Search found 23 matches

by trident
Wed Feb 25, 2009 9:30 pm
Forum: General Discussion
Topic: Help with comparator design
Replies: 38
Views: 4991

Re: Help with comparator design

1)First off, 0.0032v into 8 ohms only produces about 1.2 watts of power. I doubt you can hear this over the sound of the fan itself 2)Your new circuit isnt too bad either though, but again if you use a -2v Vgt for Q1 you might run into the problem of pin 7 not being able to go high enough to turn Q...
by trident
Sat Feb 21, 2009 8:32 pm
Forum: General Discussion
Topic: Help with comparator design
Replies: 38
Views: 4991

Re: Help with comparator design

Hello The suggestion from MrAl about adding a bias voltage to the low-pass filter got me to thinking and I just may have arrived a the minimal configuration that meets my design goals. I originally chose a comparator to eliminate any offset voltage from the mother board output. If this new and hopef...
by trident
Thu Feb 19, 2009 11:09 am
Forum: General Discussion
Topic: Help with comparator design
Replies: 38
Views: 4991

Re: Help with comparator design

You can most likely get away without using R5 and R6 (to get a min output) if you instead connect the 100k (R7) directly to pin 1 of the comparator and connect a 150k resistor from C2 to +3v. Hello If a 150K resistor is connected to C2 would this not change the cutoff frequency of the low-pass filt...
by trident
Tue Feb 17, 2009 11:24 pm
Forum: General Discussion
Topic: Help with comparator design
Replies: 38
Views: 4991

Re: Help with comparator design

With an op amp like the one shown in the schematic (1/2 of LM392) the output will not be able to reach +12v ... so an idea might be to add a diode in series with the output of the op amp to force the output to always be one diode drop above the max output. Hello Switching the amp to negative negati...
by trident
Mon Feb 16, 2009 12:22 pm
Forum: General Discussion
Topic: Help with comparator design
Replies: 38
Views: 4991

Re: Help with comparator design

http://www.postimage.org/Pq2lL12i.jpg After putting this project aside for some other priorities, I stumbled on to a data sheet for the LM392 amplifier/comparator dual. This got me thinking about the limitations of the discrete transistor circuit. A comparator input can give better noise immunity, ...
by trident
Sat Jan 24, 2009 4:21 pm
Forum: General Discussion
Topic: Help with comparator design
Replies: 38
Views: 4991

Hello I occurs to me that I did not see the forest for the trees. I redrew the circuit including the motherboard and realized that the amplifier is sourcing current to the motherboard. Insert Homer Simpson d'oh! I believe what needs to be optimized is the low-pass filter and the pull-up resistor. Co...
by trident
Fri Jan 16, 2009 6:23 pm
Forum: General Discussion
Topic: Help with comparator design
Replies: 38
Views: 4991

Hello everyone MrAl is this what you envisioned for the LM393 configuration? I have changed the threshold to 1.0 volts and reduced the hysteresis, from the original circuit. C2 was added per the data sheet. As I explained before, the LM4040 has a turn-on time of 10 microseconds and was misapplied in...
by trident
Thu Jan 15, 2009 10:43 am
Forum: General Discussion
Topic: Help with comparator design
Replies: 38
Views: 4991

Hello sghioto I concur with Bigglez, very elegant solution. Correct me if I am misunderstanding. The AD822 you suggest has a slew of 3 volts per microsecond. With positive logic PWM, a 25 KHz square wave of 30% duty cycle would have a pulse width of 12 microseconds and a space width of 28 microsecon...
by trident
Tue Jan 13, 2009 4:11 pm
Forum: General Discussion
Topic: Help with comparator design
Replies: 38
Views: 4991

Hello to All The fan control is single loop, when the temperature increases above threshold the software I am using, SpeedFan, increases the speed command to the fan. It does not check to see if fan speed increased. SpeedFan waits a time interval and checks the temperature again and adjusts the spee...
by trident
Mon Jan 12, 2009 11:08 am
Forum: General Discussion
Topic: Help with comparator design
Replies: 38
Views: 4991

Hello Bigglez You have framed the problem well, what I require is a circuit that detects the duty cycle of the PWM signal and ignores the amplitude. Ideally it will give a 12 volt DC output for 100% duty cycle. My desired output range is 5 to 12 volts to the fan, assuming that the output is proporti...
by trident
Sun Jan 11, 2009 7:29 pm
Forum: General Discussion
Topic: Help with comparator design
Replies: 38
Views: 4991

Bigglez The PWM signal varies, some use 5 volt logic others use 3.3 volt logic and I found one that uses 4 volts as logic high. One of the first designs I experimented with is similar to what you describe (see schematic), but it has a serious shortcoming, at some speed settings the fan will "sing" w...
by trident
Thu Jan 01, 2009 3:43 pm
Forum: General Discussion
Topic: Help with comparator design
Replies: 38
Views: 4991

Hello, sghioto The INTEL initiative is all well and good for motherboard designers, it eliminates the need for high current output transistors on fan headers. Many of the newer motherboards now only support four wire fans for the CPU. However if you are a person who wants you PC to be seen and not h...
by trident
Mon Dec 29, 2008 5:12 pm
Forum: General Discussion
Topic: Help with comparator design
Replies: 38
Views: 4991

Help with comparator design

My goal for this project is to control a thee wire brushless fan with the PWM signal from a four wire fan header. I have looked at number motherboards and discovered implementation the control signal varies. The INTEL specification is mute on a logic hi value, but requires a maximum logic low of 0.8...
by trident
Sun Nov 09, 2008 1:03 pm
Forum: General Discussion
Topic: help with op amp circuit
Replies: 16
Views: 3248

Hello

The pull up was added as a failsafe for a missing control signal, this will keep the fan running and prevent loss of cooling. The control signal is from an open-collector / open drain-output so I think 10K will work.

Regards, trident
by trident
Sun Nov 09, 2008 10:36 am
Forum: General Discussion
Topic: help with op amp circuit
Replies: 16
Views: 3248

Pin 4 of J1 cannot source more than 5mA with a supply voltage of 13.2 volts, the 2.2K pull up resistor [R2] takes care of that, but I am not certain how to factor in the zener voltage tolerance to size the shunt resistor [R3] for minimum supply voltage of 11.4 volts. Would it be: [11.4V - 3.5V] / [ ...