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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, 08:11 AM   #11
00940 is offline 00940  Belgium
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No r4 would have no effect beside heating up a bit without reason. It wasn't a matter of r4 affecting the speaker, just that the isn't any reason to worry about connecting or disconnecting the load.

It goes without saying on c6 voltage rating... except that I apparently missed the obvious.

Edit: for the higher power resistors, I usually use the vishay pr02 serie.
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Last edited by 00940; 8th December 2017 at 08:29 AM.
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Old 8th December 2017, 08:52 AM   #12
AndrewT is offline AndrewT  Scotland
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Have another look at your 2n7000 switch on and off.
The 220k will give ~4.4V on the gate, if the electro does not leak and Vcc=48Vdc.
If the electro leaks half the current passing through the 2M2, then you'll only have ~2.4V at the gate.
the 2n7000 with 4.4Vgs will easily switch 50mA for the relay, but with only 2.4Vgs might struggle.

As Tom says 220k & 100uF is slow and has an RC time constant of 22seconds. You need a more sophisticated relay ON/OFF.

Q1.
why do you need RC = 220seconds for start up? (2M2*100uF)
Q2.
what time delay does your circuit need?
Q3.
Why use a big leaky electro, when a low leakage plastic (MKT) would do?
Q4.
Have you estimated electro leakage and it's effect on timing?
Q5.
Why no "loss of AC" detect?
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Last edited by AndrewT; 8th December 2017 at 09:19 AM.
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Old 8th December 2017, 09:06 AM   #13
blu_glo is offline blu_glo  England
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I did also think his time delay would be 220s too but looking at the gate circuit it would only get to 1/10th supply voltage anyway as there is divider formed by R7/R11 and anyway the mosfet will turn itself on much earlier than that at a couple of volts or less .....
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Old 8th December 2017, 09:17 AM   #14
AndrewT is offline AndrewT  Scotland
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the Vgs @ 5V needs ~ 0.16 of the RC to fully switch on. 220seconds * 0.16 gives a fully on @ ~35seconds. Does he need to wait half a minute for the circuit to stabilise?
Even if the 2n700 switches @ 3Vgs, it will still be a delay >20seconds.
Somewhere between 1second and 5seconds seems more appropriate. That's why I proposed he ask himself Q2.
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Old 8th December 2017, 10:27 AM   #15
blu_glo is offline blu_glo  England
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Agreed 5 seconds should be more than sufficient.
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Old 8th December 2017, 11:04 AM   #16
AndrewT is offline AndrewT  Scotland
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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.
or change the circuit to use a voltage dropper so that you use more than 20% of the RC to turn on the mosFET.
Replace 220k with a Zener to protect the gate from excessive voltage.
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Old 8th December 2017, 12:03 PM   #17
00940 is offline 00940  Belgium
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I just put the delay system into LTspice to check the values. Here are the results.

Q1: with the values given the relay will fire up at about 10 seconds, not 220. It only draws 10mA btw, not 50mA. The 2n7000 will start conducting enough current at about 2V vgs.

Q2: Actually 5 sec delay is not sufficient. For 4700uF into 330r to settle under 0.5V, I need more than 7 seconds. 1V is still 6 seconds. So a simulated 10 seconds delay is not unreasonable for thumpless switching. I can always reduce the value of the 2M2 resistor if it is slower in practice.

Q3: MKT are expensive in the values I need and a 100uF/10V cap is hardly a "big" electro.

Q4: the amp will be powered by an of the shelf smps. So no isolated AC available for AC loss detection.

So, in line with post #5, I attached what I plan on using finally.
Attached Images
File Type: png rc delay.PNG (34.3 KB, 107 views)
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Old 8th December 2017, 03:40 PM   #18
asuslover is offline asuslover  Romania
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Single supply LM3886 layout
Akitika Preamp and Power Amp Kits
gt102
gt101
These are single supply lm3886 amps.
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File Type: jpg IMG_6377.jpg (496.4 KB, 112 views)
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Old 8th December 2017, 04:23 PM   #19
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by AndrewT View Post
the Vgs @ 5V needs ~ 0.16 of the RC to fully switch on. 220seconds * 0.16 gives a fully on @ ~35seconds.
The voltage across the cap will grow exponentially, not linearly. Also, the MOSFET will start to turn on before Vgs reaches 5.0 V.

Rather than a large RC, I generally use a smaller RC with a zener diode in series with the base of a bipolar transistor to drive the relay. With a low-current zener, the base current of the bipolar will bias the zener. This will make it possible to have the relay engage some time after the supply voltage reaches the zener voltage.

The circuit shown in the post above is horrendously complicated for what it does. I think there is some value in minimizing the noise of the input reference, but the relay driver circuit is just complicated for the sake of being complicated in my view.

Tom
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Last edited by tomchr; 8th December 2017 at 04:26 PM.
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Old 8th December 2017, 04:55 PM   #20
AndrewT is offline AndrewT  Scotland
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1mF charged through 1k0 will reach >95% of final voltage in ~ 5times RC or ~ 5seconds.
Taking longer than that to get to less than 1volt (~96%) makes me suspicious.

The easy way to allow a shorter delay is to reduce the 1k to 100r, making RC = 100ms and 5RC = 500ms
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