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Battery powered 230VAC Inverter, safety precautions?

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Battery powered 230VAC Inverter, safety precautions?

Postby Sam Spoons » Sat Dec 07, 2019 2:56 pm

I mentioned this on the S1 Pro thread but thought it probably worth a thread of it's own.

With the prevalence of battery powered inverters providing 230VAC in campervans, caravans, boats and as standalone power supplies, what safety precautions are prudent when using such a device to power, say, a guitar amp or small PA system?

I have a couple of them, one built into my Epiphone Valve Junior with a 12V SLA battery and a newly acquired standalone Power Pack which supplies 230VAC in addition to 5VDC USB power.

The other issue is that audio amps powered by either of these supplies are more prone to picking up EMI which makes their use a little hit or miss in practical situations. Just testing and, so far, I have been able to get it down to manageable levels with appropriate gain structuring but it would be nice to find a way to reduce it further. Earthing one part of the system doesn't seem to help, nor does moving the Power Bank further from the device. It only evident when a cable is plugged in (so for the VJ and AER C60 I guess that's because the input socket shorts signal to earth when there is no jack plug inserted). Any suggestions as to how should I go about improving things?
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Re: Battery powered 230VAC Inverter, safety precautions?

Postby Hugh Robjohns » Sat Dec 07, 2019 3:06 pm

The safety rules are basically the same as for using petrol/diesel generators.

In most of the intended applications -- like for a campervan -- would probably operate without a ground earth connection, relying on the fact that nothing else is grounded so the whole system can float safely. The underpinning question is always whether a fault in the inverter/generator or equipment it is powering can lead to a return fault current flow through the user. In many cases, running the inverter/generator without an earth bond is probably the safest option... But that won't always be the case...

As for the interference thing, a lot of inverters do not provide a true sinewave output. Many are squarewave of 'modified sinewave. These will result in a lot of noise on the mains supply that won't go down well with most audio equipment. Inverters may well also generate and radiate a lot of interference themselves, just because of the nature of their internal electronics.

In general, working with a grounded system should help... but it may not always be sufficient if a lot of the noise is travelling via the mains supply itself.

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Re: Battery powered 230VAC Inverter, safety precautions?

Postby Guest » Sun Dec 08, 2019 4:41 am

What are you two referring to when you use the term a grounded system?
DC battery circuit/ AC 3 pin plug socket inverter or the audio system?
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Re: Battery powered 230VAC Inverter, safety precautions?

Postby wireman » Sun Dec 08, 2019 1:09 pm

tonyztigger wrote:What are you two referring to when you use the term a grounded system?
DC battery circuit/ AC 3 pin plug socket inverter or the audio system?

Electricity distrubution systems (the wiring to the power socket on the wall) generally support a ground (earth) as a safety measure. Where this comes from can be quite complicated and confusing, it could just be a tie to the Neutral at the fuseboard for example.
Creating a supply independently (via battery or a generator) means that a ground would have to be supplied by some other means if required, so for example that could be an earth rod.

In some cases you deliberately make the supply float (and not have a potential to earth) by isolation with a transformer (110V supplies for builder power tools in the UK for example).

I don't know enough about this to say any more, the earthing has safety and sheilding (for noise) implications.
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Re: Battery powered 230VAC Inverter, safety precautions?

Postby Guest » Sun Dec 08, 2019 2:00 pm

I have had a few inverters (the cheaper ones though) and non of them have a means of attaching a ground wire to the floor. But would you really want to in a boat or caravan when the negative of the battery is grounded anyway
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Re: Battery powered 230VAC Inverter, safety precautions?

Postby merlyn » Sun Dec 08, 2019 5:36 pm

I've used power inverters when busking. Electronically they're quite delicate. It's wise to observe the correct order of plugging everything in then switching it on, inverter first. If you get the battery terminals round the wrong way it blows the blade fuse in the battery wire -- it's wise to have spares.

I wouldn't have thought the battery lasts long powering a valve amp.

The output is noisy on cheaper models -- it's not a sine wave. You can get inverters that put out a fairly clean sine wave but they're more expensive. I think that may be what you're hearing, rather than it picking up EM.
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Re: Battery powered 230VAC Inverter, safety precautions?

Postby Sam Spoons » Sun Dec 08, 2019 7:04 pm

#merlyn

The Valve Junior is 5 watt 1x8" combo with a single EL84 OP valve. The 12V SLA battery is 7AH (similar to this) https://www.toolstation.com/sealed-lead-acid-battery/p79562?gclid=EAIaIQobChMIyPSp7M6m5gIV2obVCh1aLAwLEAAYAiAAEgKgdvD_BwE and runs the VJ for around 2½ to 3 hours at full chat (though being Class A, running it with no input makes little or no difference).

The noise may well be inherent on the mains output from the inverters but in most situations it's at a tolerably low level.

Yes, cheap ones are electronically fragile but the ones I have now have performed ok so far.

#tonyztigger

The all in one Power Pack I have just bought has no earth obvious/accessible connection but both my separate (300 watt) inverters and my petrol genny have an earth screw terminal, obviously this is not much use indoors (difficult to get a safe connection to a painted CH radiator without incurring the wrath of the venue manager).
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Re: Battery powered 230VAC Inverter, safety precautions?

Postby ef37a » Mon Dec 09, 2019 9:13 am

All interesting stuff. We used to run valve amps in fields from rotary converters and an earth stake was almost mandatory to keep things quiet. The safety implications never dawned!

