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Chip Amps Amplifiers based on integrated circuits

Single supply LM3886 layout
Single supply LM3886 layout
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Old 8th December 2017, 05:18 PM   #21
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by AndrewT View Post
Assume the gate needs 6V.
63% of 48V = 30V. That's the voltage that will have charged up the capacitor that does not leak when 1*RC has expired.
6V is ~20% of 30V so you are looking for ~ 20% of the RC as your timer value.
Thus for 5seconds delay you need an RC of 5seconds/20% = 25seconds.
2M2 & 11uF = 24seconds
You could use 10uF low leakage electro, or if you can tolerate a shorter delay use a 4u7F MKT.
Let's straighten out the math. The capacitor voltage increases exponentially over time starting at t=0, which is when the power supply is turned on. The capacitor voltage can be calculated from the following equation:

Vc = Vs*(1-e^(-t/RC)),

where Vc is the voltage across the capacitor vs time and Vs is the supply voltage.

Solving for t yields:

t = -ln((Vs-Vc)/Vs)*R*C

Plugging your values of Vs=48; Vc=6; R=2.2M; C=100u results in:

t = 29.38 seconds

You can verify this using your favourite spice simulator.

Tom
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Last edited by tomchr; 8th December 2017 at 05:23 PM.
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Old 8th December 2017, 05:23 PM   #22
AndrewT is offline AndrewT  Scotland
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I am glad you are good at the maths required.
I am only good with simpler arithmetic.
Thus I have often used approximations that get confirmed, or otherwise, during testing.

Can you provide a similar non approximation set of formulae that allow for calculating a time delay when the electrolytic leaks?
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Last edited by AndrewT; 8th December 2017 at 05:26 PM.
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Old 8th December 2017, 05:31 PM   #23
tomchr is offline tomchr  Canada
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Nothing wrong with estimates. I just found it a bit odd that in one post you worry about the leakage current of the capacitor changing the charging time, whereas in another post you're trying to make a piece-wise linear approximation of the capacitor voltage.

Tom
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Old 8th December 2017, 05:47 PM   #24
AndrewT is offline AndrewT  Scotland
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The electrolytic leakage current will seriously lengthen the time delay and may even prevent triggering.
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Old 8th December 2017, 08:10 PM   #25
00940 is offline 00940  Belgium
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While I do appreciate the concerns around the delay circuitry, they don't bother me as much right now as the concerns Tomchr raised around grounding and decoupling in post #4. I would really, really appreciate if they were made clearer.

Thanks for any clarification on this.
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Old 8th December 2017, 09:28 PM   #26
00940 is offline 00940  Belgium
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And I've redrawn the schematic to account for the mistakes pointed out and the improvements suggested.
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Old 8th December 2017, 11:56 PM   #27
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by AndrewT View Post
The electrolytic leakage current will seriously lengthen the time delay and may even prevent triggering.
So you're saying that the leakage current is enough to cause (48-6) = 42 V drop across 2.2 MΩ? That would be 42/2.2E6 = 19 uA. Which electrolytic caps are you aware of that leak that much? I'm not aware of any...

Quote:
Originally Posted by 00940 View Post
While I do appreciate the concerns around the delay circuitry, they don't bother me as much right now as the concerns Tomchr raised around grounding and decoupling in post #4. I would really, really appreciate if they were made clearer.
I suggest reading my Taming the LM3886 article series: Taming the LM3886 Chip Amplifier

You can ignore the page on Stability as you have that covered in your design already. Also, that section is in serious need of an update.

Tom
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Old 9th December 2017, 06:37 AM   #28
00940 is offline 00940  Belgium
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Tom: I've read those pages a dozen times. My point is that I just don't get what's the problem here.

Wrt decoupling, I can hardly put the decoupling caps closer and I use the exact same caps you suggest in your article.

Wrt grounding, the signal section has its own groundplane, connecting to power ground at the output.

So what's wrong exactly?
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Old 9th December 2017, 10:20 AM   #29
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by tomchr View Post
So you're saying that the leakage current is enough to cause (48-6) = 42 V drop across 2.2 MΩ? That would be 42/2.2E6 = 19 uA. Which electrolytic caps are you aware of that leak that much? I'm not aware of any... ..........
if one takes a fairly usual leakage value from the manufacturers' datasheets
then we find that leakage should drop to less than the specified value within 2minutes.
Ileakage <= 0.003CV
for a 100uF charged to 48V that would be >14uA

the 2M2 with 48Vdc applied will initially pass 21.8uA and this falls slowly to 20uA when the the charge has reached 4Vdc

If half of this charging current leaks through the capacitor, the capacitor effectively charges up half as quickly.
That was a worst case because initially the capacitor has very low voltage applied.

It is for this and other reasons I suggest that polar electrolytics be slowly reformed BEFORE they are fitted to a circuit.

I find that slow reforming can reduce leakage to << 1% of specified 2minute values and get there almost instantaneously.

This becomes even more important in speaker protection filters where slow, or non-triggering could result in damage. Here the capacitors lie dormant possibly for years and initial leakage could/will prevent the trigger action, just when really required.

That's why I much prefer an MKT. For timers, try to scale values such that reasonable MKT values are practical.
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Last edited by AndrewT; 9th December 2017 at 10:23 AM.
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Old 9th December 2017, 11:44 AM   #30
indianajo is offline indianajo  United States
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Quote:
Originally Posted by tomchr View Post
Which electrolytic caps are you aware of that leak that much? I'm not aware of any...

Tom
Old ones. I'm working on a 1980 organ that thumps at turn on because Allen used an electrolytic 100 uf capacitor in the timing circuit, and the "serviceman" bypassed the timing module to provide instant 120 vac to the amps. I haven't changed it yet because a lot of other things are broken, like one amp & one speaker, and the DC power supplies & boards are chock full of 37 year old ecaps just like the ones that failed in the amps.
I've used a ceramic 1 uf cap in power up reset circuits since 1976 when I designed boards for JSC MSC. Then I had to use an exotic 96S02 IC to sense a 1 megohm pull up circuit, but now nfets are $.38 and free in junk TV's, there is no reason to use any electrolytic in a timer circuit. If 00940's new circuit uses a plastic film 4.7 uf cap or the $5 ceramic version, he should be fine.
Thanks 00940 for doing this IMHO, Most protection circuits have a dozen parts if fet or a metal contact relay not rated to break 200A DC current. I don't connect $600 speakers to any transistor without one. A series capacitor for protection is stupid proof for 20 years anyway. The layout in the lm3886 datasheet perhaps is not experience tested. If you provide gerber files when done, or sell boards, it would be nice.
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Last edited by indianajo; 9th December 2017 at 11:53 AM.
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