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Old 11th Jul 2018, 10:38 pm   #1
Oldcodger
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Default 12V to USB convertor.

not sure if this is right place, as it could be circuits, or Modern or here.
I know that the "car " word is disliked, but it's where the kit is used, so it features and is very relevant.
Dashcam uses a USB supply derived from 12v supply to drive it. In normal use this is from a cigarette lighter plug to a USB socket.
From what I've read ( USB voltages etc are much of a mystery to me) , voltage is 5v, at varying current depending on units supplied ( for usb2 it's 100mA per unit) , and I'm getting sick of connection and fuse failure problems. And tthere's fact that my 12v socket is on permanantly. So simplest solution to me would be to use a 7805 from a switched supply. I've got stacks of 2A 7805, so anyone see any problems of the old 7805 circuit ,with something 330nF in and 100nF out ,or would it also need something like a 10-22uF on the output.
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Old 11th Jul 2018, 10:44 pm   #2
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Default Re: 12V to USB convertor.

You can buy cigarette lighter to usb converters for pennies

Poundland have done them in the past.

Most cars in the last 8 years ( well japanese and korean) have had USB as standard.

My Dashcam runs off just such a supply.

https://www.ebay.co.uk/itm/Dual-Port...gAAOSw~gRVqJ~v

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Old 12th Jul 2018, 12:19 am   #3
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Default Re: 12V to USB convertor.

COBALT- my dash cam has it's own ciggie plug, but it causes problems, + it is on a non switched supply ,and other problems. I ws thinking of getting rid of plug + convertor and connecting dashcam directly hardwired from a supply that's on with ignition but via a 12-5v convertor . I've no USB in car ( it's one of the lower spec VAG MODELS, and 14 year old).
I didn't want to bring it into the post, but I'm also looking at converting my system to remote central lock ( an old ,"when I get round to it" project) , and as part of th PSU board, it would be a good idea to provide a 5v (USB) supply ,that is on with ignition. Supply to the remote locking would be disabled till ignition off.
But I have little ideas of USB supplies, or what smoothing is needed, hence my post.
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Old 12th Jul 2018, 12:36 am   #4
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Default Re: 12V to USB convertor.

Standard USB is 5V 500mA, though higher currents are possible if both the supply and device support it. The cheap Poundland type cigarette lighter plugs are nominally 500mA. Devices need not draw the full 500mA available, and it's perfectly normal for them not to do so.

You can build your own hard wired supply with just a 7805 and a cable with a female USB socket ('receptacle' in USB terminology) on the end of it. Info on USB cable connections is easily available if you google. There are only four wires in a USB 1.1/2.0 cable, a twisted pair for the data and +ve/-ve. You don't need smoothing as such but you will need to decouple the supply with something like 220uF. It's good practice to add a diode to protect the regulator, as shown in the standard datasheets.

https://en.wikipedia.org/wiki/USB_(Physical)
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Old 12th Jul 2018, 8:45 am   #5
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Default Re: 12V to USB convertor.

I bought an adapter to wire permanently into the fuse box of my car rated at 1.5 amp. Came with long leads, fuse adapters etc, and all in a small box - ready made and cost about 7 inc p&p. I hate the fag lighter adapters as the wires always seem to get in the way.
Not worth the effort making it.
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Old 12th Jul 2018, 9:33 am   #6
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Default Re: 12V to USB convertor.

If you look up 'Toyota USB adapter' on Ebay it's a nice design that you can hard wire into your ignition switch and cut/adapt a suitable aperture on your dashboard, although I fitted into the glovebox so that I can store and charge a phone out of sight.
Many ignition switches have an extra tag that allow you to switch on with just the key inserted without rotating to ignition ..if that is any use.
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Old 12th Jul 2018, 9:36 am   #7
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Default Re: 12V to USB convertor.