Also back in the day I had problems with 'cracks' from fridges getting into the audio circuits of the new-fangled transistor kit. I made up some filters. 100nF across the mains in, a Radspad RF choke in live and neutral, another 100nF. This was fitted to the fridge and usually stopped the noise.

I therefore wonder if you could wind some chokes on some chunky ferite rings and make a filter with some X rated caps? All would need to go into a metal box. I used dad's 2oz baccy tins.

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Re: Battery powered 230VAC Inverter, safety precautions?

Postby Sam Spoons » Mon Dec 09, 2019 10:32 am

:thumbup:
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Re: Battery powered 230VAC Inverter, safety precautions?

Postby CS70 » Mon Dec 09, 2019 10:50 am

This is very interesting as I've been looking of powering an amp with a battery.

Reading up, I had understood the need of a "pure sine wave" converter, but I'm not sure of a couple of things:

a) Can manufacturers be trusted on this?
b) How much power output from the inverter is it needed for, say, a 15W output amp? (I'm thinking of bringing around my AC15)?

I found for example this - 300W max output - on amazon

https://www.amazon.co.uk/power-inverter ... 74&sr=8-15

but not sure if it's suitable (but hey, I'd get to use the one UK-plug power cable I have..)
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Re: Battery powered 230VAC Inverter, safety precautions?

Postby Sam Spoons » Mon Dec 09, 2019 11:27 am

My 18 watt combo draws around 0.34A from the mains at normal/gig volume so you should be well within the capability of that inverter. I might stick the 'scope on the output from my new power pack later and see what the 'modified sine wave' looks like.
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Re: Battery powered 230VAC Inverter, safety precautions?

Postby Folderol » Mon Dec 09, 2019 12:00 pm

Sam Spoons wrote:My 18 watt combo draws around 0.34A from the mains at normal/gig volume so you should be well within the capability of that inverter. I might stick the 'scope on the output from my new power pack later and see what the 'modified sine wave' looks like.

I can tell you now. It will look horrible and nothing like a sine wave! Especially if only lightly loaded.

A true sinewave will actually be more efficient (as well as electrically quieter) as the transformer is not getting slammed with hard step sequences.
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Re: Battery powered 230VAC Inverter, safety precautions?

Postby Guest » Mon Dec 09, 2019 12:40 pm

CS70 wrote:
I found for example this - 300W max output - on amazon

https://www.amazon.co.uk/power-inverter ... 74&sr=8-15
The problem I found with the inverters I have used is they cut out well well before the battery is dead. But this was with typical car battery which are designed for cold cranking and not long drain so I don't know if that was why
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Re: Battery powered 230VAC Inverter, safety precautions?

Postby Sam Spoons » Mon Dec 09, 2019 1:09 pm

A typical inverter will shut down when the battery voltage reaches 10V which is a safe level for the battery to help prevent damage. This will probably represent 50-60% total battery capacity.
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Re: Battery powered 230VAC Inverter, safety precautions?

Postby CS70 » Mon Dec 09, 2019 1:16 pm

tonyztigger wrote:
CS70 wrote:
I found for example this - 300W max output - on amazon

https://www.amazon.co.uk/power-inverter ... 74&sr=8-15
The problem I found with the inverters I have used is they cut out well well before the battery is dead. But this was with typical car battery which are designed for cold cranking and not long drain so I don't know if that was why

Yeah from what I read yo’ve got to use a more heavy duty type of battery designed for continuous use, as in boats and caravans for example
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Re: Battery powered 230VAC Inverter, safety precautions?

Postby Sam Spoons » Mon Dec 09, 2019 1:27 pm

Deep cycle batteries are designed to cope with being discharged to lower voltages than cranking type batteries, I don't think it makes much difference to the effective capacity but they last many more charge/discharge cycles used correctly. Car starter batteries can supply much higher amperages, typically several hundred amps, which would damage a deep cycle battery.
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Re: Battery powered 230VAC Inverter, safety precautions?

Postby Sam Spoons » Mon Dec 09, 2019 1:35 pm

Folderol wrote:
Sam Spoons wrote:My 18 watt combo draws around 0.34A from the mains at normal/gig volume so you should be well within the capability of that inverter. I might stick the 'scope on the output from my new power pack later and see what the 'modified sine wave' looks like.

I can tell you now. It will look horrible and nothing like a sine wave! Especially if only lightly loaded.

A true sinewave will actually be more efficient (as well as electrically quieter) as the transformer is not getting slammed with hard step sequences.

How do SMPSUs react to being supplied power as a 'modified sine wave'?
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Re: Battery powered 230VAC Inverter, safety precautions?

Postby Folderol » Mon Dec 09, 2019 2:50 pm

Modern ones don't care. The first thing they do is rectify and smooth it. The cap will have a slightly lower peak ripple current, so in theory will last longer, but whether that is measurable is moot. It's the EMI suppression that will have most trouble - all those lovely harmonics :tongue:
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Re: Battery powered 230VAC Inverter, safety precautions?

Postby Sam Spoons » Mon Dec 09, 2019 2:55 pm

Thanks Will. :thumbup:
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Re: Battery powered 230VAC Inverter, safety precautions?

Postby ef37a » Mon Dec 09, 2019 3:40 pm

Can we not build a 50Hz resonant filter like the tank circuit for a class C transmitter?

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