Looking for information on usb chargers

Discussion in 'General Electronics Chat' started by geekoftheweek, Apr 23, 2015.

  1. geekoftheweek

    Thread Starter Member

    Oct 6, 2013
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    I am wondering if anyone has come across any reviews or has any knowledge of automotive usb chargers / power supplies. I'm looking for specifics on voltage ripple and the like. Would one that's designed to run something that doesn't use a battery be any better that one that's designed to be used as a charger?

    I have hacked a few for some projects so far and have noticed differences... the one Motorolla charger I've been using has two inductors as opposed to the one from Wal Mart that only has one. My knowledge of switch mode power supplies is pretty fundamental, but due to what I've read I assume the two are better than one. I don't have access to a scope so I thought I would see if anyone has actually looked into this or not. I do know for a fact the A/D converter on the PIC I'm using seems to like the Motorolla version over the other one. The values seem to hold steady with the Motorolla whereas they seem to fluctuate when powered with the other one.

    I'm working on a few gizmos to make work and life easier that need to be portable and easily powered. I've considered making it battery powered, but since my gizmo will either be near truck or a battery already it would be smarter to use a converter. Roughly calculated I'll be drawing just under one amp. I'll be using a two amp converter to have the extra capacity available and to allow for a margin of safety.

    I know I may be looking for a simple answer to something that doesn't have a simple answer. I'm better at trial and error than my wallet is so any ideas will be appreciated.

    Thanks in advance.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Some of the switchers rely on the presence of a battery to filter the output. Running one into a non-battery load might cause lots of ripple, and maybe even oscillation (bursts of pulses)... Hanging a big electrolytic across the output may cure all of the above...
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    If you are using these to power your circuits (generally a good idea) then stay away from the chargers: they are meant to charge batteries, not provide a fixed voltage over a wide range of currents.

    As far as one or two inductors go I will not comment on a circuit unless I can see the whole thing.
     
    Roderick Young likes this.
  4. OBW0549

    Well-Known Member

    Mar 2, 2015
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    I haven't looked at any automotive USB power adapters, but I imagine their output characteristics are not very different from the AC powered USB blocks I've tested, or the USB cellphone "booster batteries". All had excellent load regulation (<10mV change from no load to full load), good output voltage accuracy (the worst one was about a percent off), but rather crappy ripple. If you use one of these things to power a device like a PIC with A/D converter, plan on having to deal with as much as 200 mV ripple, at frequencies which vary with load current and range from a few hundred Hz to over a MHz. Filtering will be needed.

    I'll second EarnieM's advice about staying away from units that are meant as chargers. If you do use one, test it first to make sure the output behaves the way you need it to behave.
     
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  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Of the dozen of so automotive USB critters I've opened up, all are simple buck regulators, and almost all used variations of the same Motorola chip. Whatever brains are needed by the battery to control its charger usually are in the device or even inside the battery package. My LG phone, Samsung tablet, and all of my wife's Apple goop look out and expect to see a specific charger and put up some kind of warning if it isn't out there, but they charge up just fine with generic chargers.

    If you want really quiet power, whip up an LM317 regulator. Stable output, no added ripple, no inductor math... Depending on the current draw, you might have to get rid of up to 10 watts of heat, but it's quiet heat.

    ak
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
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    Ripple should be very low on the supply from the 12V lead acid battery - the manufacturers chose the 3-phase design for the alternator because the phases almost overlap.

    However - automotive is a very noisy electrical environment, you can get some pretty big spikes from motors, solenoids etc.

    There are plenty of off the shelf protection devices specifically for this reason.
     
  7. hspalm

    Active Member

    Feb 17, 2010
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    USB "chargers" are mostly 5V power supplies. The charging circuit sits in the phone or whatever you connect it to.
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    That sums it up well enough - but to expand on that a little:-

    A USB charger must equal or exceed the USB power specification, it must have a well regulated 5V output that can deliver at least 500mA.

    I've seen units rated as high as 2A in some shops.
     
  9. hspalm

    Active Member

    Feb 17, 2010
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    Just to expand a little more:
    I own a nexus 6 which comes with turbo charging by qualcomm. The charger actually steps even the voltage up if you connect a phone that support turbo charging (12v 1.2A) but keeps it at a safer 5v 1.6A if no turbo charger capable device is detected. Still using USB ports in both end and works with standard cables.
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    Hopefully each device will only negotiate for a higher voltage if they actually need it!
     
  11. geekoftheweek

    Thread Starter Member

    Oct 6, 2013
    42
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    Thanks for the replies all!! I haven't checked back in the last few days...

    I was kind of guessing since the chargers are actually meant to charge batteries they wouldn't have as tight of tolerances as something designed to be powered directly from the converter... in terms of ripple and such. That is what kind of what lead to the question in the first place. I also noticed besides two inductors on the one there was several additional parts. I think I'm going to look for another one of those (I just wish I would have kept the shell). It works well enough for what I'm using it for at the moment.

    I gave the 317 some thought, but then I would have to deal with transients and such that are already taken care of by the off the shelf chargers. I never thought of the charger / battery combination. I'm thinking I need to look into batteries and see how hard it would be to implement a battery into the mix. I just assumed all batteries had external charging circuitry that I have no ambition to try to sort out. If I can find one that has internal charging circuits at a decent price it would actually solve a problem that should never happen, but will sooner or later at the worst possible time. Would I be right to assume then if I were to use a battery it would be more or less clean power not affected by the charger? I'm building a tool that uses a couple PICs to take a some measurements and provide some feedback. I have a working version I want to expand on, but also want to do what I can to ensure as clean of power as I can.

    Time to start poking around digikey again for ideas....
     
  12. geekoftheweek

    Thread Starter Member

    Oct 6, 2013
    42
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    So I poked around... thinking I'm going to go with an external usb battery. I never knew they existed! Besides the hope of keeping the power supply relatively clean it opens the door for some design improvements.

    I still need to look into some sort of power converter for another project so the hunt isn't over yet...
     
    Last edited: Apr 26, 2015
  13. geekoftheweek

    Thread Starter Member

    Oct 6, 2013
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