I'm trying to design a 25W stereo amplifier for my senior project using the LM3886 IC but seem to be running into some problems with my power supply.
Here is the design I made in LTSpice, its mostly taken from the datasheet. The power supply is on the same PCB as the amplifier itself.
I am powering this amplifier with a 240VA transformer (40VCT, 6A) (https://www.mouser.com/datasheet/2/410/F_273U-780989.pdf) . I've done some general unloaded voltage testing on the transformer and have found no issues. I chose this transformer rating through calculations done with the equations provided at the bottom of the LM3886 datasheet.
Most recently when I plug the transformer's secondary winding to my PCB I blow my 10A fuse that is located on the primary winding. How do I go about troubleshooting the problem if I am unable to put power into the board and test things properly? I have triple checked the MUR860 diodes that are located in the center of the board and everything appears to be biased correctly.
I do not know if my transformer and wall socket models are very accurate, I set the series resistance to 10 ohms since it resulted in a realistic current for a wall socket in the US. Because of the voltage step down however (120V wall socket to 40VCT), my current is 3 times what is in the primary winding in LTSpice and is leaving me extremely confused on the next steps. My only idea is to add a current divider circuit between the secondary winding of the transformer and the voltage rectifier in order to get a more ideal current entering my LM3886 chips.
Another thing that I am confused about is when I calculate the voltage and current that my power supply should be feeding into the IC (based on the datasheet and my desired power output), I get about 20V (positive and negative) and around 3A per amplifier circuit. When I run tests on LTSpice however, it seems that the input impedance on the IC power pins is so high that I am inputting current in the order of micro amps. Is this a result of simulating the IC in rest mode (powered on but not playing music)?
Here is the design I made in LTSpice, its mostly taken from the datasheet. The power supply is on the same PCB as the amplifier itself.
I am powering this amplifier with a 240VA transformer (40VCT, 6A) (https://www.mouser.com/datasheet/2/410/F_273U-780989.pdf) . I've done some general unloaded voltage testing on the transformer and have found no issues. I chose this transformer rating through calculations done with the equations provided at the bottom of the LM3886 datasheet.
Most recently when I plug the transformer's secondary winding to my PCB I blow my 10A fuse that is located on the primary winding. How do I go about troubleshooting the problem if I am unable to put power into the board and test things properly? I have triple checked the MUR860 diodes that are located in the center of the board and everything appears to be biased correctly.
I do not know if my transformer and wall socket models are very accurate, I set the series resistance to 10 ohms since it resulted in a realistic current for a wall socket in the US. Because of the voltage step down however (120V wall socket to 40VCT), my current is 3 times what is in the primary winding in LTSpice and is leaving me extremely confused on the next steps. My only idea is to add a current divider circuit between the secondary winding of the transformer and the voltage rectifier in order to get a more ideal current entering my LM3886 chips.
Another thing that I am confused about is when I calculate the voltage and current that my power supply should be feeding into the IC (based on the datasheet and my desired power output), I get about 20V (positive and negative) and around 3A per amplifier circuit. When I run tests on LTSpice however, it seems that the input impedance on the IC power pins is so high that I am inputting current in the order of micro amps. Is this a result of simulating the IC in rest mode (powered on but not playing music)?
Yeah. There's something screwy with that power supply. Does your transformer really only have three wires on the secondary (so a true centre tap)? That's pretty rare these days.
If it does, then you can use a two-diode full-wave rectifier:
From: https://www.electronics-tutorials.ws/diode/diode_6.html
Tom
If it does, then you can use a two-diode full-wave rectifier:
From: https://www.electronics-tutorials.ws/diode/diode_6.html
Tom
The center tap is incorrectly connected and there are 4 extra diodes. If you insist on using 8, CT should directly form the ground leg, and not connect to any part of the rectifier. The other four diodes generate the actual rails.
First schematic on the page is what you need:
https://hackaday.com/2014/12/03/scope-noob-bridge-rectifier/
First schematic on the page is what you need:
https://hackaday.com/2014/12/03/scope-noob-bridge-rectifier/
Pin 8 is grounded...that is going to keep the chip muted. Please read the data sheet...you need to use that pin to make it work. Also, that power supply is ridiculously complex, and cannot work as shown. If all you have is an actual center tapped tranny, the center tap must be connected to circuit ground, not like that crazy thing you got going on with the too many rectifiers...all you need is one bridge rectifier.
