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DC To AC Inverters For Paying Off-Grid

Discussion in 'Amps and Cabs [BG]' started by Chuck McGregor, Aug 20, 2017.


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  1. While the use of a dc to ac inverter was addressed for a specific circumstance in a recent thread, I think it useful to expand this to the general case for those who want to or need to play off-grid.

    There are a number of battery powered devices that convert the battery’s dc voltage to 120 V ac. The most common, least costly, and most readily available types work off of 12 V batteries. They typically have either a “cigarette lighter plug” to plug into a vehicle’s accessory 12 V power outlet and/or battery clamps to connect directly to the vehicle’s battery or to a standalone 12 V battery. Others have internal batteries. These latter include UPS systems for computers, the most costly type.

    If you use a stand-along battery for one of those 12 V dc to 120 V ac inverters, always use a sealed lead acid battery for safety reasons. There are also “gel-cell” batteries but those are not addressed here as they are different animals being far more costly and requiring specialize chargers. You will also need an automotive-type 12 V charger to recharge it. Always use an “automatic” charger to avoid over-charging. Here’s how to select what you need

    1. Select an inverter with at least 1.5 times the ac power input in watts listed on your amplifier.

    2. Select a battery with the Ah rating you need for the hours of playing time using the calculations below by trying different Ah values.

    3. Select a “automatic” 12 V charger based on how many hours of charging time you can tolerate. Calculate this as follows: Battery Ah rating / Charger rating = Charging time Hours. Add 10% of extra time to totally top off the battery.

    There are several things you should know before powering your bass or guitar amplifier from one of these devices.

    1. Most of the reasonably priced devices output a “modified sine wave.” This is essentially a square wave. In more costly devices it can be a multi-step square wave that better simulates a sine wave. Those that output pure sine waves, like most UPS devices, are even more costly.

    2. A basic, modified sine wave inverter works best with amplifiers that have a switched mode power supply (SMPS). This includes most any Class D amplifier. If the power input says 120 V – 240 V or similar, like 110 V – 220 V, it has an SMPS. If it lists only one voltage there is no way to tell if it is an SMPS except from the specs or by contacting the manufacturer.

    3. Amplifiers with linear power supplies (transformer / rectifier / capacitor) are generally not a good choice for modified sinewave inverters. One drawback is the transformer will likely run hot. The other is that some of the harmonics of the square wave input can get reproduced in the audio = buzz or hum.

    Trivia: an SMPS can be run off an ac input or a dc input, meaning it doesn’t care if the input voltage is an ac sine wave, a dc voltage, or anything in between like a modified sine wave (= square wave).

    The BIG question is how much playing time can you expect from the battery? Here is the way to calculate the worst case, meaning you’ll likely get longer playing times than this.

    This calculation makes several assumptions.

    1. The efficiency of the inverter is 75% meaning the power from the battery will be 1.3 times the power out at 120 V ac. This is a conservative value.

    2. The peak to average value of the signal from your instrument is 10 dB.

    3. The signal is continuous, i.e. you are playing constantly with no breaks and at close to the amplifier’s maximum output.

    4. There is no accounting for the idling power draw for the amplifier when there is no signal.

    5. At typically 100 mA each, the addition of stomp boxes with a 120 V ac to 9 V dc power supply (almost all use an SMPS) will only trivially decrease the playing time unless you use bunches of them.

    To do the calculation you need to know only three things.

    1. The voltage of the battery.

    2. The Ah or mAh rating of the battery.

    3. The rating for the amplifier’s ac power input in watts found next to the ac cord or socket for the ac cord.

    So here goes and it’s simple math:

    1. Amplifier ac Power Input in Watts / 10 = Average Power Input to Amplifier in Watts

    2. (1.33 x Average Power Input to Amplifier in Watts) / Battery Voltage = Amperes from Battery

    3. Ah Rating for Battery / Amperes from Battery = Hours of Use

    Or

    4. mAh Rating for Battery / (1000 x Amperes from Battery) = Hours of Use

    So that’s how you can figure your playing time off-grid with a dc to 120 V ac inverter. Playing at lower volume levels and/or with breaks between songs and/or sets and your playing time will be longer. As I said, this is a conservative, worst case calculation.

    As to cost, the least expensive and most versatile option is a 12 V to 120 V ac inverter and a standalone battery. Here’s an example for a 200 W ac input amplifier and 6 hours of continuous (i.e worst case) playing time with some typical prices found on-line.

