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Noisy 12v circuit from 7" LCD Screen?

Chrisbitz

Feb 20, 2016
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I have a system that comprises of a 12v battery, a little 7" LCD screen, and a cheapo chinese audio player and audio amp.
Everything works fine.

I want to replace the MP3 player and audio amp with the sparkfun MP3player and amp. As soon as I connect it, I get a thumping pulse from the speaker, at a frequency of about 2 or 3 per second.

It stops if I turn off the LCD screen, so I'm assuming its from that?

I've tried putting a 2200uf cap across the power at the amp end, and it very slightly muffles the pulsing, but not significantly.

I'd rather not run the audio from a separate battery for weight, so is there a 12v - 12v power noise reducing widget I can put in between the screen and the audio?

Or (as usual) :) it it not as simple as that?

Thanks for any advice!

Chris.
 

davenn

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Sep 5, 2009
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I have a system that comprises of a 12v battery, a little 7" LCD screen, and a cheapo chinese audio player and audio amp.
Everything works fine.

I want to replace the MP3 player and audio amp with the sparkfun MP3player and amp. As soon as I connect it, I get a thumping pulse from the speaker, at a frequency of about 2 or 3 per second.

It stops if I turn off the LCD screen, so I'm assuming its from that?


my first guess would be -- maybe the battery isn't capable of supplying both devices

tell us more about the battery and the current requirements of both devices
 

Chrisbitz

Feb 20, 2016
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Thanks very much for the reply!

The battery is a 12v 5ah motorbike (I think) battery.
and the old soundboard is an 25w fs-k51 Don't know the model of screen, but it's a fairly standard 7" one like this one.

The new sound system is the mp3 trigger and a 40w amp, both from sparkfun.

The battery was fully charged.
Would a 40w amp require so much more power than a 25w one?
 

davenn

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Would a 40w amp require so much more power than a 25w one?

what do you think ? 40 > 25 isn't it

25W / 12V = 1.01A

40 / 12V = you do the math ;)

the MP3 trigger is only a few 10's of mA ... shouldn't be a problem,
the battery should last up to an hour

looking at the parts you linked to, I don't see where the LCD screen fits into this ?
 

Chrisbitz

Feb 20, 2016
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It's for a large puppet - it makes noises, and the screen is connected to a camera, so the operator can see what's going on outside! :)
 

davenn

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and the screen is connected to a camera,

OK so is the screen and camera being powered from the 12V battery or just the screen
do you have a link to the screen ?
 

Audioguru

Sep 24, 2016
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The stereo audio amplifier does not produce 40W all the time, only during the loudest parts of the sounds, when the amplifier is blasting at its maximum ratings. But if it is oscillating at a high frequency then it might be oscillating at 40W continuously.
.
The amplifier is class-AB then its heating is about 25W when its output is 40W so the total average maximum draw from the battery is 65W (5.4A) or 7.7A momentary peaks.

I suspect the original Chinese amplifier that has no detailed spec's produced 25 Whats or 20 Watts only when its supply is its maximum of 30V then its actual power with a 12V battery is only maybe 2.5 Watts.

Therefore I believe that the battery had no problem with the Chinese amplifier but is extremely overloaded with the new amplifier. Maybe the battery needs to be replaced? Measure its voltage when the new amplifier is playing.
 

Chrisbitz

Feb 20, 2016
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Thanks Audio Guru, that's interesting - From my diagnosing though, when the screen is off everything works fine, and the second I turn the screen on, the pulsing happens.
I haven't checked the voltage though, and won't be able to till the weekend. It's a newish 5ah motorbike battery - would the amplifier really be able to hammer a fresh battery that much straight away? If I power the amp directly from the battery, it also works fine. I guess it's relevant about how much power the screen uses, but I don't think it's *that* much, as in normal use the battery lasts for about 5 hours constant use! (screen on and occasional sounds from the soundboard)

I have a video recording of the pulsing sound if that might help?

The stereo audio amplifier does not produce 40W all the time, only during the loudest parts of the sounds, when the amplifier is blasting at its maximum ratings. But if it is oscillating at a high frequency then it might be oscillating at 40W continuously.
.
The amplifier is class-AB then its heating is about 25W when its output is 40W so the total average maximum draw from the battery is 65W (5.4A) or 7.7A momentary peaks.

I suspect the original Chinese amplifier that has no detailed spec's produced 25 Whats or 20 Watts only when its supply is its maximum of 30V then its actual power with a 12V battery is only maybe 2.5 Watts.

Therefore I believe that the battery had no problem with the Chinese amplifier but is extremely overloaded with the new amplifier. Maybe the battery needs to be replaced? Measure its voltage when the new amplifier is playing.
 

Chrisbitz

Feb 20, 2016
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Everything's being powered by the same 5ah 12v motorbike battery, and I don't have a link to the screen. but it's similar to the one I linked to above? It's just a cheapo generic screen.

OK so is the screen and camera being powered from the 12V battery or just the screen
do you have a link to the screen ?
 

davenn

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and I don't have a link to the screen. but it's similar to the one I linked to above?

ohhh I missed the "this " one link sorry

OK so it's drawing an amp
and I don't know what the camera is drawing ??

As AG said there is more juice (current) being drawn by the amplifier that I initially thought about .... I in my early morning haste, forgot about efficiency

All added up, your poor battery just isn't hacking it

Measure the battery voltage with nothing connected ... you should be seeing around 12V
then again with everything connected and running, I suspect you may not be seeing 12V from the battery any longer, it's likely to be dropping in output


Dave
 

Audioguru

Sep 24, 2016
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Since the new amplifier works fine for 5 hours with the screen turned off but produces pulsing when the screen is turned on then the screen is causing it.
Maybe it is a lack of shielded audio and video cables? Then the unshielded video signal causes interference in the nearby unshielded audio wires?
When you connect or disconnect the screen from the battery are you disturbing the amplifier connections to the battery?
 
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