[HacktionLab] 12 volt calculator

Mon Apr 11 22:19:40 UTC 2011

Hi

I really admire Mike's piece for hitting all the key points in a 
digestible length document, and once I got away from the crappy 
computers in my place of work the Javascript calcs worked really nicely.

But it did spur me into thinking about the voltage issues Jimdog refers 
to, and power amp stuff kriptick has mentioned.

The Allen & Heath amps are brilliant for convincing customers that it's 
a "real" PA, and there's a lot to be said about all that, but I kind of 
think the future is with Class H amps. I have some 24v modules which I 
intend to try out soon on an unsuspecting public...

Speaker efficiencies also vary massively, as do, to a slightly lesser 
degree, cabinets of differing designs...

I was also about to hack into some lights I have that have 24v switch 
mode supplies in them, then I realised that whenever I'm using them 
there'll almost always be a big inverter running anyhow and what mains 
is really good for is power transmission over (medium) distance (that 
skinny cable thing) so probably not worth the outlay in fat cable and 
chunky connectors just to miss out on the losses in those PSUs.

There are also lots of issues around how much power you get out of a 
lead acid battery vs. the discharge rate, which might seem too detailed, 
but if someone gets the idea that a 100Ah battery will run their 
operation for ten hours, they might go on to assume that a 10Ah battery 
will run the same system for an hour (which you might want to do it 
riding around on a bike) but it's unlikely to as ratings are typically 
given as C10 (meaning you'll get 10 amps for 10 hours out of a 100Ah 
battery, but not 100A for one hour, and certainly not 1000A for six 
minutes).

I have some supposedly 270Ah batteries which always seemed a bit small 
(not three times the size of 90Ah batteries, which you'd expect them to 
be) until I found out they were rated at C100 - 2.7A for 100hours, but 
somewhat less than 27A for 10hours.

Anyhow, like I said, spurred me into thoughts, started writing it down, 
turned into War & Peace before I'd finished the introduction, gave up. 
Bit like this email.

Congrats to Mike for actually delivering some output!

Protag

On 11/04/2011 19:32, JimDog wrote:
> We use 48v Allen and heath amps with 2 pairs of batteries at +24 and 
> -24 volts with solar panels paired up.
>
> This drives a 5k rig nicely as well as led lighting - bypass the 
> transformers to minimise power wastage.
>
> JD
>
> http://www.yorkshiresolarsound.co.uk
>
> ----- Reply message -----
> From: "kriptick" <kriptick at riseup.net>
> Date: Mon, Apr 11, 2011 17:24
> Subject: [HacktionLab] 12 volt calculator
> To: "Mike Harris" <mike at mbharris.co.uk>, <hacktionlab at lists.aktivix.org>
>
> Prob very handy although at the moment, the calculator part doesn't 
> actually
> do anything on my (likely broken) browser. I'm not too up on current PA
> stuff but one thing I do know is that we live in a very imperfect world.
> Anyone else ever noticed this? ;-) Amplifiers never convert all the 
> ingoing
> power to output power and how much is lost depends on their class.
> http://en.wikipedia.org/wiki/Class_C_amplifier#Class_B
> Traditional class B can never be better than ~78% but class D can be well
> into the 90s so maybe have a control for class or else directly type 
> in any
> stated efficiency given by manufacturer? Or am I years out of date & just
> about all PA stuff is class D? If you want to get picky then you could 
> have
> a music/speech control as harsh rave music is almost continuously loud
> whereas speech is much more peaky with significant lulls so would prob
> consume something like 3 times less power though I'm sure there are 
> figures
> for this somewhere? Also seems to me you shouldn't dis 24V gear as being
> inappropriate as, to my mind 24V should be significantly more efficient as
> well as capable of far higher output power than 12V.
> The other thing that should be emphasised in the text is that low voltage
> systems need to have all power & spkr cables as short as poss & seriously
> fat - I squared R etc. Not easy I know these days when scrap copper is a
> fiver a kilo. Many activisty people I've spoken to think 12V is mostly 
> about
> safety & assume low voltage means you can use skinny wires whereas you 
> should really be using welder cable.
> hth
>
>
> ----- Original Message ----- From: "Mike Harris" <mike at mbharris.co.uk>
> To: <hacktionlab at lists.aktivix.org>
> Sent: Thursday, April 07, 2011 3:41 PM
> Subject: [HacktionLab] 12 volt calculator
>
>
> > Hey All,
> >
> > Some friends were asking me the other day about 12 volt stuff: basically
> > how many batteries would they need to run their mobile amp for X hours
> > for a little rave up.
> >
> > I've been asked it before and started to explain it, but thought it'd be
> > better on a web page, so I put together the following article, featuring
> > some javascript calculators.
> >
> > I submit it to the hacktionlab list for peer-review and any comments,
> > additions, corrections, etc, much appreciated:
> >
> > http://mbharris.co.uk/12-volt-sound-system-battery-calculator/
> >
> > I could move the text content to the wiki if that made it more open....
> >
> > Cheers,
> >
> > Mike.
> >
> > -- > Mike Harris
> > w: http://mbharris.co.uk
> > t: +44 7811 671 893
> > 0: http://mbharris.co.uk/keys/pgp.html
> >
> >
> > _______________________________________________
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> > HacktionLab at lists.aktivix.org
> > https://lists.aktivix.org/mailman/listinfo/hacktionlab
>
>
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