Mike
Mike
Yeah, I'm on a very tight budget and the transformer I selected was the cheapest I could find that still had the power rating I was looking for. I would like to not have to spend more money on a different transformer so I will likely try to bread board the schematic you attached and see if it makes a difference.
The reason I have two bridge rectifiers is because I had designed a mono amp circuit then copied and pasted the entire circuit to make the design stereo. In terms of the capacitors, the 100uF capacitor is meant to act as my ripple capacitor while C3, C2, and C1 and all mirrored sets of these capacitor values are meant to be power supply decoupling capacitors. I read about this concept from the following page: https://www.circuitbasics.com/desig...er-lm3886/#Power-Supply-Decoupling-Capacitors
While you have too many rectifier diodes in the power supply( for center tap transformer) you have an interesting minimalistic amount of capacitance.
Basically 1000uf +100uf +22uf each rail for each channel.
That’s not the best, hopefully you could replace those 1000uf caps with larger values.
Basically 1000uf +100uf +22uf each rail for each channel.
That’s not the best, hopefully you could replace those 1000uf caps with larger values.
Folllow the path through D2 to D9 and you will see that they are shorting the transformer output. Too many diodes. Ground the secondary center tap and remove all but two diodes and try that. Keep D3 and D8 only. You would need to cut and jump to put in two more diodes properly to make full wave rectification.
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Then you need only 4 diodes or 1 single bridge rectifier package. I did the same mistake in my first build, tranny became super hot. From what I see, you are shorting secondary winding thru 2 diodes. during each AC half cycle
Use series dim 40W - 100W incandescent bulb on the primary and try to build an external power supply using tranny , 4 diodes or 1 single bridge rectifier package. and 2 caps. Measure everything twice and only then connect it to amp PCB.
Beware of DC at output of the amp, don't connect B&W Nautilus for testing , use any cheap speakers with 47R to 100R series resistor
read-
https://www.circuitbasics.com/design-hi-fi-audio-amplifier-lm3886/
There's no negative rail with two diodes in the way you've drawn.
You have to either reconfigure the amp for single-supply operation, or use two more diodes to get the negative rail, or use half-wave rectification with one diode per rail.
Here is the very common four diode arrangement for full wave bridge with a center tapped transformer. Ignore the regulators and part values for your application. It looks like the OP's schematic was captured in LTSpice in an earlier post. I'm guessing that the transient simulation was never run as it would not work as drawn. Correct the schematic and run the simulation before making changes to the board to avoid more problems. Verify the supply rails provide the desired voltage in the simulation. D1 and D4 in this schematic correspond to D3 and D8 in the OP's schematic. With cuts and jumps on the existing PCB the D2 and D3 diodes shown below could be added to the existing PCB. Looking at the amplifier schematic, I would cut and add jumpers to the left side of D2 and D8 to connect the left side of each of those diodes to the positive and negative filter capacitors to form the schematic below. I hope that makes sense. So in the end you would keep only D2, D3, D8 and D9 after modifying the PCB as described.
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That can be made with a diode bridge like the KBPC2510 as well.
Yeah, quite possibly. Though it'll probably work for a desktop amp, especially if you add a small fan.
Tom
"That can be made with a diode bridge like the KBPC2510 as well."
The PCB has individual MUR860 diodes installed already. Pulling out four diodes and adding two cuts and jumps as I explained above will make the existing layout work perfectly with the existing diodes. There's no reason to buy a diode bridge unless the other diodes are blown. There's no room for it on the PCB anyway.
"The heatsinks are too small" There's no way those are too small for normal listening levels. Maybe if they were run at full power sinewave output they would be too small, but those amps have over temp and current limit built in, so it hardly matters. They will survive drunken parties with those heat sinks just fine. I run them with smaller heat sinks just fine pushing 35 Watt peaks from music program per amp.
The PCB has individual MUR860 diodes installed already. Pulling out four diodes and adding two cuts and jumps as I explained above will make the existing layout work perfectly with the existing diodes. There's no reason to buy a diode bridge unless the other diodes are blown. There's no room for it on the PCB anyway.
"The heatsinks are too small" There's no way those are too small for normal listening levels. Maybe if they were run at full power sinewave output they would be too small, but those amps have over temp and current limit built in, so it hardly matters. They will survive drunken parties with those heat sinks just fine. I run them with smaller heat sinks just fine pushing 35 Watt peaks from music program per amp.