12V to 6V (3ma max)

Discussion in 'General Electronics Chat' started by gregthegeek, Apr 10, 2013.

  1. gregthegeek

    Thread Starter Member

    Apr 10, 2013
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    I'm new to circuit building but have always been good at figuring out electronics. I'm hoping to get pointed in the right direction.

    I have a bunch of police scanners that operate on 4 AA batteries (4.8V or 6V). I'm sick of replacing the batteries every day (use them for my job). I want to hook up a huge 12V car or lead acid battery to power them. I used my multimeter and measured 1.4ma with the light and speaker off, and up to 2.9ma with the light and speaker on full volume.

    What would I need to build a voltage regulator to go from a 12V battery to 6V with a maximum of 3ma? Will the 7805 voltage regulator work or is there something better?

    I think my biggest confusion is that I don't understand how to shop for a voltage regulator based on the 3ma. Do voltage regulators work in a "up to X milliamps" mindset? For example, if a voltage regulator states "up to 1 amp", will this work if I only draw 3ma or do I need one specific to 3ma?

    I would prefer to keep the circuit as simple as possible in the beginning. I can improve it in time. I'd like to keep the heat down as there will be as many as 4 of these under my car seat. If I need to add capacitors/resistors to reduce noise (either literally audible noise or radio frequencies), then please let me know. I can't stand listening to electrical humming and don't want any interference being generated near my police scanners.

    This is what I'm thinking, but I need 6 volts (I think? Or will 5 volts work fine? 4 AA batteries equivilent): http://www.youtube.com/watch?v=WTylbNtZW3A
     
    Last edited: Apr 10, 2013
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    LM 7806 also exists. It will output 6 volts for you up to 1 amp.
     
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  3. joeyd999

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    Jun 6, 2011
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    The 7805 will not handle an automotive load dump, I believe. Such an event will destroy the 7805 and possibly your equipment. You're much better off purchasing an off the shelf 'battery eliminator' that plugs into the cigarette lighter.
     
  4. YokoTsuno

    Member

    Jan 1, 2013
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    Yes, there is no minimum current required.

    This depends on what you mean by better. There's a 78L05 which allows up to 100mA, comes in TO-92, and is therefore a lot smaller.

    You can also consider a small transistor, zener and resistor. Not sure if you see this as "better".
     
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  5. gregthegeek

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    Apr 10, 2013
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    If AA rechargables are 1.2V (4.8V for 4) and the one time use ones are 1.5V (6V for 4), and my police scanners work perfectly fine on either one, would I be perfectly fine to use the LM7805, or would providing 6 volts with the LM7806 be better? I ask because I'm at the mercy of what I'm able to buy locally (at least while I prototype/play around).

    By load dump I believe you are referring to (for example) when I start my car, the starter sucks the battery for a ton of amps, and therefore there is a large voltage/amp drop? The battery I will be using will not be connected to my vehicle. My theory is that if 4 little tiny AA batteries can power my police scanner for 4-8 hours, then a huge (seperate) car battery should be able to power it for days or even weeks before I need to recharge it.
     
  6. gregthegeek

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    Apr 10, 2013
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    I had to Google what "to-92" looks like. I don't see a screw hole for a heat sink. Does the 78L05 not require a heatsink and/or generate less heat? Physical space is not a concern. In fact, bigger is a little easier for me to solder anyway. What would be some pros/cons of the LM7805 vs the 78L05?

    As for a transistor, zener or resistor.. can you elaborate? I fully expect to need a small circuit board with wire terminals or some sort of solder plastic connector (for ease) and capacitors to handle the "power up" draw, so going with something smaller is not a concern for me. I'm clueless as to the pros/cons of a voltage regulator vs the 3 options you mentioned?

    Thanks in advance to everyone for the quick responses! I'm like a sponge right now, learning more minute by minute. :)
     
  7. gregthegeek

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    Apr 10, 2013
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    Per Wikipedia and my understanding of what I read, TO-220 is basically TO-92 but is larger and capable of mounting a heatsink, and therefore capable of dissipating more heat?

    I suppose the question (in regards to 220 vs 92) would be, do I NEED to dissipate a lot of heat? I don't know if I should expect the voltage regulator to be as hot as an oven or as warm as the palm of my hand.
     
  8. YokoTsuno

    Member

    Jan 1, 2013
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    Are you sure about this 3mA? The idle current for the 7805 is probably more.

    This means that the scanner would only consume 15mW. That seems low to me.

    On a 40Ah battery your scanner would run for a full year if the battery would hold its charge.
     
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  9. joeyd999

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    Jun 6, 2011
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    Please read the link I provided for an explanation of load dump. If the battery is not actually connected to an alternator, then a 78xx or an LM318 should work, assuming your current measurements are correct.
     
  10. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I haven't actually measured the power consumption of a scanner, but something tells me it draws more power that this. Assuming 6V and 2.9mA, that is only 17.4 milliwatts, or .0174 watts. That doesn't seem enough to hear on a speaker, even if the radio is 100% efficient, which it is not. Are you positive about this? Could it be that you had your meter set wrong, and it was 140mA and 290mA? That seems much more realistic to me.

    If so, then you are outside the range of a range of a low power regulator, but well within the capability of a regular LM7806. You probably wouldn't need a heat sink.

    Here is a schematic for a regulator circuit that would work for you. You could use an LM7805 or an LM7806, whichever you prefer.
     
