Voltage regulator, multiple loads?

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gregthegeek

Joined Apr 10, 2013
39
Do you have a 120Vac jack in your vehicle?

FYI - describing what your envisioning, and your requirements helps you and us immensely!
Yes but it's an inverter ran off my car battery. And I'm often sitting in my car for hours on end with the engine off, so I only allow myself to use it with the engine on. Otherwise my car battery is dead in 10 minutes. It powers a Macbook Pro right now. The Macbook Pro battery lasts me usually while I'm sitting, and recharges when I drive. But it can last hours, my car battery can't.

My goal is to be "off the grid" (by grid I mean car battery/alternator), but in an emergency I can charge the lead acid by my car, by means of a 120V 6V battery charger temporarily plugged in to my inverter.

I would probably own two big 6V batteries so I can charge one at home.
 
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Thread Starter

gregthegeek

Joined Apr 10, 2013
39
u need a dc - dc converter. from 12 volts to 6v. they use the same switch mode circuit system.

example here:
http://www.ebay.com.au/itm/Waterpro...6V-15W-Max-3A-Power-Supply-E063-/280922899627
I've seen these, but doesn't this generate heat too? It was my understanding that if I broke one of those open, it would just be a LM7806 and some capacitors inside. Heat is my only reason for not using a 12V->6V drop down, and instead just using a 6V battery to power my 6V device (which can operate down to about 4.2V).
 

Thread Starter

gregthegeek

Joined Apr 10, 2013
39
If I bought five of these, would these work?

My scanners label states that it requires 6V via AA batteries. But my rechargables (when brand new and fully charged) are only 4.8V. Additionally, I've tested an LM7805 with my scanners and it works fine. So I know my scanners will operate at 5V.

My question is, if it's taking 12V and making 5V from it, doesn't that generate heat? If not, then why does anyone bother buying (I think "linear voltage regulators is the word) anymore?

http://www.pololu.com/catalog/product/2107
 

Thread Starter

gregthegeek

Joined Apr 10, 2013
39
And I guess as a third option, I could buy just one of these ($25) 5V SMPS that offers up to 7A. All five of my scanners, worst case scenario, would be 2.5A.

So couldn't I just buy this one board (linked below) and run all five scanners off it from a single 12V battery?

http://www.pololu.com/catalog/product/2111

Although I'd prefer 6V, this may be my simplest option... no? Ready-made (basic wire/soldering), reasonably priced, and can power all five radios?
 

tindel

Joined Sep 16, 2012
936
Let's do some math!

Equations I'll use
Ohms law: P=V*I
Efficiency calculation: Pout/efficiency = Pin
Battery life
: Capacity/current = time

The only thing I'm clear on is that at your 9V input the current is 500mA that equals 4.5W Without knowing the schematics for sure, I suspect that at your battery voltage that you will need a similar input power of 4.5W, or this calculates to be 900mA, or 4.5A with 5 scanners. This equates to you needing in the neighborhood of 22.5W of power to power all 5 scanners.

I'm guessing this is pretty close since 4 high quality AA NiMH batteries at 2.3Ahr gives you 9.2Ahr of capacity, or at 900mA, about 10 hours of life. That makes them last about one day, and will surely not work two days. So, I'm guessing I'm in the right ballpark, at least.

So let's evaluate your options:
6V battery - 100% efficient. But this battery is only 4.5Ahr, so will only last about 1.2 hours powering all five scanners.

http://www.globalindustrial.com/p/e...mpaignId=T9F&gclid=CICj_IvFhrcCFSdgMgodxlAASg

12V battery with your 6V linear regulators
- Your output power will be 22.5W. Your input power will be 45W, so your efficiency will be 50%. Assuming your 12V battery is about 9Ahr, it will last you about 2.5 hours.

http://www.apexbattery.com/12v-9ah-..._content=pla&gclid=CIvxxs3GhrcCFcFAMgodZn4AKw

12V battery with 5V SMPS - The SMPS you found is 90% efficient with a 12V input (good job finding this thing - I looked for hours last night and couldn't find one I was satisfied with!). Your power output is still 22.5W, and at 90% efficient your input is 25W, or 2A out of your battery. This solution (assuming a 9Ahr battery) will last you about 4.5 hours. Still not enough to get through an entire day. You could switch out batteries at lunch time though. Or get a higher capacity battery - I did find a 20Ahr one that would get through most of the day, but it's was about $100, and much larger.

Some valuable lessons in this post:
Battery capacity per volume of AA batteries is incredible compared to Lead Acid
The improved efficiency of a SMPS increase battery life significantly over a linear regulator.

( Being very careful to not upset the powers that be: )
Your real problem is you don't have a charge source for your battery. You really need two (or more) batteries in your car if your car will only last 10 minutes with your key in it and not running. It is against the terms of service for me to offer suggestions on how to do this. There are other forums that specialize in these types of modifications that will help you though.
 
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Litch

Joined Jan 25, 2013
85
Do yourself a favour and stay the hell away from linear voltage regulators when dropping voltage by 6V and sucking 0.5A - they will burn out in seconds without an appropriate heatsink (and still get rather hot with one) and the circuit to alleviate this is just complicating your needs.

There are easy drop-in parts like: "TSR 1-2465" (or similar) that will suit your needs just fine - They're basically (non-isolating) mini-SMPS that are designed to be drop-in replacements for typical voltage regulators - I use a 5v 1.5A model to power a Rasberry Pi and some peripherals (900mA) and it doesn't even get warm dropping from 9V.

They are efficient and all you need is a handful of these, some capacitors and presto - as many 6.5v @ 1A power rails as you need (or get the cheaper 5v 1A models).
 

Georacer

Joined Nov 25, 2009
5,182
@ gregthegeek,

You have started 4 different threads for the same project. This not only clutters the forum, but most importantly makes it very difficult for the other members to help you effectively.

All threads will be closed, except for the original one. Please continue the discussion there.
Do not open another thread for the same project.

The threads were not merged, as it would cause confusion from the overlapping answers.
 
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