potentiometer wiring problems

[joeloek]

ummm... what does it mean when my potentiometer bursts into flames? meaning it smokes and sparks. i'm only using a 9 volt battery. what am i doing wrong?
here's the schematic:

[Ripthorn]

Which one is burning up?  There are two in the schem.  If something is sparking, that means there is something on the other side of the pot that is drawing way too much current.  That means that even with 9V, you can get a lot of power going across your (most likely) 1/4W potentiometer.

[joeloek]

dammit i always forget something lol
the logarithmic one
and yeah, 1/4 watt.
what can i do to remedy that?

[mtripoli]

Taking a guess... that pot on the output in series with the speaker is wrong. If you want to control the output volume the pot has to be on the input side. Secondly, the output needs a series cap to remove the DC bias at the output of the LM386. Without it you have DC flowing through the pot and the speaker all the time (bad). A 100-250uF cap with the positive lead going to pin 5.

Pin 3 is the (+) non-inverting input; why is it connected to the rail? Input the signal (with a volume pot) here.
Pin 2 ties to ground (pin 4).
Pins 1 and 8 control GAIN in the circuit (not volume level). You can use a fixed cap or a series cap and resistor to set the gain.

Have a look at the sample circuits in the datasheet:  http://www.national.com/ds/LM/LM386.pdf

Hope that helped...

Mike Tripoli

[joeloek]

oh wow thanks
that helps immensely
i thought i read somewhere that you could put a variable resistor between pins 1 and 8 and have gain control?
gah back to the drawing board lol
thanks alot =D

[P.S.]
the pot's probably useless after that, huh?

[mtripoli]

Have a look at this, excerpt from National Semi data sheet:


Fire from the pot? Yeah, I'd say it's junk...
Use a pot and CAP between 1 and 8.
Use the bypass (pin7) to keep some noise out of the ciruit.


Mike Tripoli

[joeloek]

mmhmm... i see...
i read through the datasheet and yeah got a whole new perspective on the chip and how to wire it
thanks for the link
so the 250uF cap takes the DC bias off the speaker
the resistor/capacitor series between pins 1 and 8 sets the gain
signal from guitar into #3
i want the + end of the power supply(9v) going into pin 6, right?
pins 2 and 4 are grounded
5 goes to the electrolytic cap i see...
what exactly does the bypass pin do?

[mtripoli]

mmhmm... i see...
i read through the datasheet and yeah got a whole new perspective on the chip and how to wire it
thanks for the link
so the 250uF cap takes the DC bias off the speaker - Yup
the resistor/capacitor series between pins 1 and 8 sets the gain - Yup
signal from guitar into #3 - Yup
i want the + end of the power supply(9v) going into pin 6, right? - Yup
pins 2 and 4 are grounded - Yup
5 goes to the electrolytic cap i see... -Yup
what exactly does the bypass pin do? -Look at the equiv. circuit on the first page of the data sheet. - It places a cap (10uF or more) through a 15k resistor on the positive rail.

[joeloek]

okay cool
- It places a cap (10uF or more) through a 15k resistor on the positive rail.
what's that for, just out of curiosity?

[markeebee]

okay cool
- It places a cap (10uF or more) through a 15k resistor on the positive rail.
what's that for, just out of curiosity?

It stops noise from the power supply getting onto the audio line.  You don't HAVE to include it, but the output of your amp will have less hum & buzz if you do.

Fair play to you for designing something from scratch, but maybe sometimes it can be a good idea to 'cheat' a little bit and use existing designs for reference.

If you're not averse to a little leg-up, and if you haven't already checked them out, have a look at these for inspiration:

http://www.beavisaudio.com/projects/NoisyCricket/MarkII/NoisyCricketMarkII_RadioShack_Rev2.pdf
http://www.runoffgroove.com/ruby.html
http://www.runoffgroove.com/littlegem.html

They've become the 'standard' designs for LM386 guitar amps.  The Beavis site is especially wonderful, Dano has a real knack for explaining things in a simple and entertaining way.

[anchovie]

Note that the Little Gem does have the volume control on the output of the chip, but in this case it's a rheostat rather than a regular pot so therefore shouldn't be a fire hazard!

[joeloek]

@markeebee:
oh cool. i'll definately heed to include it then
and yeah... i'd actually modeled it a little off the ones on runoogroove

[Hupla]

The Pot in pins 1 to 8 is wrong. One of the lugs shouldnt be going to ground. you connect the middle one to one of the other lugs and then connect tht lug to 1 and the other to 8. Dunno if that was the one that caused you problems though.

[mtripoli]

You MUST use a cap on the ouput of a LM386... The output "sits" at 1/2VDD...