Diode running into the resistor

[mearvk]

http://www.itisff.it/dip_eln/tda2030.pdf

Couple questions.

1. In the Typical Application diagram on the left there is Vi w/ a 22k resistor. Appears that there's a diode running into the resistor and then the resistor goes to ground. The question is how should I interpret that? The connection isn't on either side of the resistor but literally going into it. How to wire this? Why is this portion dotted? The diode should allow the input signal to pass thru but no current to pass back out, right?

2. The diodes on the circuit between 3 and 5. Over http://images.the12volt.com/12voltimages/12voltdiodes.gif is a diode cheat sheet. What purpose does the circuit between pins 3 and 5 serve? Should current be flowing towards 3 from 5 or to 5 from 3?

Thanks.

 

[the_emi_guy]

1. It is a 22K potentiometer for volume control

2. Protects output of the amplifier from going above Vs or below ground.

 

[mearvk]

Thanks.

The diodes should be letting current flow from 5 to 3 or 3 to 5?

Thanks.

 

[gnurf]

The diodes should be letting current flow from 5 to 3 or 3 to 5?

None of the above.

 

[AlephZero]

The diodes should be letting current flow from 5 to 3 or 3 to 5?

The diodes are there to protect the IC against faults. In normal operation they do nothing.

If you apply incorrect voltages to the circuit (for example connecting the power supply the wrong way round) the diodes limit the incorrect voltages applied to the IC.

 

[mearvk]

1. How do I know if I have them soldered correctly?

2. Can you guy more into explaining how the fault protector works?

My understanding of diodes is that they are a one way street for current. They can be used for rectification precisely because of this. I don't understand how they'd be used for FT.

Thanks/

 

[gnurf]

1. How do I know if I have them soldered correctly?

2. Can you guy more into explaining how the fault protector works?

1) The cathode is marked on the device, see the datasheet.

2) Look at the I-V curve of a diode. What happens when the voltage across the terminals increases? The diodes in your circuit perform the same role as the overflow drain on your bathroom sink.

 

[mearvk]

Top view:

Bottom view:

More pics

Don't mind the missing ground connection. I took it off for diagnostic purposes. The amp isn't working either way. Assume it's there.

The dots represent connections. The arrows indicate a circuit running under another area. For instance, ---> component <--- indicates that there's a wire under the component but not connected to it.

I've used the Typical Application diagram as my basis. I'm feeding it 18 volts (verified) from a simple AC-to-DC rectifier I made. I'm using pin 3 (Vs-) as my ground in all cases.

Thanks.

 

[the_emi_guy]

Check you input circuitry again closely. The 100K resistors are not supposed to be connected directly to the input.

 

[mearvk]

Thanks EMI.

I made the resistor a 2nd path, which I think is what we were aiming for. Added back the ground connection for input. Still no music but I do hear the soft popping when I connect the headphones.

 

[the_emi_guy]

Where is your 1uf input capacitor?

 

[mearvk]

Thanks EMI; 2nd set of eyes and all that.

Ok so I added the 1uF and the 22uF (bottom left of circuit diagram).

I checked the output twice, after adding the 1uF and then after the 22uF. The sound actually got worse after adding the larger capacitor. Having said that there was an audio signal initially albeit buried under static and popping. Currently it's unlistenable.

There's a couple decoupling capacitors in the diagram just after the Vs+ but before the first 100k resistor. I don't have those. And there's a circuit from pin 4 (output) to ground before the 2000 uF capacitor. Don't have that either. Finally, I'm using a 1000 uF in place of the 2000 uF since I just don't have one right now. That's full disclosure.

 

[the_emi_guy]

Add the decoupling caps.
Sometimes it's hard to get amps like this to work unless proper layout guidance is observed. Leaving decoupling caps off may aggravate the situation.

You can also try lowering the gain (reducing resistor between pins 2 and 4).

 

[mearvk]

After adding the decoupling capacitors for the Vs+ it works. Weird thing is that the voltage between pins 5 and 3 is like 5.83 when I have the potentiometer fully open. With no current being pulled the voltage is like 17ish.

Probably going to get a stronger power supply tomorrow at Radio Shack. For anyone coming behind me I left out the 220nF and 1 ohm circuit and it still works. Not sure what purpose that serves, be glad to know if anyone cares to relate that data to me.

Thanks!

 

[NascentOxygen]

I left out the 220nF and 1 ohm circuit and it still works. Not sure what purpose that serves, be glad to know if anyone cares to relate that data to me.

Thanks!

Your application note says that is a filter to protect against oscillation if the speaker is inductive. (It might also help to limit any speaker thump when power is first applied.)