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  11. gregthegeek

    Thread Starter Member

    Apr 10, 2013
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    Oddly enough, I had to have my red probe plugged in to the 10 AMP jack or else I just got 0.00 and the circuit wouldn't close. If the multimeter was looking for 10 amps, then possibly 1.4 is really 14 and 2.9 is really 29? I'll try to take some pictures.

    I apologize for my noobness, and am very very appreciative of everyones help and patience. :)
     
  12. takao21203

    Distinguished Member

    Apr 28, 2012
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    1W resistor, Zener diode. Problem solved.

    40 dollar ticket for presenting automotive gimmick circuit.
     
  13. gregthegeek

    Thread Starter Member

    Apr 10, 2013
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    Here is a photo showing the police scanner, the battery holder, and the settings I used on my multimeter.

    To recap, I need to use a huge 12V battery wired to an empty battery holder. The huge 12V battery will act as 4 AA batteries, and the police scanner should never know the difference.

    And yes, the police scanner is on when I took this photo.
     
  14. tracecom

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    Apr 16, 2010
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    If you look at the label on the back of the scanner, it tells you what size power supply you need. I think it says 6V, 500mA, but it's too blurry for me to be sure.

    And you have the red probe plugged into the 10A socket, but the meter is set to read mA. It won't work accurately that way.

    ETA: Just noticed that you already pointed out that you were using the 10A socket. Don't try the 200mA socket because if you draw much more than that, you will blow the fuse in your meter.

    ETA 2: Maybe you already did try the 200mA socket and blew the fuse, which would explain why you can't measure any current there.
     
    Last edited: Apr 10, 2013
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  15. #12

    Expert

    Nov 30, 2010
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    Let's quit asking about maximum current and assume it is impossible to run these at 3ma. The 7805 chip in TO-220 is a safe bet. It will make heat to the tune of 7V times amps used. I can't believe anyone would expect more than an amp out of AA batteries, so the LM7805 can cover that nicely. 7 watts would need a bit of a heat sink. Try this circuit and feel the regulator with spit on your fingers. If it is painful, add a heat sink.

    The 4 diodes on the left are leftover from when I used this drawing for an AC input. If you leave them in, it will make the circuit work properly, even if you connect the battery backwards. Otherwise, connect the battery correctly.

    The capacitor on the left is .33uf, ceramic, and the one on the right is .1uf, ceramic.

    Edit: if you decide on a 6 volt supply, just use the LM7806 chip in TO-220 package.
     
    Last edited: Apr 10, 2013
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  16. gregthegeek

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    Apr 10, 2013
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    Yes the label says 6V and 500MA. I disregarded that number because I thought that was for the AC charger jack on the side. I suppose I was doing things the hard way trying to read the milliamp consumption, as I was obsessing on simulating the 4-AA battery holder.

    And incase anyone is wondering "why doesn't he just use an wall charger or cigarette lighter charger?". Cause I don't wanna. :p I have my reasons but I won't bore you guys with a long explanation of why, since it's irrelevant.

    Is it just a matter of moving the decimal point, or does doing it this way generate a totally whacked out reading?


    I may be wrong, but I believe my multimeter is either "DC MilliAmps" or "DC 10 Amps" modes? I'll open 'er up sometime and see if I blew the fuse.
     
    Last edited: Apr 10, 2013
  17. tracecom

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    Apr 16, 2010
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    The circuit I previously posted will work with either an LM7805 (5 volts) or an LM7806 (6 volts.) You can feed it with the 12 volts from your battery, and you don't need the 1000 μF electrolytic or the 10 μF electrolytic.
     
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  18. gregthegeek

    Thread Starter Member

    Apr 10, 2013
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    I can understand the flow of the diagrams, but not the specifics.

    [​IMG]

    These are my assumptions:

    U1: LM7805 (1 = input positive, 2 = ground, 3 = output positive)
    TB1: positive/ground of the 12V source
    TB2: positive/ground of 5V to my load
    J1: ???
    D1: silicon rectifier diode 1amp?
    C1: 1000uf capacitor
    C2: 0.33uf capacitor
    C3: 0.10uf capacitor
    C4: 10uf capacitor
    LED1: 6V LED (confirming 6 volt?)
    R1: 1000ohm resistor?

    If I don't want an LED, can I disregard the LED1 and R1?

    What are the 1 2 and 3 in the lower left corner?

    Why are there so many capacitors, and why are they of different numbers?

    What is the purpose of D1?

    The diagram looks a little intimidating.
     
  19. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    If you want the bare essentials, with a battery source, you can try just U1, C2, and C3 (which is what #12 posted.)

    Yes, on U1 pin 1 is the input, 2 is ground, and 3 is the output. With an LM7805 or LM7806, the pins are numbered from left to right with the writing on the chip facing you.

    And with just those three components, you don't need a circuit board. Just solder everything to the LM7805 or LM7806 leads.
     
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  20. gregthegeek

    Thread Starter Member

    Apr 10, 2013
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    [​IMG]

    Ohh, so the equals sign is a capacitor. I understand the upside-down triangle is ground.

    What is the benefit of adding the C1 and C4 capacitors? Is D1 simply for reverse voltage protection?

    Is there a difference between ceramic, electrolytic and metalized capacitors? Google search says ceramics are better?
     
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