low current you can make a thing called a jewel theif there are plans on the net.

[xman]

This is the "kit" I made to charge car batteries from small 12volt 35 watt solar panels for remote location pumps etc that run off an electronic timer I made to control when the pump works and for how long. Basically the timer counts a couple of thousand pulses from a RC network and once the count is reached, the chip switches on a transistor that drives a relay and the relay applies power to the bore pump which is actually a 220 volt pump so there's an inverter in there as well and the pump pumps for about an hour which equates to a couple of hundred liters into a water tank on a hill and from there gravity does the rest. It does this every day of the year and in theory requires no maintenance providing it doesn't blow up.
http://secure.oatleyelectronics.com/fil ... Anotes.pdf

And this is the "kit" I made for charging the batteries on the machines in the shipping container while I'm not at the farm. As you can see it isn't exactly a bare bones buck inverter but the theory is the same. My mod to this unit was to add a 10 stage stepper circuit on it's output that uses a very similar circuit to the bore pumps timer however instead of using the output from the chip to drive one transistor circuit driving one relay, this output pulses a 10 stage stepper chip and each of it's 10 stages drives the transistor circuit that in turn drives it's relay so there are 10 relays in all, one driven on at a time that charges the battery in that machine and then steps on to the next machine's battery and so on meaning I only use one charging circuit to keep up to 10 machines batteries fully charged via trickle charging it instead of one charger for each machines battery. There is another gate using in the quad gated chip I use and that is to stop the RC pulses at sunset. That one gate is set up as a voltage trigger so no sun, the timer stops and resumes the next morning when the sun comes up. After about 5 days, the 1st battery is being trickle charged again. It costs me a couple of dollars to build the timing and stepper circuits and being the tight wad I am, this works well.
http://secure.oatleyelectronics.com/fil ... Anotes.pdf

I did look for the "Joule Thief" but could only find ones suitable for simple LED circuits. PJ can you get a link for me that can supply a bit more current?

[asbo]

I'll explain what I'm trying to do.
We've finally got planning permission on our much redesigned extension, huzzah!
We're going to end up with a rather long upstairs corridor with very little, if any, natural light. So I was thinking of having some LEDs running down its length, light it up a bit without having to switch on a mains light.
My idea was to have a few smallish solar panels power the LEDs and keep some 18650 cells charged so they stay on at night.
I have a couple of small panels here that give about 1.3v in the shade and about 4v in the sun. So if I got a load more of them(or maybe built a panel with solar cells), and a/some boost buck converter(s) to keep the voltage at 4.2v to charge the 18650 cells would that work?

We could just leave a mains LED bulb on constantly but where's the fun in that?

Being in the UK the panels are unlikely to be in direct sun light very often, if at all, and I was thinking of just having them on the windowsill and up the sides. That way all the wiring is inside and I can actually do it/supervise. I'm pretty much housebound so messing around with little projects like this keep me sane, well just about

[xman]

Sounds like a job for PJ's Joule Thief......Have a look and see what you think. Sounds perfect to me for your application........
http://www.instructables.com/id/Make-a-Joule-Thief/

Personally I'd hook onto the mains via a plugpak to charge a 12vDc battery and use a white LED light strip solely because it will cost nothing to run and when the sun isn't out for a week as is probably the case in the UK from time to time, your light will continue to work.

[asbo]

I might be wrong but when the sun does come out wouldn't whatever is connected to the joule thief get fried?

I've bought some solar cells off ebay that all together produce 15v 3.6a in the sun, so minus a couple I'll probably break and the lack of direct sunlight I reckon I'll get 8-10v and maybe 1a. Should be able to use just a buck regulator to get it down to ~4.2v and charge some 18650 cells and power one of those cree torch LEDs. They're pretty epic if you haven't used one

[xman]

Seeing as you already have solar panels how about something like this as a controller. I have a few of these type that seem to work fine for lighting when the sun goes down............
http://www.ebay.com.au/itm/CMP-Solar-Pa ... 4172c1d144

Or something like this because it has a cool timer feature and is cheap as shit....
http://www.ebay.com.au/itm/Juta-Mini-So ... 3a7d964c64

[professor_jonny]

DO NOT charge those lithium batterys cells with a constant voltage you must use a proper lithium battery charger do you remember the pictures of buzz's laptop that could happen to your house!!!!

On all-battery.com there is a range of battery bmc's that you can use to charge lithium batterys with auto low voltage cut off and cell ballancing:
http://www.all-battery.com/protectivepc ... packs.aspx

A standard solar regulator will not charge lithium batterys or be safe to charge lithium batterys you could try one of these circuits and use the buck boost or some sort of regulator to provide voltage to charge the batterys using this circuit.

