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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 23rd Oct 2018, 4:15 pm   #1
60 oldjohn
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Default Measuring current, Class D ?

How can I measure the battery drain on a class D amp ? it uses 2x 1.5V batteries.


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Old 23rd Oct 2018, 5:34 pm   #2
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Default Re: Measuring current, Class D ?

Measure the current being drawn? I must be missing something.
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Old 23rd Oct 2018, 5:58 pm   #3
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Default Re: Measuring current, Class D ?

Hi Paul, its described as a 3w amp, IIRC should draw around 10 ma at 5.5v
I am running it at 3v. Inserting a 100ma then 1ma analog meters then a digital meter in the + battery lead, (one at a time) the amp stops working and just clicks.


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Old 23rd Oct 2018, 6:04 pm   #4
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Default Re: Measuring current, Class D ?

They do that with a 'loose' power supply, try decoupling with a 1000uF (or so) capacitor after the meter.
 
Old 23rd Oct 2018, 6:06 pm   #5
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Default Re: Measuring current, Class D ?

Sounds as if it's pulling a lot more than 10mA then. The meter resistance is dropping enough voltage to stop the chip working. 3V is already low - these are usually strictly 5V devices. It should be a lot happier with a 4.5V supply.
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Old 23rd Oct 2018, 6:16 pm   #6
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Default Re: Measuring current, Class D ?

2.5v is min on this chip. With 3v there less output but he distortion is very small. I will try the 1000uf cap tomorrow.


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Old 23rd Oct 2018, 6:42 pm   #7
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Default Re: Measuring current, Class D ?

Quote:
Sounds as if it's pulling a lot more than 10mA then.
Oh, lots more peak, datasheets often recommend at least 220uF next to the chip (I recently designed in one for a product), hobby PCB offerings only have a small local capacitor hoping that the upstream supply has the rest!
 
Old 23rd Oct 2018, 6:57 pm   #8
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Default Re: Measuring current, Class D ?

To dismiss the sceptics that a digital amp is rubbish I bought one of these https://www.ebay.co.uk/itm/Nobsound-...696640809?var= it is very good indeed, it will "go away" for my workshop amplifier, bluetooth is handy for podcasts from a tablet etc..
 
Old 23rd Oct 2018, 7:45 pm   #9
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Default Re: Measuring current, Class D ?

This is a bit smaller than the cased amp pictured above. A stereo version of this chip is available it's only slightly bigger then a penny. Pictured, mono 3W version with a one pence piece for scale.


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Old 23rd Oct 2018, 8:04 pm   #10
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Default Re: Measuring current, Class D ?

It does need a "bulk cap" no room for one on that PCB. The bulk cap is the two cells in your case.
 
Old 23rd Oct 2018, 10:48 pm   #11
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Default Re: Measuring current, Class D ?

If the amp is delivering 3W output and the dc supply current at 5V is less than 600mA someone's finally made a perpetual motion machine.......... hide, quick!

Presumably we're talking about the amp's quiescent current draw here?

Also, what exactly is a "digital" amplifier? Class D isn't- it's a linear analog amplifier that happens to use HF switching techniques to get the efficiency up.
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Old 24th Oct 2018, 8:45 am   #12
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Default Re: Measuring current, Class D ?

AFAIK, all class D amplifiers require a low impedance power supply to function properly. In this one, there is no supply decoupling on the board, as there is no board space. It relies on the low impedance of the battery to keep things stable (and keep the price down to a minimum!).
I would put a low E.S.R. cap. of at least 220uf across the supply at the board. Then introduce whatever measuring devices you wish between the board (with cap. attached), and the power source. This should keep things stable, and enable accurate measurements to be made. Tony.
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Old 24th Oct 2018, 11:04 am   #13
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Default Re: Measuring current, Class D ?

No success to report yet. More experimenting called for.


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Old 24th Oct 2018, 1:06 pm   #14
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Default Re: Measuring current, Class D ?

Where have they hidden the output filter?
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Old 24th Oct 2018, 2:53 pm   #15
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Default Re: Measuring current, Class D ?

That's left as an exercise for the user....... use short wires to the speaker and it won't radiate (that) much!
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Old 24th Oct 2018, 3:35 pm   #16
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Default Re: Measuring current, Class D ?

Quote:
Where have they hidden the output filter?
Most type specify that 1m or less is OK without a filter. Given that it is a balanced output and if you use a twisted pair not much will get out.
 
Old 25th Oct 2018, 7:51 pm   #17
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Default Re: Measuring current, Class D ?

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Originally Posted by 60 oldjohn View Post
No success to report yet. More experimenting called for.
Is it oscillating maybe? Can you put a 0.1uF ceramic cap right across Vcc and ground pins?
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Old 25th Oct 2018, 9:58 pm   #18
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Default Re: Measuring current, Class D ?

I would not say its oscillating, just clicking about 2 or 3 three times a second. When I used the 1ma meter needle flicked up to 0.9ma.


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Old 25th Oct 2018, 10:12 pm   #19
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Default Re: Measuring current, Class D ?

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I would not say its oscillating, just clicking about 2 or 3 three times a second.
Hello John,

It might be oscillating at RF...I'd still decouple it as described.

Can you post a pic of the whole set-up including your wiring to the board, battery and speaker? What are you putting into it?
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Old 25th Oct 2018, 10:50 pm   #20
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Default Re: Measuring current, Class D ?

It could be, seems to change speed depending on music or speech input. I will take a photo tomorrow evening. Its just a breadboard lash-up, input being taken from either a Crystal set or a working RDA5807M.

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