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Smoothing 12v
scudderfish - 30/8/08 at 08:51 AM

I've bought one of these http://www.viglen.co.uk/viglen/Products_Services/Product_Range/Product_file.aspx?Type_Info=Description&eCode=XUBUMPCL&Type=Desktops&GUID= to stick in my car for various control and logging tasks. It requires a 3A 12v supply and I'm sure if I connect it directly to the car the magic smoke will soon be let out of something important inside. Does anyone have any advice on how best to feed a nice smooth 12v into it?

MikeRJ - 30/8/08 at 10:08 AM

You need to find out if it uses the raw 12v for any internal circuits. Many of these micro PC use an internal switching regulator to reduce the 12v down to 5v and 3.3v and can be used from an unregulated 12v source.

If this does need a regulated 12v, then I don't know of any off-the-shelf solutions

ReMan - 30/8/08 at 10:09 AM

You need to check the tech data, for it. You will probably find that it gives a voltage range around 10-15v in which case the car will be fine.
Failing that a simple voltage regulator chip and a couple of other components should be only a few quid to build if it is vital that it is exactly 12v

MikeRJ - 30/8/08 at 10:20 AM

quote:
Originally posted by ReMan
Failing that a simple voltage regulator chip and a couple of other components should be only a few quid to build if it is vital that it is exactly 12v


Unfortunately it's not that simple. A simple linear regulator can only reduce voltage, and needs some overhead to work properly. For the regulated 12v, you may need 13-14 volts which may be ok when the engine is running, but once it's stopped, and especially during cranking the regulator will drop out of regulation. It will also needs to be capable of 3amps which rules out 7812 and LM317T solutions.

The ideal solution is a SEPIC switcher which can reduce and increase voltages, but as I say I don;t know or any off the shelf solutions.

ReMan - 30/8/08 at 11:03 AM

quote:
Originally posted by MikeRJ
quote:
Originally posted by ReMan
Failing that a simple voltage regulator chip and a couple of other components should be only a few quid to build if it is vital that it is exactly 12v


Unfortunately it's not that simple. A simple linear regulator can only reduce voltage, and needs some overhead to work properly. For the regulated 12v, you may need 13-14 volts which may be ok when the engine is running, but once it's stopped, and especially during cranking the regulator will drop out of regulation. It will also needs to be capable of 3amps which rules out 7812 and LM317T solutions.

The ideal solution is a SEPIC switcher which can reduce and increase voltages, but as I say I don;t know or any off the shelf solutions.

I am fairly sure you can get 5A voltage regs?
I don't know what he spec for the regulator is, but even at standstill a fully charged battery is at least 12.5 volts anyway, so I don't see why it wouldnt work.
As you say at cranking, it will drop considerably, but it would probably also drop below that of the input of a dc to dc converter to maintain the output too?

So unless a completely independant supply , similar to a UPS is used with internal battery, which in itself is compicated and expensinve, then surelya a simpler solution is better.
As said, it may be that the input voltage of the PC is tolerant of car battery voltage variation anyway?

novacaine - 30/8/08 at 11:39 AM

its not an elegant solution but how about getting one of those 240V inverters, and a regulated 12v power supply (maplin do a suitable one) and install that,

not elegant but it does the job

scudderfish - 30/8/08 at 02:08 PM

quote:
Originally posted by MikeRJ
quote:
Originally posted by ReMan
Failing that a simple voltage regulator chip and a couple of other components should be only a few quid to build if it is vital that it is exactly 12v


Unfortunately it's not that simple. A simple linear regulator can only reduce voltage, and needs some overhead to work properly. For the regulated 12v, you may need 13-14 volts which may be ok when the engine is running, but once it's stopped, and especially during cranking the regulator will drop out of regulation. It will also needs to be capable of 3amps which rules out 7812 and LM317T solutions.

The ideal solution is a SEPIC switcher which can reduce and increase voltages, but as I say I don;t know or any off the shelf solutions.


There is the LM138 which is good for 5A http://www.national.com/mpf/LM/LM138.html, would that be a suitable starting point?

Maplins do a laptop PSU which would do, but at £30 when the PC only cost me £80 seems a bit steep.

scudderfish - 30/8/08 at 02:11 PM

Ouch, that 5A regulator is circa $40!

scudderfish - 3/9/08 at 01:58 PM

I think I've found a solution. The Eee 901 laptop uses 3A @ 12v so I just bought a cigarette lighter adaptor for it from eBay for £6 delivered. I'll crack it open and wire it permanently into the car. Fingers crossed, the smoke will stay in.