Voltage regulator + resistor in parallel? Why?
- Thread starter elnaldo
- Start date
..... I'm sure with a new daughter and wife he's 'tired' i'm sure of that.
Oh, before you move on, make sure you put back the resistor from the input to output and look at the output with the scope. Important.
The reason is there's another possibility. The circuit powered by the regulator does not draw enough current to overcome the current through the resistor. Those regulator can only SOURCE current, it cannot sink current for $hit. This means if the current provided by the resistor is MORE than the circuit need, then the excess current has to find a path to go. The regulator cannot sink current, the excess current will pull the voltage at the output above 10V. The regulator will lost control, the ripple will go from the resistor to the circuit and create the hum and buzz.
So it is important to put back the resistor to test and scope the output voltage, make sure it is rock solid 10V with less than 50mV of anything.
The reason is there's another possibility. The circuit powered by the regulator does not draw enough current to overcome the current through the resistor. Those regulator can only SOURCE current, it cannot sink current for $hit. This means if the current provided by the resistor is MORE than the circuit need, then the excess current has to find a path to go. The regulator cannot sink current, the excess current will pull the voltage at the output above 10V. The regulator will lost control, the ripple will go from the resistor to the circuit and create the hum and buzz.
So it is important to put back the resistor to test and scope the output voltage, make sure it is rock solid 10V with less than 50mV of anything.
elnaldo
Lunatic Member
While testing all this, discovered thet freezing a TDA7313 stops the buzzing too...
http://html.alldatasheet.es/html-pdf/25118/STMICROELECTRONICS/TDA7313/1621/1/TDA7313.html
I think I'll wire the regulator with some longer leads to put an Anmeter at the output and measure the current.
All this , "tomorrow". Today I need to move on to other unit.
I'm measuring the output right now before putting the unit apart.
Edited: the output looks perfect, just some noise but it's the same noise with the unit unplugged, seems something the scope is picking from the air or the AC lines.
http://html.alldatasheet.es/html-pdf/25118/STMICROELECTRONICS/TDA7313/1621/1/TDA7313.html
I think I'll wire the regulator with some longer leads to put an Anmeter at the output and measure the current.
All this , "tomorrow". Today I need to move on to other unit.
I'm measuring the output right now before putting the unit apart.
Edited: the output looks perfect, just some noise but it's the same noise with the unit unplugged, seems something the scope is picking from the air or the AC lines.
Last edited:
You still get 10V DC at the output?
If so, move on. Your problem is somewhere else.
First do a DC measuring and write it on the schematic. Look at the voltage whether it make sense or not.
Then use the scope to look at where the noise comes from starting with the input of the TDA7313, then at the output of the TDA. Find out where the noise starts.Go through the whole signal path and find where it starts.
Do it step by step logically, don't just jump into conclusion so fast. Find the source of problem and talk about it. Don't pull parts out so quick. You might have just pulled out a good regulator already.
BTW, don't border to measure the current drawn, If you have the clean and 10V, you are ok with the regulator.
If so, move on. Your problem is somewhere else.
First do a DC measuring and write it on the schematic. Look at the voltage whether it make sense or not.
Then use the scope to look at where the noise comes from starting with the input of the TDA7313, then at the output of the TDA. Find out where the noise starts.Go through the whole signal path and find where it starts.
Do it step by step logically, don't just jump into conclusion so fast. Find the source of problem and talk about it. Don't pull parts out so quick. You might have just pulled out a good regulator already.
BTW, don't border to measure the current drawn, If you have the clean and 10V, you are ok with the regulator.
Last edited:
elnaldo
Lunatic Member
Hello. Back to this unit, I've closed the cover with the problem "solved" by now.
Following your advice, I've focused to find the source of the noise. It was the TDA7313 causing the noise (the unit has 2 TDA7313, managing "zone1" and "zone2" outputs, and the Zone 2 was silent, just the Zone 1 was noisy. Since it was very intermittent, it was hard to catch it. Many times, with all the test setup connected, it didn't do the noise. Then I discovered that moving the volume up or down could trip the noise, specially when going to zero and keep going down with the "down" button.
What cheated me 1st time was that freezing the regulator, the noise stopped, but I think it was due to limiting the current or the voltage to the TDA. Freezing the TDA also stopped the noise.
What I did:
(after searching for a replacement locally, not available, found just the through hole version, not the smd IC)
The TDA is fed by the 10V regulator and a 15 ohm resistor, which drops around 0.4V (around 27mA at that resistor) The absolute max supply voltage for that IC is 10.2V, and supply voltage is 6 to 10.2V. Since freezing the 10V regulator stops the noise, I decided to drop the supply voltage a bit more to make the IC to run cooler. Replaced the 15 ohm with a 56 ohm resistor, and supply voltage at the IC came down to 8.5V.
Added a heat-sink to the 10V regulator, added a heat-sink to the TDA7313, and it worked silently for about one week, so I think the problem is fixed by now, in the meantime I'll get a replacement IC from the web, or if needed, swap the Zone 1 and Zone 2 chips (zone 2 is not used by the owner), but desoldering and resoldering those 28 pins ICs in that tiny board is not easy without the proper smd reworking tools.
BTW, what I write in 10 lines of text, took me weeks of going to the board, testing, assemble, disassemble, measure here, there, let it warm up, test it for weeks, and so. A couple of weeks ago seemed solved with just the heat -sinks, but putting an ear close to the tweeter, the noise was still there at a very low level.
Binkman
Lunatic Member
Glad you got that headache cured. I certainly understand chasing ic rabbits for weeks. Many don't realize that v in min max was a good clue and addressing the nominal v plus heat sink will keep that in check. orig. ic was probably pushed too hard, too or poor reliability. Even though ic appear small but inside is a crap load of micro circuits that can become unstable generating it's own little heated circuit inside.
elnaldo
Lunatic Member
The IC was working at approx 9.5V, being the max 10.2V . Looks safe to me, but seems it was too close to the max. This same problem was reported on the web (a guy from India had the same problem, high pitch noise caused by that IC).
I think at 8.5V (or less) the IC will be more relaxed. I don't know it this affects the distortion, or whatever, but the datasheet specs 6 to 10V working voltage. 8.5 is fine within the range, I could even fit a 100 ohm resistor and drop it to ~7.5V.
I think at 8.5V (or less) the IC will be more relaxed. I don't know it this affects the distortion, or whatever, but the datasheet specs 6 to 10V working voltage. 8.5 is fine within the range, I could even fit a 100 ohm resistor and drop it to ~7.5V.