A proper solar regulator is designed to charge lead batterys but you could use one of the regulators above that xman suggests with a 4 cell bmc and that would be bang on 12.8v with the lifepo4 cells you plan to use and would charge nicely from the 13.8 to 14 volt output from the 12 v solar regulator.

[asbo]

DO NOT charge those lithium batterys cells with a constant voltage you must use a proper lithium battery charger do you remember the pictures of buzz's laptop that could happen to your house!!!!

I remember and don't worry I'm researching and planning carefully before I put one of those cells into anything other than a 18650 AC charger

Here's what I was thinking, use a buck regulator to get the voltage down to 4.2v and then feed it into one of these, ignoring the 12v car bit. Assuming it actually works as its meant to it'll charge the cell safely, not charge them if they've been discharged below ~3.6v, not charge them above 4.2v etc etc.
However I don't know if it would be safe to have an LED drawing power from the cell and charger in parallel whilst it was being charged?
I'm not quite sure on a minimum charging current? The charger might stop charging below a certain amount of current but maybe not.
Some of the cells are already protected but I'd add a protection circuit to the ones that are not before using them.

The charger I use from DX was really cheap, from what I remember, and it charges them perfectly, I check them with my multimeter and they're always bang on 4.2v when they're done.

[xman]

How about a battery like this one.....


This one is 12vDC and is rated at 1.4 amps for under $20. It will hook directly up to the solar regulator and your panel providing they add up to more than 14-15 volts will have it charged fully in no time. Then to power your LEDs simply wire the LED power lead through the contacts on the solar regulator that "make" after dark and the system will run off this battery while it is dark outside and charge the battery during the day. The solar charger I linked to without the timer feature has a neat system where when the battery is getting critically flat it automatically kills the power to the load, in your case, the LEDs so it doesn't destroy the battery that will happen if you drain a dry cell battery below about 10.5 volts. I do like complete automated systems but "KISS" plays a big part in my designs. Keep It Simple Stupid means reliability in the long run. The less parts the less parts to go wrong. Of course you would be using either 12vDC LEDs or standard LEDs with a dropping resistor in line so they don't fry by being feed 12volts. You may choose a regulator to drop the voltage or maybe even a LED light strip that run on 12vDC but these will flatten the battery rather quickly as they pull close to 1/2 amp an hour. For this sized battery I'd go a couple of white LEDs like 4-6 and this battery should power them for several hours quite easily.

[professor_jonny]

The charger I use from DX was really cheap, from what I remember, and it charges them perfectly, I check them with my multimeter and they're always bang on 4.2v when they're done.

Just to throw a spanner in the works the batterys you mention come in 2 flavours or li-ion or lifepo4 and they have different terminal voltages and should not be charged beyond 4.2 (li-ion) or 3.6v (life)

I think you would be best to use one of the circuits I mention if you are going the lithium path, as you iliminate over charge and discharge and it ballances while charging the most common cause of lithium failure.

If you stick three li-ion cells in series you will end up with 12.6v perfect to run strip lighting and will charge off a 12v solar pannel dirrectly of a suitable bmc with no regulator.

If you paralell the pack by 2 cells you will get 4.2amp hour depending on the cells you buy and will end up about the a 1/4th of the size of an equalivant lead battery, but in saying that it will be probally be on par or a little bit more than a lead battery and solar regulator.

[asbo]

The charger I use from DX was really cheap, from what I remember, and it charges them perfectly, I check them with my multimeter and they're always bang on 4.2v when they're done.

Just to throw a spanner in the works the batterys you mention come in 2 flavours or li-ion or lifepo4 and they have different terminal voltages and should not be charged beyond 4.2 (li-ion) or 3.6v (life)

Shhh
The two protected cells have information printed on them, max charge voltage 4.2 etc.
The unprotected ones I've charged to 4.2v and they don't even get warm so I'm sure they're ok too

I think you would be best to use one of the circuits I mention if you are going the lithium path, as you iliminate over charge and discharge and it ballances while charging the most common cause of lithium failure.

If you stick three li-ion cells in series you will end up with 12.6v perfect to run strip lighting and will charge off a 12v solar pannel dirrectly of a suitable bmc with no regulator.

If you paralell the pack by 2 cells you will get 4.2amp hour depending on the cells you buy and will end up about the a 1/4th of the size of an equalivant lead battery, but in saying that it will be probally be on par or a little bit more than a lead battery and solar regulator.

I'll definitely use a protection circuit but I think I'll keep them at ~3.7v, either 1 cell per LED or a few in parallel powering several LEDs. I'm planning on using those 3.7v CREE torch LEDs, have you seem then? crazy bright although I'll use the lowest brightness I can find.
Then again I have one here that could easily light the whole corridor on its own so maybe I'll use that.