clean 12v from battery?

It is also the "guaranteed" versus the chancy. If the dc-dc conv (~$30?) isn't much more than timed relay, IMO play safe. But a 120W dc-dc conv may be >$100. There is also the UIBI idea - the alternator charge light circuit energises an ordinary relay. In most cases, that means relay energisation AFTER the engine has started. But otherwise, a DIY timer may only add a few $ to the relay price, hence be worth a try. (I'm not au fait with those fancy relays - the 520t's or whatever...) Alas I often overlook such timer-delayed solutions, more so because I am unaware of those fancy relays and their prices and availability. It is certainly a good suggestion that may fit the bill. As usual, it depends on what the load tolerates, and what the user wants (ie, occasional reboots ok, or guaranteed ride thru).

Peter, a DEI 528t goes for about US$15-20 on fleabay.

That's pretty good. Even though I am Mr Cheapskate DIY using common & standard parts, I'd go for that. Why other taking the time and effort to build a circuit. And the DEI 528t has settable 0-90sec delay and polarity options... Put it this way, if I ever see some going cheap, I'm likely to buy a few. (Unless I build my "standard" PIC-08M2 relay etc, but the USA will have gone metric by then...) Let me know if I forget to include delay solutions in the future! (As if my replies aren't long enough already!)

It's just occurred to me that this whole thread is pointless. I'm just starting a project to install GPS linked cameras with SD card storage in London taxis, two cameras can be separated, look up Novus vehicle cameras, about 2 x 1/2 x 4" draws about 30 mamp.

It may well be. But as I recall, the cameras are plugging in to a DVR and there is no info about whether the DVR can power the cameras etc. AND the issue is presented as one of voltage tolerance of the loads (DVR or cameras) as distinct from power requirements. (The string is twice the length from its center to an end.)

All done! Took a while for everything to arrive from the far east. I decided to use the DC-DC module in the end, I had nothing to lose bar a few coins. I haven't got a scope but my DMM tells me a steady 12.0V is coming out of the DC-DC convertor even with the starter motor cranking. The camera according to the specs can take an input voltage of 8-16V and draws that power separately to the device the video signal is sent to. Despite this I hooked it up to the DC-DC module for good measure. I'm still unsure of the input voltage range the DVR can take, I've measured the AC adapter that it came with to have a no-load voltage of 12.4V which falls to 12.2V when powering the DVR. Better to be safe than sorry I suppose, especially if there's some camera footage that I really don't want to lose. All in all the project set me back £45, a success in my book because an equivalent off-the-shelf option would be at least £100. Anyway, thanks for the help!

I'm glad is was a success. I especially like the £55 saving - you could have two for one or maybe even 3 depending on the converter capability. (Maybe a rearward facing camera, and one for your GPS or speedo to prove you were weren't speeding.) But that's usually the advantage of DIY - cheaper cost. Also that you may have more freedom in sourcing parts, carrying spares, or a system that better suits you. The camera(s) could be moved off the converter since it seems designed for car 12V systems, but it's fine on the converter. Besides, the converter should ensure it works during voltage dips (assuming the converter can operate at a lower voltage then than camera) and it provides extra protection against spikes etc. Alas for the DVR, its PSU doesn't say much about what what the DVR itself will tolerate. But you played safe. I can only reiterate that there is a big difference between a 12 volt device and an "automotive 12V" device. (If it comes standard with a cig-socket power lead, then it is (or should be!!) automotive 12V compatible.) FYI - some convertors can be noisy. They use high-frequency switching that can interfere with radio & video etc (the M2 & M4 ATX supplies are renowned for that). But they shouldn't - they should be fully metal enclosed and have appropriate input & output filtering. I'm glad the12volt could assist. Maybe let us know if the setup saves you conviction or claims in the future, or if our - I mean your - video hits "World's Worst Drivers" or youtube etc.