Hi guys!
I need some help with a comparator circuit.
How do I find out what resisters to use?
Here is an example

I used 2 quad's and 8 leds to make an led thermometer. The lowest voltage I will have is 2.73v and the highest is 3.13v .5v for each led. each led is for 5c.

Thanks!

 

Hello,

In your example the voltage divider is dependend on V+.
This can be changed by replacing R1 with a current source.
To get 2.73 Volt , you can calculate the resistor this way.
R = U / I . If I = 10 mA (for example) the resistor will be 2.73 / 0.01 = 273 Ohm.
Each step has a voltage 0.05 Volts. so the intermittend resistors are all 0.05 / 0.01 = 5 Ohm.

Greetings,
Bertus

 

 

But what if the current is not 10mA?

 

--------
I= R/v
--------

 

The relationships in bertus' examples still apply. Do the math using the actual (or desired) current.

 

but then I need to know resistance...
so it keeps going around not knowing either the resistance or voltage.

Beenthere:
how would I find out the actual current without knowing the resistance?

Im so confused now.. XP

 

Hello,

The 2.73 Volts resistor will be 2.73 / I .
So if I = 20 mA R = 2.73 / 0.02 = 136.5 Ohm.
If I = 5 mA R = 2.73 / 0.005 = 546 Ohm.
The same goes for the intermittend resistors.
So if I = 20 mA R = 0.05 / 0.02 = 2.5 Ohm.
If I = 5 mA R = 0.05 / 0.005 = 10 Ohm.

Greetings,
Bertus

 

http://www.ee.ucl.ac.uk/~amoss/java/vdiv.htm

 

Hello,

Here I made a little schematic.
The values may not exact, but will bw close.

Greetings,
Bertus

 

I had the same circuit except the 2 diodes and transistor. What are they for?
I did some tests in multisim and if I change the 1k resister, the one after the 2 diodes, to 150 it works right!
Can I use a different diode and transistor?
Thanks!

EDIT**
Actual I replaced the whole diode transistor thing with a 190ohm resister.

The circuit works when I replaced the LM335 with a pot and used a pot on the voltage divider for tuning, so I will have to get some more temperature sensors. What would be better? A LM335 or a thermocouple?

 

Hello,

As I told you when the powersuplly changes (this can happen when you use a battery as powersupply), the voltages for your comparators will change too.
Thats why I recommended a constant current source.
On this page the constant current source is descibed.
http://www.jefspalace.be/analog/input stage/constant current source.php
The only differecnce with mine is that this source goes to powersupply.
I have changed this by mirroring it and use a PNP transistor.

Greetings,
Bertus

 

I dont understand how that works. If you have a voltage of 5v and a constant current of 1A, then if you use ohms law your resistance should be 5ohms. But what if you use a bigger resistor? like 10ohms, would the voltage become 10v?

And in the third picture,why connect ground to the battery positive?

 

Hello,

I was concentrating on the firtst 2 pictures.
I also do not understand te tekst of picture 3 , they are using other numbers for the resistors that do not match the picture.

I have redrawn my thermometer schematic with the new current source.

Greetings,
Bertus