Smoke at LM1876's output resistor having inductor
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 Chip Amps Amplifiers based on integrated circuits

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diyAudio Member

Join Date: Jan 2009
Thank you for very detailed information..

Quote:
This time I measure and found 0.9R resistance.

I build this LM1876 first time because I found several thread related to LM1875 and read somewhere LM1876 has two LM1875 having mute/stand-by function. In fact mute/stand-by function I could not build. At first impression I like LM1876 but long time listening I think I will not be able to live with it. My opinion it is just for pc multimedia speakers. If I push more voltage/gain chip becomes angry and over-heated. Or may be the reason I could not build as it should be.

Thanks & Best Regards.

diyAudio Member

Join Date: Nov 2006
Location: Indiana
Blog Entries: 1
Quote:
 Originally Posted by pra3718 10 Ohm Resistor wound with 1mm enamelled copper wire 10 turn. When I check resistance of the inductor what value should show by my digital multi meter ?
0 Ohms is incorrect, if your meter goes down to 0.1 milliohm, or even if it goes to only 1 milliohm and the wire is greater than about one inch in length.

Any metal wire with non-zero length has a resistance greater than zero.

Resistance of any metal conductor is

R = pL/A,

where R is in Ohms, p is the resistivity of the metal (0.0000000168 Ohm-meters for copper at 25 C), L is length in meters, and A is cross-sectional area in square meters.

For your 0.0005-meter-radius copper wire, i.e. with A = .000000785 square meters cross-sectional area, that becomes

R = (0.02139) x (Length in meters), in Ohms.

For one inch (2.54 cm) of that wire, the resistance should be about 0.0005533 Ohms, at 25 C.

Note that if you measure it with the resistor that it's wound on, you would be measuring the parallel combination of them both (and anything else that's in parallel with them), which would change the expected measurement value.

Last edited by gootee; 4th March 2012 at 01:57 AM.

diyAudio Member

Join Date: May 2008
Quote:
 Originally Posted by pra3718 This time I measure and found 0.9R resistance.
Have a look at the deviation of your meter. It has a certain percentage calculated from the maximum value of the measuring range plus a certain amount of digits. The lower the measured value, the more difficult it is to get a precise reading with a standard DMM.

Quote:
 Originally Posted by pra3718 read somewhere LM1876 has two LM1875 having mute/stand-by function.
That information is wrong. The circuit is much more similar to the LM3886, except that the muting function works differently.

Quote:
 Originally Posted by pra3718 In fact mute/stand-by function I could not build.
Are you using single or split power supply? The 2,5 V refer to half supply, which means only with split supply they refer to ground. If you use single supply, you have to use a level shift circuit according to figure 5 from the datasheet.

Either that or the IC is a fake.
__________________
If you've always done it like that, then it's probably wrong. (Henry Ford)

diyAudio Member

Join Date: Jan 2009
Thank you gootee

Thank you pacificblue, due to not getting expected results I was decided to move to LM4766 but your post stop me and try again.

Quote:
 Are you using single or split power supply?
I am using split power supply. i.e. 20 (+/-) VDC.

As per datasheet figure 5 : VCC = +20VDC , Logic Input = +5VDC and 10uf cap is for delay. Is this correct ?

 4th March 2012, 11:23 AM #25 diyAudio Member   Join Date: May 2008 So what do you have? Split power supply means no need for figure 5. In that case check, whether the amp is muted, when you connect Vcc to the mute pin. If it does your IC is OK and you have to check the mute circuit you tried before. If it does not, your IC is probably not an original LM1876. Single power supply means you need figure 5. 100k and 10µ are for delay. __________________ If you've always done it like that, then it's probably wrong. (Henry Ford)
diyAudio Member

Join Date: Jan 2009
Quote:
 whether the amp is muted, when you connect Vcc to the mute pin.
Yes,

 4th March 2012, 11:58 AM #27 diyAudio Member   Join Date: Jan 2009 When No signal power supply shows at chip pins 25(+/-)vdc and music starts with moderate volume it shows 20(+/-)vdc. Is this normal ?
 4th March 2012, 01:51 PM #28 Audio Engineer diyAudio Member     Join Date: Jul 2003 Location: Cambridge UK Hi pra3718, I think your amplifier is probably unstable. This is the most likely reason for lots of heat to be dissipated in output inductor. Check all your decoupling is correctly placed and soldered in. If possible borrow a oscilloscope so you can diagnose what is wrong. If you put a Zoble network on you have a good chance of stabilising the amplifier. Regards, Andrew
diyAudio Member

Join Date: Jan 2007
Blog Entries: 3
Quote:
 Originally Posted by pra3718 This time I measure and found 0.9R resistance.
You don't need fancy error calculations, when dealing with low resistances and cheap DVMs. Even cheap DVMs are surprisingly accurate.
Did you followed Andrews advice and substracted the wire loop resistance?
If so, there is probably a problem with the enamelled wire. Soldering it is difficult and I usually use a blade and sand paper to remove the coating.

diyAudio Member

Join Date: Jan 2009
Quote:
 If so, there is probably a problem with the enamelled wire. Soldering it is difficult and I usually use a blade and sand paper to remove the coating.
I have faced this problem if soldering is not perfect then resistance value shows 10R.

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