Running amp off 24 Vdc

PoweredByDodge

Hello there my Cheekey lil Monkeys (ala Craig Ferguson)...

Designing a solar powered event trailer and I've run into an oddity. I've got to run a 12 V car amp (that's used to 12.6 Vdc batteries at 10.5 to 16 Vdc) off of a 24 V source (25.2 Vdc nominal , with a range from 21.5 Vdc to 27.6 Vdc).

I haven't been able to find any European or Chinese amplifiers that have 24 V input capability, and even if I did, I'd be skeptical of even trying them out.

What I have found is a 24 V to 12 V buck that'll provide 30 A @ 12.5 (regulated), accepting an input of 22 to 30 Vdc, which is pretty much in my range of supply voltage.

What I'd like to know is -- has anyone ever tried something like this before?

And what were the results?

I am worried that on big music peaks the amplifier may try to draw more than the available 30 A ((its a 200 W amp @ 75% efficiency = about 300 W draw .... so that's 24 A)).

I realize that I could 'buffer' the system by placing a 12 V battery in parallel with the amplifier... so that the buck would both power the amplifier and keep the battery charged... but I would like to avoid this if at all humanly possible.

If not possible, then I guess that's my backup plan.

Thoughts?

sophie

i think if you have two of the same amps you can wire them in series and that will give 1/2 the voltage to each. check with someone els before you try this but i dont see why it wouldent work. also you could use a cap in place of the battery, might be smaller.

Jstas

Wiring two amps in series will not halve the available voltage to each. You can't really wire power circuits in series without fancy regulating circuits. A resistor like a speaker, yes, you'll get a voltage drop when wired in series but you also get an impedance drop. But, the amp will not present a total resistance load to the circuit because the power transformer and the power circuit isolate the rest of the amp from the power circuit fed by the battery. That essentially hides that entire mismash of circuits and thier combined ressitance from the power circuit. However, wiring amps in series will not drop voltage mainly because you are not wiring a ground back to the battery and that's not series wiring. It's not even parallel wiring. You'll just be daisy chaining amps on make-shift power drops like fish hooks on a trawling line. It's bad for sound quality, a wiring nightmare and generally considered unsafe.

To supply an audio system with power, you'd need a capacitor the size of a refidgerator to power it. It would also be charged at 24 volts, not 12 and he'd still have the same problem.


Vinny,

I think the only way to do it safely is a step-down transformer like you already stated. You could do the battery thing but, you run the risk of that battery failing in a spectacularly catastrophic way. Those batteries also emit very toxic fumes and keeping it under constant charge without discharging it is a good way to boil that battery.

If the buck can handle a 30 amp draw and the max you are going to push is 24 amps, you don't have an issue. The only issue I would see is things like slew rate and discharge rate. As long as the buck can support those, it should work just fine. Otherwise, a step-down transformer or voltage regulator is the way to go.

Bill Ayotte

I would assume that you are looking to wire an amp up in a diesel truck of some sort?

PoweredByDodge

Bill,

No, it's a solar powered cargo trailer... the solar panels produce 24 Vdc nominal (it'll range from the mid 20's through low 30's but will be 'throttled' by a charge controller that'll output 27.6 to a bank of 12.6 Vdc batteries wired in series -- so a bank of 25.2 Vdc batteries.)

So my available power is 25.2 volts (24 volts).

John,

I hadn't thought about the effect of not discharging a battery. I was of the understanding that discharging and charging cycles created heat in the battery due to the battery's internal resistance. ... now, I realize that not cycling a battery may lead to decreased lifespan, but not grossly decreased (maybe 20 - 30%). What's the 'Cliff's Notes' deal as to why lack of discharge causes damage? (Not bustin your balls, I just want to understand).

Also - here's a link to the item I was considering (the 30 A buck)... http://www.samlexamerica.com/products/productdescription.asp?ProductsID=60041

Now that you mention it, I think it's a step down transformer rather than a buck, because it claims to be isolated... and buck/boost devices aren't isolated. I've just gotten in the (bad) habit of calling all high to low dc products bucks and low to highs boosts.

PS -- regarding wiring power circuits in series... I agree with you completely John. Not to mention the fact that, depending on the type of power supply circuit used by an amp, it will most likely not be differential with regard to power input. So... even if it did 'work', the amp's output would have a 12 Vdc offset... and you'd likely fry the voice coils.

killerb

why dont you just build a simple voltage divider circuit with 2 power resistors and tap off one of the resistors for 12v. half the voltage will drop on each resistor. sounds like a nice project. also you cant use a transformer for d.c. voltage. you might also want to build a dc to dc switching power supply. you can get some plans for them off the internet.