I've gone round this loop several times re. satnavs and dashcams needing 5v via a USB adaptor fed from a switched vehicle 12 volt supply.
'Scotchlok'ing off the radio supply will provide the switched feed; the fiddly bit comes with trying to work out tidy and unobtrusive routeing USB-side wiring for the device/s in question.
Please note that some dashcam product importers/suppliers have associated 'Volkswagen' [Auto Group] with high failure-rate on the plug-in adaptors normally included with the device. No detailed explanation; it just gets flagged up in the Frequently-Asked Questions bit - as here, for example:

https://www.quesh.co.uk/products/?product_id=384

I think the '7805'-based solution is a good one. At least you're more in control of the quality of the end-result

Best wishes
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Old 12th Jul 2018, 9:51 am   #8
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Default Re: 12V to USB convertor.

Quote:
Originally Posted by Nymrod121 View Post
'Scotchlok'ing off the radio supply

NO,NO,NO! Those things are the spawn of the devil and should be put straight into the bin. Lots of ways of making a good and safe connection including piggy back Lucar, strip solder and insulate or the solder/heatshrink tubes that do an excellent job.

Did I say never use Scotchloks? Please don't even give it a passing thought.
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Old 12th Jul 2018, 10:33 am   #9
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Default Re: 12V to USB convertor.

Scimitar, you're absolutely right. I withdraw the suggestion, having read this:

https://advrider.com/index.php?threa...no-good.44025/

Interesting caveat posted on June 9, 2004 (about half-way down) - everything therein is sound and presented in a forthright yet diplomatic and competent fashion.

I have some "SOTD" in my garage - I'll get rid of them with a new-found (and extreme) prejudice without delay

Best wishes
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Old 12th Jul 2018, 1:49 pm   #10
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Default Re: 12V to USB convertor.

there is a 12v to usb supply on e-bay https://www.ebay.co.uk/itm/12V-to-5V...r=461700380520
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Old 12th Jul 2018, 9:56 pm   #11
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Default Re: 12V to USB convertor.

Quote:
Originally Posted by paulsherwin View Post
Standard USB is 5V 500mA, though higher currents are possible if both the supply and device support it. The cheap Poundland type cigarette lighter plugs are nominally 500mA. Devices need not draw the full 500mA available, and it's perfectly normal for them not to do so.

You can build your own hard wired supply with just a 7805 and a cable with a female USB socket ('receptacle' in USB terminology) on the end of it. Info on USB cable connections is easily available if you google. There are only four wires in a USB 1.1/2.0 cable, a twisted pair for the data and +ve/-ve. You don't need smoothing as such but you will need to decouple the supply with something like 220uF. It's good practice to add a diode to protect the regulator, as shown in the standard datasheets.

https://en.wikipedia.org/wiki/USB_(Physical)
Thanks, Paul- I knew USB was based on 5v at unit currents ,but not having seen any circuits using a 7805, I was wary .
The idea I had in mind was to use a 7805 from a switched supply ( possibly heater fan supply ,as cigarette lighter is permanently on ) , via a fuse and diode. Dash cam only needs the +& - to run and the cigarette lighter is right next to handbrake and every time handbrake is used, the plug gets moved with possibility of shorting the plug end. I've lost count of number of plug end and car fuses I've replaced. Current won't be a problem as I bought a stack of Maplin 2A 7805 when I had the chance.

As for Scotch locking- a car is not the best place to do IDC type connections. Those of us that have seen the old BT "jelly crimp" and how it corrodes outside the crimp have seen what happens only too well. Far better to either strip back the wire and solder ,only problem is that you need to tape over the joint. I prefer to cut the wire and either solder the three wires together and cover with heat shrink, or crimp two wires into one end and double the other into the other end ,covering the joint with heat shrink.

Last edited by Oldcodger; 12th Jul 2018 at 10:03 pm.
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Old 12th Jul 2018, 10:02 pm   #12
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Default Re: 12V to USB convertor.