    12 V dc to 120 V ac 300 W modified sine wave inverter $30
    12 V 15 Ah sealed lead acid battery $25
    1.5 A automatic 12 V charger $30

    Cheers,
    Chuck McGregor
     
    superheavyfunk likes this.
  2. OR...you could buy one of these which I subsequently ran acroos:
    Black & Decker PPRH5B
    With a 300 W ac input amplifier you get:
    At least 6 hours of worst case playing time, more with smaller amplifiers
    PLUS ---
    120 psi air compressor (hose with standard Schrader nozzle included)
    12 V 5 A dc output (use a cheap 12 V to USB adapter, you may already have one, to charge tablet, smartphone, etc).
    Car battery jump starter (cables included)
    Built in "automatic' charger for the internal battery
    USB outlet (sadly only 500 mA)
    LED Area light

    $ 99.00

    At only a few $ more, I must ashamedly disavow anything about my "separate component" approach :cool:

    DUH! I use the previous version, a B&D Powermate without the USB, for all of the above. Like mine, there will be some "livable" fan noise using it as an inverter. Looking up mine is how i discovered this new version.

    So unless you need to power your stadium rig, for most this would seems an obvious choice to play off-grid. If you want to calculate worst case playing time for your rig, using the math in my previous post, the numbers you don't have and need are 18 Ah and 12 V.

    Then just imagine....If you're using a much smaller amplifier lower, playing levels, whatever to use less power you could probably still jump start your car after the gig cause you left your lights on or put enough air in the flat to get to a gas station or or see what your're doing with no other lights around (for changing that flat because you actually DO have a spare) or charge your cell phone, tablet, whatever, even during the gig.

    Most all this stuff has happened to me before or after gigs. Flat tire a couple of weeks ago in fact, in the dark but for my Powermate light.

    Waiting for my check from B&D :hyper::D:D
    Chuck McGregor
     
  3. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    The biggest problem with modified sine wave inverters, and this applies to SMPS power supplies as well, is that the step transients of the tier switches are generally fast enough (to improve efficiency) conduct through the power supply in the amp and when this noise is present on the amplifier rails (especially class D amps, but all amps to a large extent) it transmits through to the speaker as buzz. These switching transients can also cause mechanical noise in transformers, cause increased ripple current is supply capacitors, and in general should be avoided.

    With the cost of pure sine wave inverters (noting more than a specialized high power class D amplifier with a 50/60Hz generator built in) dropping as they have, IMO it's well worth investing a little bit more for a better quality approach.
     
    BassmanPaul and Killed_by_Death like this.
  4. What you say can happen which I noted is a likely issue for amplifiers with linear power supplies. Though only anecdotal, I've used my B&D with two different amplifiers which have switched mode supplies with no audible issues.

    As to what you said here is good advice from Street Musician – Guitar Blog:

    "As for running musical equipment such as fx racks, keyboards, amps and guitars etc. with the modified wave inverter, the advice is generally the same. Depending on your set up, and how robust your equipment is you may find you get away with it, and your equipment runs fine, but you could experience noise, equipment buzz, overheating and reliability issues. It could also affect the life span of some of your more delicate gear.

    "A better quality supply producing a true sine wave will certainly run 99% of your digital fx equipment, amps, synths and laptops etc. with no hassle at all, just as you would expect if you were plugging in at home.

    "The best advise if you are unsure as whether to take the risk and go for a cheaper MSI would be to test it out before you buy. Any respectable dealer will allow you to test the inverter on your set up and give you the opportunity to return it, if it’s not right for your system."

    Cheers,
    Chuck
     
  5. I'm not sure if I like the idea of getting battery powered so I can make payments off-grid.
     
  6. superheavyfunk

    superheavyfunk

    Mar 11, 2013
    Toronto
    This is all great information, especially as we get near the end of summertime in North America. I don't know about the rest of you but around this time, I start getting lots of calls for barn blowouts, outdoor wedding receptions in semi-remote locations, and other kinds of parties where power can often be an issue. I am not a smart man (lol) and have nothing that I can contribute to the thread but I am glad to read it, bookmark it, and share it with my other musician friends who - for some strange reason - have no problem counting beyond 1-2-3-4
     
  7. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Agreed, though I have seen enough noise issues with SMPS that the general caution is I think still a good one. The problem is that the switching transitions are far slower, which fall farther from the typical filters used in SMPS.
     

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