12V batteries voltage step down

Franny

Howdy!
What's the best way to step voltage down when using the propeller with 12V SLA or 12V (or higher) Lithium battery packs?

Franny

I'm using the LM317 right now but not sure if that's the best way to do it.

kwinn

Franny wrote: »

I'm using the LM317 right now but not sure if that's the best way to do it.

Definitely not the best way. Almost 75% of the power the battery provides is wasted as heat. A small switching regulator module would reduce that to less than 10% wasted.

Mike Green

An LM317 or LM78xx will work, but a lot of power will be dissipated as heat and you'll need a good heatsink. Better would be to use some kind of buck switching regulator. There are all sorts of these on the market with anything from just parts plus application notes to ready-to-go drop-in replacements for an LM78xx. I use a Dimension Engineering 3.3V or 5V fixed regulator ... a little over $15. They have adjustable and higher current versions as well.

gis667en11

I put together a list of Standard ICs and Solutions on reddit.com/r/electronics awhile back; I found this switching regulator was pretty common with low power, low cost regulators:

MC34063A- 8 pin IC for buck, boost, or inverting switching power supply design. It appears that the external components set the output voltage with a 1.25V comparison between two pins- much the same as designing a circuit for the LM317. Available for 10 cents each on ebay. (Or 50 for about $2.50)

I'm not 100% it can take a 12V input though, so that might be a deal breaker.

EDIT: Just checked- it can take up to 40V input. This thing's a freakin beast. Must be why I have a drawer full of them in my office.

Cluso99

This is a nice switching regulator in a package
R-78E5.0-0.5 from Mouser, Digikey, Element14

gis667en11

The R-78E5.0-0.5 is a 5V regulator; the R-78E3.3-0.5 will output 3.3V. That's a pretty cool little packaged regulator, though!

altosack

The propeller project board has an excellent switching power supply on it that can accept 16V. I use it to connect to 12V SLA batteries; it goes just high enough to allow equalization.

Franny

something like this maybe?

http://www.ebay.com/itm/LM2596-DC-Power-Supply-Step-Down-Adjustable-Buck-Converter-Switching-Regulator-/331327483066?hash=item4d24a82cba:g:z~UAAOSwPe1UIRbX

Franny

it says "Conversion efficiency: Up to 92% (output voltage higher, the higher the efficiency)"

AV_Tech

Hello Franny.

I use a similair bord with the same IC LM2596 on it, with very good result. But the input limit is 32 Volt. Not a problem for you, but think about this in an other prodject.

Greeting Abraham.

LoopyByteloose

I have had a LM2596 device running a Propeller 24/7 for many months now (maybe a year or so). It is supplied with 13.5VDC that comes from a wall wart and also trickle charges a Lead acid Gel cell in parallel. The Propeller Proto Board takes 5.5VDC 5.0VDC and runs it down to 3.3VDC.

I communicate with the Propeller via Bluetooth and am currently changing over to Wifi.

Works fine. There is a blocking diode to prevent excessive discharge of the gel cell when the wall wart is connected but power is off.

The beauty here is that you can buy a 13.5VDC 1 amp wall wart and directly hook it up for a 24/7 trickle charge -- nothing fancy to create a charger. Whenever there is a power outage, the gel cell will keep the Propeller running.

Lead Acid batteries will behave nicely with a trickle charge 24/7 of about 13.5 VDC. A bit less due to a blocking diode is no big deal. The same cell will fast charge at 14.2VDC, but I never bother with fast charging.

Linear regulators of any sort will work, but create a lot of heat that may cause other problems, including just wasting more electricity.

Mark_T

And LM2596 DC-DC converter boards are very cheap on eBay - you can get a panel of boards for
not much at all, each adjustable with a 10-turn present. There are one's with a 7-seg LED voltmeter
built in too.

Franny

Thanks everyone, I got this one for now

http://www.ebay.com/itm/LM2596-Buck-Step-down-Power-Converter-Module-DC-Voltmeter-4-0-40-to-1-3-37V-/262136734501?hash=item3d08912325:g:uOEAAOSwEgVWRFLz

Franny

I wonder if I need a fuse between the battery and board...

DigitalBob

I always use the LM2940-5.0 and the LM2937-3.3 the same ones you get in the prop education kit. The prop data sheet says the prop alone can suck up to 300 ma of power depending on your set up, configuration , accessories etc. So I would calculate my total load and then select the battery pack size ma hour and reg. Etc.

Franny

I use them too, however, this project has four modules and one of them works on 6V. How do you go about calculating your total load?

LoopyByteloose

Franny wrote: »

I wonder if I need a fuse between the battery and board...

Fuses are protective devices. The basic question is what do you desire to protect?

In higher voltages and higher power situations, we use them to protect ourselves from fire. In some cases, they are used to protect a device from extended damage if a wrong situation occurs - like a motor is stalled.

So it seems that many low-voltage, low-power situations we just omit the fuses and accept our losses.

kwinn

Franny wrote: »

I use them too, however, this project has four modules and one of them works on 6V. How do you go about calculating your total load?

For linear regulators the current draw from the battery is the same as the current drawn by the circuit it is powering plus the tiny bit the regulator draws.

