Roughly how much energy is wasted in a 24vdc pot controlling 2 0.1A fans rated at 24vdc, Power supplys output is 24vdc 1.5A also powering a 24vdc 700ma buck driver

 

The lower the rpm the more energy is wasted provided that the pot can withstand the power dissipation which is highly unlikely for standard pots

 

You should be using PWM to control the fan speed.

 

i see you always recommend a pwm, but how much more efficent is a pwm? how much is it going to cost? fan operates at 14-26 vdc, input power 2.40 watt

 

If you want to go that way, you need at least an emitter follower to buffer the potentiometer. The disadvantage, is that any voltage not dropping on the fan is going to drop on the transistor.

 

i see you always recommend a pwm, but how much more efficent is a pwm? how much is it going to cost? fan operates at 14-26 vdc, input power 2.40 watt

I don't always recommend it. However in your particular application, PWM just makes good sense.

If you simply use a pot/rheostat (or some form of linear regulator) to limit the current flow through the fan, the pot/rheostat/linear regulator will dissipate power as heat.

With PWM, the connection to power is either ON or OFF, so there is comparatively little power loss in the circuit.
[eta]
Cost is minimal; a wide-range PWM circuit can be built from just a few components; a 555 timer, a timing cap, a pot, a resistor, a couple of diodes and a power MOSFET. Since your supply voltage exceeds the limit of a 555 timer, you would also need a voltage regulator and associated caps for the input and output.

 

got a diagram and parts list? fans are 2 24vdc brushless .15 amp

 

Search the forum. AAC has plenty

 

got any searching tips?

 

for god sake use the search button or click all the link on top of this page

 

See the attached.

Sure, using a pot and a voltage follower is initially much cheaper to build and quite a bit easier. However, you'll pay for the inefficiency in energy costs and poor reliability.

 

thanks, never made a circuit before this will be my first

is the lm317 going need to be heatsinked?

 

An LM317 almost always needs to be heat-sinked.

However, not in this case. You might get a 15°C rise internally, but the power dissipation is only about 240mW after the start-up.

 

do you know where i can order all these parts in one place?

 

Hello,

If you want to order parts, it is good to know where you are.

You could take a look at digikey or mouser.

You can edit the profile and set the location.
It can be found under User CP (at the left almost top).
Then "Edit Your Details", in the Additional Information paragraph.

Bertus

 

im in nebraska, usa

 

hey sarge, do you have a partlists for that circuit?

 

--- Bill of Materials ---

Ref. Mfg. Part No. Description
C1 -- -- capacitor, 1nF
C2 -- -- capacitor, 100nF
C3 -- -- capacitor, 100nF
C4 -- -- capacitor, 100nF
C5 -- -- capacitor, 10µF
D1 -- 1N914 diode
D2 -- MURS320 diode
FanMot -- -- inductor, 1mH
M1 -- IRF510 MOSFET
R1 -- -- resistor, 1.2K
R2 Linear Technology Rtot=100K wiper=.5 integrated circuit
R3 -- -- resistor, 47
R4 -- -- resistor, 1K
R5 -- -- resistor, 120
R6 -- -- resistor, 10K
R7 -- -- resistor, 47, 2W
U1 Linear Technology LM555 integrated circuit
U2 Linear Technology LM317 integrated circuit

 

I dont know how to find or what this part is Linear Technology Rtot=100K wiper=.5 integrated circuit here is my shopping cart http://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=4a07d2116f can you check it over to see if its all the right parts?

 

Have you considered basket weaving for a hobby?

Perhaps you should.