Monday, 16 November 2009

regulatory authorities

My last ever post on Canalworld Forums, I recall, was suggesting that if electrical kit sold for car use (in the context of radio, tv etc) was used on a boat there'd be no need to worry about overvoltage when the engine was running, in response to the original question. This was responded to by one of the resident gadget-obsessed trolls 'experts' that I should 'bugger off and get myself an education'. At this point I quietly considered my bachelor's & master's degrees in electronic engineering, and twenty years as a broadcast engineer, and decided the education I should get myself is not not bother with that particular website*.

Anyway, this is vaguely relevant as I've been building regulators, to prevent equipment being damaged by a battery charging voltage. Not for me, of course a friend on a liveaboard installed lots of nice LED lights (more or less MR16s in a custom flush mount fitting, hence designed fr a regulated 12v supply rather than a constant current supply) two years ago, and they've largely failed, due to being exposed to nigh volages from a constantly running charger. A temporary solution was a 7812 regulator meant it was safe to fit replacements, which to be fair ran happily (and brightly) on the resulting 11.5v, as said IC does introduce a significant voltage drop, well over a volt. For a tiny load it's hardly worth buying in a switch mode regulator, but a quick search found me this article about the LM2940 type regulator (that only drops 0.5v) on the very useful Renewable Energy UK site. They're only rated for a 1A current, If I needed more I guess I could do something with MOSFETs instead - if I needed to... ;-)



Needing only two components per circuit, I twisted & soldered the legs together, and mounted them straight into 3A terminal block, then bolted the whole lot to be bit of aluminium (heatsink tags handily connected to ground, needing no pesky insulation kits). Heat generation is pretty minimal, but will be monitored. The volt drop is pretty impressive - with a 12.3v battery it still managed 11.96v - the 7812 would have been barely over 11v.

I've built myself one too, just in case, but I haven't needed it yet. Even domestic kit that happens to have a 12v power supply is often fine on a boat - if supplied with an unregulated power supply, it'll be faced with voltages of up to 17v off full load. I'm happy to look inside kit and see if the power goes straight into am internal regulator IC anyway, but of course there are times when it's better to be safe than sorry. ;-)

* I must admit that many people are there are perfectly respectable individuals (and may even know what they're talking about - Tony Brooks posts there, after all), but surely it's a function of true wisdom that there is always more to be learned, so those who think they know everything are stupid indeed. Another favourite incident was where I advised that a new 12v spur should be fused if run straight from the battery - I was then told off for perpetuating the 'myth that fuses stop fires'. If only I'd suggested that he short out all his fuses on his undoubtedly immaculate boat, then say, short out the cabling at his water pump (probably the other end of the boat to the batteries), and then see what happened, just to prove himself right. I could continue...

4 comments:

nblazydays said...

Well this is a coincidence, I was thinking along the same lines as you and will be looking into it over the next few days. It seems to me that most of my mains equipment runs off 12 volts and it is such a waste to upconvert and then downconvert back to this voltage.

Simon said...

I'm currently - albeit slowly - oputting together some web pages about this kind of thing; most kit is actually more robust than you'd think, but it's very much at your own risk. '12v' can be a very moveable feast... ;-)

Halfie said...

Simon, I've just rediscovered your blog. This is all very interesting. I've blogged myself about building a regulator circuit (using the LM317T) for LEDs, but other things intervened and I never quite finished the project. You've given me a nudge - now I'll have to carry on!

http://jhalfie.blogspot.com/search/label/electronics

Simon said...

cheers Halfie. The problem with knowing you can do something yourself is that it gets put off, as you know you can do it any time. Watch that volt drop though... ;-) Some LED MR16s I was looking at claimed to be rated up to 13v anyway, so if they're not used [much] when the engine is running (or the batteries are otherwise being charged) they may be OK...