For switching regulators it is based on the power draw of the circuit and the regulator efficiency. For example, if the circuit draws 100mA at 3.3V, the regulator is 90% efficient, and the input voltage is 12V the circuit draws 3.3/0.1 = 0.33W. At 90% efficiency the 12V supply would need to provide 0.33/0.9 = 0.367W. The current draw from the supply would be 0.367/12 = 0.031mA.

DigitalBob

You can calculate each module in watts. Example volts X amps will give you watts. So 6 volts X 500 ma would give you 3 watts. ( 6 X .5 = 3) The 6 volt would be ok for the 3.3 volt devices. I would design my power pack for the highest voltage module. So if the highest module is 12 volts then I would use a 12-14 volt power pack and use separate regulators to drop then voltage for the other modules. Like car and truck systems are 12 volt but the alternator actually maintains the system at 14.5 volts. Look at the data sheets for all of the devices in each module add up all of the Ma or amps for each module. Remember in power systems you can have high voltage and low amps and low voltage and high amps but the same wattage. 12 volts X .250 Ma = 3 watts vs. 6 volts X .500 Ma = 3 watts. Or use all 3.3 volt devices in all of your systems like an IPad. Those Apple battery packs do have a pretty good cycle life.

Franny

Interesting, once I get the total wattage for all the modules how do I pick the right fuse?

Peter Jakacki

12V batteries are normally some type of lead-acid and therefore are capable of many hundreds of amps, I've even cranked a stranded 4WD with a small 7AH job. So it is never a matter of voltage no matter how high or low, it is a matter of safety and hundreds of amps is a fire or explosive hazard. Rate your fuse much higher than your load, you don't want it blowing when you draw a few amps too much but if there is catastrophic failure it needs to work then. However I personally hate fuse and prefer polyfuses or even just a suitable non-flammable resistor on a higher voltage supply.

So using a 5A fuse for a 1A circuit is more than fine and besides regulators tend to limit the current anyway, except of course in the case of catastrophic failure.

Franny

So basically about 5 x the amperage for the fuse rating?

Peter Jakacki

IMO you could even make it 10A but certainly not close to your 1A requirement, so at least 3A and up to 10 even.

Franny

And what about the volts rating on the fuse?

gis667en11

Fuses blow based on current. In theory, a fuse would have fairly little resistance, right? You wouldn't want to put a fuse on the positive lead from your 9V power supply and have 8.2V on the other side going to your components. The voltage rating on fuses represents the hold off limit for after the fuse pops. If you're thinking of putting a fuse on the positive lead from a 12V power supply, I'd think any fuse rated 30 or 40VDC and up would be just fine. Most fuses are rated much higher than that, though, and the higher the better (so long as the cost is comparable).

LoopyByteloose

Electricians size fuses in housewiring according to size of the house wiring. The idea is to prevent the wires in the walls from getting too hot and actually starting fires. I think you won't find many easy to replace fuses rated 30 or 40 volts, more likely they are rated 125-250VAC or higher -- it is all about having enough insulation in a failure mode. But you may find automotive fuses in lower voltage ratings.

On a gel cell, it is just wise to limit the battery's output to something sane in case you make a wiring error. If you are using 18 gauge copper wire or larger for primary power wiring, you can easily fry small devices while your distribution remains intact.

I have used a 3 amp fuse on a gel cell, just to deal with any really stupid wiring errors - short circuits and solder bridges in constructions. I could just as easily use 5 amps. I have blow a few too. If i were to have 10 amp fuses in a microcontroller project, there would have to be an obvious reason to need that additional 7 amps, such as driving motors from a standstill. If you get into large values, you may end up with a bag of fuses you never use.

The ideal fuse would be zero ohms, but that will never happen in the real world. The small resistance is not an issue in most situations.

My multi-meter uses a 250ma or 1/4 amp fuse which is about as low as one can go without having some problems with the fuse actually becoming an added resistance element as well. So don't bother with really tiny ones unless you have a specific device that requires them.

Such small fuses really can create more problems than they are worth. Diodes, transient suppressors, and current limiting resistors seem to take on the role of protection devices in the low voltage, low current world of ICs, or driver chips end up the scraficial element.

kwinn

For many years I have followed the 2X rule for components. Selecting components rated for approximately two times the maximum expected current, voltage, or power dissipation has resulted in very reliable long lasting projects for me.

Peter Jakacki

Fuses rarely protect the electronic circuit to which they are connected as when it is a device failure then it has failed and the fuse then disconnects the power but in terms of electronic components it takes an eternity to do so. If the fuse wasn't in there though then you could have cascade failures and a very good chance of excessive heat, smoke, and possibly fire. So a 2A fuse won't protect a 1A regulator circuit any better than a 5A one will.

Franny

So basically what you are saying is that it's better to have a 5A fuse so that it doesn't go out too soon on ya?

kwinn

Peter Jakacki wrote: »

Fuses rarely protect the electronic circuit to which they are connected as when it is a device failure then it has failed and the fuse then disconnects the power but in terms of electronic components it takes an eternity to do so. If the fuse wasn't in there though then you could have cascade failures and a very good chance of excessive heat, smoke, and possibly fire. So a 2A fuse won't protect a 1A regulator circuit any better than a 5A one will.

True, the fuse does not protect the components on the board, but having a correctly rated fuse makes it less likely that the traces and circuit board will be damaged by excessive heat or fire.