PoweredByDodge

a) A transformer, by strict definition is anything that 'transforms' or changes a voltage into another voltage. The typical coil transformers that we associate with power are, as you say, only viable for AC voltage. However, step-up converters, step-down converters, buck converters, boost converters, etc etc etc... are all transformers, and while it may not be the best grammer, 'step-down transformer' is a viable term.

b) a voltage divider? --- in order for a voltage divider to have any efficacy, it is required that the load impedance be dramatically above either leg of the divider impedance. ... at full power output, the relative input impedance of the amplifier's power terminals will be around 0.5 ohms. In order to maintain a stable voltage, the divider legs would have to be roughly 1/100th of this resistance. The result would be dropping over 55,000 watts across the voltage divider, just so that I could pull 200 watts off for the amplifier. ... by the way, since the amplifier's relative imedpance is going to change, dumping a series resistance isn't going to solve it either.

c) just about every amp you find is going to have a dc/dc switching supply as its power 'half' ... If I felt like modifying one, I'd just rip the amp open and get to work.

killerb

the word your looking for is "transducer". like a pressure "transducer" converts a pressure signal into an electrical signal.your right any car amp has a dc/dc switching supply and there easy to build. so just build one and your problems are over.

PoweredByDodge

it wouldn't be UL approved -- we have to follow the National Electric Code for this project... so everything has to be listed, or else we can't use it. As far as modding the amp -- I'd love to, but there goes the warranty, and then 6 months from now when I'm not around and something happens to it, somebody's gonna be pissed when the warranty is invalidated.

killerb

good luck with the project, i wish you well.

disneyjoe7

PoweredByDodge wrote: »

Hello there my Cheekey lil Monkeys (ala Craig Ferguson)...


I realize that I could 'buffer' the system by placing a 12 V battery in parallel with the amplifier... so that the buck would both power the amplifier and keep the battery charged... but I would like to avoid this if at all humanly possible.



Thoughts?

Have you considered using that Buck converter with a large cap instead of the battery?

PoweredByDodge

honestly, I hadn't. I've always been of the opinion that caps were to stiffen voltage, not offset a lack of current, although I suppose they can be made to do both provided that they're matched properly to the load.

Off the top of my head, I'm thinking that I'd need at least 5 to 10 farads to make any sort of real difference.

exalted512

kinetik 600...
-Cody

disneyjoe7

I think a cap may give you filter current like a battery.

Jstas

PoweredByDodge wrote: »

John,

I hadn't thought about the effect of not discharging a battery. I was of the understanding that discharging and charging cycles created heat in the battery due to the battery's internal resistance. ... now, I realize that not cycling a battery may lead to decreased lifespan, but not grossly decreased (maybe 20 - 30%). What's the 'Cliff's Notes' deal as to why lack of discharge causes damage? (Not bustin your balls, I just want to understand).

Keeping a battery charged constanly is fine. Nothing really wrong with it. Over time it will develop a memory and you will get reduced life but overall, it won't hurt the battery that much. However, constantly charging a battery without a trickle charger or some other sort of regulation that stops the flow of juice when the battery is full is bad. If the battery can't hold any more juice, you essentially end up with infinite resistance and the charging system and battery both heat up. You'll start boiling off the acid and if the gases aren't vented, it goes KABLAMMO! If the gases are vented, eventually the battery burns and starts a fire. The gases are very toxic too. While a battery is a good way to do what you want to do, unless you regulate it's charging and still present the 12v draw to the 24v source, you're not going to get your benefits from it.

You'd probably be better off using a reduction circuit and resistors to take the place of the battery and then just cooling the resistors with fans or an oil bath or something. Then again, I'm not really sure if it would do exactly what you want it to do.

disneyjoe7

Jstas wrote: »

You'd probably be better off using a reduction circuit and resistors to take the place of the battery and then just cooling the resistors with fans or an oil bath or something. Then again, I'm not really sure if it would do exactly what you want it to do.

I don't think a reduction circuit of resistors is the best approach as an amp will draw varies current, no load 24v input, large load 0v input. We're not talking about a fan or something which a resistor maybe the best option.

Jstas

disneyjoe7 wrote: »

I don't think a reduction circuit of resistors is the best approach as an amp will draw varies current, no load 24v input, large load 0v input. We're not talking about a fan or something which a resistor maybe the best option.

Read what you quoted from my post again.

I was not saying to use resistors like you would on a fan or other constant draw like that to change voltage and such. I was saying to use a system of resistors to replace the battery in the circuit he would have to build to use the buck. The battery is going to store the energy and at some point it gets full. A resistor bleeds off that energy as heat and it doesn't get stored but it will still present a load similar to charging a battery in a circuit using a buck to step down voltage.

disneyjoe7

Are you concerned about a no load condition that the buck may see? If so I see your point, and you make a good one. I would then say a 1 amp load to buck nothing under with a large cap to store a heavy current need an amp may have. The resistor then would need to be 12 ohm 20 watt as this is 12 watt load.

I also have a concern about the large cap charge load, but I think these products already have something to prevent this. As a large cap like that may draw hundreds of amps and blow a fuse first, but a diode and a resistor built in would prevent this. Noted that their wire diagram doesn't show anything special so my thinking was this is built in the cap.