Bear in mind that a linear regulator dropping 14V to 5V at around 500mA will literally cook itself in a small case without adequate heatsinking and air flow. I assume most of those small cigar plug size units are switching regulators to get away with lower heat dissipation.
Alan.
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Old 12th Jul 2018, 10:26 pm   #13
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Default Re: 12V to USB convertor.

Biggles- One reason I'd opted for one of the Maplin 2A 7805. I'm using one in my central locking PSU ( CAR nominal 12 dropped to 5v via 7805 and converted to 12v+/- .5 via SMPU chip), and even at .5A this chip remains cool . I'll try one of these in car as a test on DASHCAM, and if getting warm , I'll simply change it for a 7905 ( reversing polarities ,of course) and use a heatsink/ some part of car as heatsink. Thanks for the warning.
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Old 13th Jul 2018, 11:19 am   #14
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Default Re: 12V to USB convertor.

On 79xx series the lead configuration is common/in/out when viewed from the package side (c.f. in/common/out on the 78xx). The 79xx heatsink tab is also connected to the input connection so an appropriate heatsink washer / insulated mounting-bolt/nut will be needed:

http://www2.ensc.sfu.ca/reference/da...ets/LM79XX.PDF

Apologies if this is 'grandma [grandpa?] & eggs' ...

O/C, if you decide to go down the 79xx route - should you need them, send me a PM with your postal address as I have a few TO-220 heatsink washers somewhere (and the little plastic 'top hat' insulators for the mounting screw) that I'd be happy to send you FoC

All the best

Guy

Last edited by Nymrod121; 13th Jul 2018 at 11:27 am. Reason: clarification re 79xx tab
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Old 13th Jul 2018, 11:30 am   #15
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Default Re: 12V to USB convertor.

Quote:
Originally Posted by Nymrod121 View Post
On 79xx series the lead configuration is common/in/out when viewed from the package side (c.f. in/common/out on the 78xx). The 79xx heatsink tab is also connected to the input connection so an appropriate heatsink washer / insulated mounting-bolt/nut will be needed:
The way I remember it (and there may be electrical reasons why this is so) is that the most negative connection is the tab (and centre lead). So for a positive regulator (78xx), the tab is common. For a negative regulator (79xx) it's the (-ve) input.

On a negative earth car you can normally have the tab of a 78xx (but not that of a 79xx) in contact with the car body.
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Old 13th Jul 2018, 11:57 am   #16
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Default Re: 12V to USB convertor.

https://electronics.stackexchange.co...-voltage-to-th
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Old 13th Jul 2018, 12:09 pm   #17
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Default Re: 12V to USB convertor.

Of course, something like a dashcam will draw much less than 500mA, so you only need to worry about heat dissipation if you're going to power something else. Some smartphones and tablets can draw quite a lot when charging.
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Old 13th Jul 2018, 5:07 pm   #18
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Default Re: 12V to USB convertor.

Although the commonly available 12v to twin USB adaptors are cheap and plentiful, they generate a frightful amount of RF interference. That radiated from the device lead connected to your dashcam/phone etc can be minimised by the judicious application of a ferrite choke but they also radiate a lot into the cars wiring, enough to make Radio 2 from Wrotham at 240kW ERP unlistenable along the A27 between Lewes and Brighton! Pulling the adaptor from the 12v socket to disconnect normal reception is resumed.
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Old 13th Jul 2018, 9:16 pm   #19
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Default Re: 12V to USB convertor.

Quote:
Originally Posted by paulsherwin View Post
Of course, something like a dashcam will draw much less than 500mA, so you only need to worry about heat dissipation if you're going to power something else. Some smartphones and tablets can draw quite a lot when charging.
Again, thanks, Paul. Your two posts have answered my question- as in can I derive a USB voltage safely from a 7805, and like me, you suggest that the current draw would be light.
And on that note, if admin agree, this post can be closed
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