# voltage not sensitive to temperature

Discussion in 'General Electronics Chat' started by z_mx, Apr 8, 2013.

1. ### z_mx Thread Starter New Member

Apr 7, 2013
9
0
Hi!
I am young engineer after college. i am trying to solve next problem:

i need to stabilize voltage between points A and B (constant 0.7V) in temperature range from -45 to +85. Haw can I do this (minimal change in the circuit)?
thanks very much
https://www.dropbox.com/s/4g59uc864z0jl1s/4A.jpg

Last edited: Apr 8, 2013
2. ### KMoffett AAC Fanatic!

Dec 19, 2007
2,584
234
Precision +/- ?
Source range?

Ken

3. ### timescope Member

Dec 14, 2011
298
44
To obtain a voltage of 0.7v across 0.33 ohms implies a current of 0.7 / 0.33 = 2.12 Amps.

No load is shown on the schematic and you will not get 0.7v without one.

This circuit is a constant current source

Timescope

4. ### z_mx Thread Starter New Member

Apr 7, 2013
9
0
I need to get 4A (+-10%) on the source ( pin 3)
thank you

Max

5. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,442
1,628
You are depending on The B-E drop of Q4 to regulate that voltage, and that drop will vary with temperature... 2mv per degree C as I remember.

6. ### crutschow Expert

Mar 14, 2008
13,509
3,385
You can build a (relatively) temperature stable constant-current source using a low offset op amp and a temperature stable voltage reference. The circuit would be similar to that shown here, but substitute you P-MOSFET for the PNP transistor and generate the reference voltage with the reference connected between the +28V and the Vref point (remove R1 and R2).

Note that Vref must be referenced to the V+ (+28V) rail, not ground. Low voltage references usually go no lower that 1.2V, so you can use a voltage divider between the voltage reference and +28V to reduce the voltage to your desired 0.7V below +28V.

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