Working out resistor values

[Spadez]

Hi.

I have a 15V supply coming on to my board and from that I need a range of 0.4V to 1.2V using a potentioneter (and resistor if needed) as a volume input on my IC.

Could someone recommend a combination I could use to achieve this? I tried a 1M pot but that didnt work out very well as it was far too sensitive and let the voltage go too high.

Regards,

James

 

[MikeMl]

Add a fixed resistor both above and below the pot, thusly:

V(out) is shown as a percentage of pot position. This only works if virtually zero current flows out the wiper.

 

[davenn]

A variable pot isnt going to work for a variable poer supply as hinted at by Mike.
you are not going to have any useable current. Any significan current from the 15V supply will just burn out the carbon track of the pot

Give a bit more background on what you are trying to achieve and maybe there are other options
as you seem to be talking about 2 different uses in the one sentance a variable power supply and a volume control
which do you really need ? --- hence clarification needed

Dave

 

[Spadez]

Hmm. Thank you for the reply. It is to vary the voltage on my amp IC to determine the volume. This is the datasheet, p.g7 shows the voltage range and another page also shows a pot being used to control it:

https://www.electro-tech-online.com/custompdfs/2011/03/TDA7052A_AT.pdf

Im guessing in this application hardly any power will be drawn so it will be ok?

 

[davenn]

Hmm. Thank you for the reply. It is to vary the voltage on my amp IC to determine the volume. This is the datasheet, p.g7 shows the voltage range and another page also shows a pot being used to control it:
Im guessing in this application hardly any power will be drawn so it will be ok?

Hi there
ok so now we have the background interestingly it doesnt state as to if the pot should be log or linear type
Log is normally used for master volume controls. but then this pot isnt wired into the cct where most vol controls would go.
Log and liner pots are denoted with an A or B on the pot by the resistance value.
I assume you wired the pot correctly with the centre pin connected to one of the outer pins and with a 1uF cap across the outer pins ?

cheers
Dave

PS. its stating a 6V power supply for that chip you probably shouldnt be using 15V, thats getting close to its max rating

 

[MikeMl]

According to fig 5 on the data sheet, the current draw from the wiper ranges from -45uA to +10ua, with -20uA at 0.8V (pot centered). This will effect the voltage at Pin 4 a bit.

This would suggest making the pot 5K or 2K and reducing the two resistors by the same ratio.

 

[audioguru]

Duh!
Figure 6 and figure 7 in the datasheet show the volume control with the IC powering it, no external voltage is feeding it!
They show that with the volume control resistance maximum at 1M then the gain is 30dB.

The written spec's and figure 3 show that the gain is typically 35.5dB when pin 4 is 1.4V which might happen when the volume control resistance is infinity. The graph has a curve of the gain control and shows that a linear 1M pot provides a logarithmic volume function.

 

[Spadez]

Hi,

Its the first time ive used these and im still a begineer so the help is very much appreciated.

Is it correct that I should be using a 1M variable pot, with pin four connected to both input legs of the pot, and then the third output pin of the pot going to ground?

 

[davenn]

Hi,

Its the first time ive used these and im still a begineer so the help is very much appreciated.
Is it correct that I should be using a 1M variable pot, with pin four connected to both input legs of the pot, and then the third output pin of the pot going to ground?

there's 3 pins on the pot... centre pin to one of the outer pins and that common connection to pin 4 of the chip
the other outer pin of the pot goes to ground.

Dave

 

[Spadez]

Sorry im not trying to make this more difficult, but im having a hard time understanding this.

**broken link removed**

A: Pin 4 as well?
W: Pin 4
B: Ground

 

[audioguru]

If your sketch of the pot is its front side where the shaft is located then pin A is connected to ground and pins W and B connect to pin 4 of the IC.

 

[Spadez]

Ah thank you.