dc fan control

[glinjik]

hi i have tried a few of these circuits most do not work so today i got yet another circuit and tried it and it works fine very sensitive but it should not work i conected the fan to the wrong place once i realised i put the conection in the right place it then wont work? strange so i put it back to my orinal error place and all works ive highlghted where ive connected in the picture what is happening here why is it working?

[mikerj]

You've just put a yellow square on the schematic, that doesn't tell us how you've connected the fan. 

If you connected the fan +v to where the yellow square is, and the -ve to ground then the fan is connected directly across the power rails.  The only way the fan speed could be controlled in that case is if you have made an error in the construction and the BD140 is actually shunting the power supply and causing the voltage to drop, though this would reverse the control law

[glinjik]

hi sorry yes yellow square is where the fan + is the ground is as it is shown ive checked all connectios transistors in the right way round even the adjustment pot works fine i dont understand why its working

[glinjik]

after letting it run for a wile the bd140 starts getting a bit warm as expected