I am looking for a very simple and cheap circuit that will take a small voltage, around 0.6-0.65v and double it. The input may increase, so the circuit should follow accordingly. It will only increase to maybe 1.1-1.3v, so the output would be 2.2-2.6v. Right now I take the source 0.6v and using a 3.3v source and a variable resistor, I bring the 3.3v down to 0.7v and add it to the original source, so if the original source increases, the output does, but its hit and miss sometimes and I would like something to add in between the input and output and not be combining voltages. I'm good at soldering, I've built a few circuit from schematics and such, but just don't understand how everything all works. The minimal component for this, the better, as I'll be making multiple ones for a specific application. Its a very low power, control voltage.

 

You're looking for a standard amplifier with a gain of 2.

This should work:
http://www.play-hookey.com/analog/experiments/non-inverting_amplifier.html

You want equal resistors for a gain of 2.

 

Nothing simpler? Even if its not a gain of exactly 2. 1.5, 1.4, 1.6 would work fine too. Something simple with some diodes and capacitors or something instead? I'm looking for a few bucks at most in components, as I will probably go through doing this a few hundred times over time.

 

Resistors are $0.01 each and a simple op-amp (including dual or quad op-amp chips) can be bought for less than $0.50

If you're working with DC you're stuck with something like this, there's nothing simpler.

If you're working with AC you could use a voltage doubler but that's inaccurate within a diode drop and your voltages are smaller than a diode drop to begin with.

 

Nothing simpler? Even if its not a gain of exactly 2. 1.5, 1.4, 1.6 would work fine too. Something simple with some diodes and capacitors or something instead? I'm looking for a few bucks at most in components, as I will probably go through doing this a few hundred times over time.

Afraid not, but if it's any consolation op amps are available in quad configurations (4 separate amps in one package) so if you're running several control voltages from one device it would help on the cost.

I suppose you could patch together some sort of transistor circuit but it would be just as hard to build and only pennies less.

 

Would it work with a LM386N-1 amp? I know I have one of those laying around to test it with and see how well it works in my situation. If not, I can get a 741 on Tuesday, (everything is close here tomorrow because of Labour Day, but I'd like to play on the breadboard tomorrow if possible. lol.)

 

The LM386 is an audio power amplifier.
Its minimum gain is 20... so no.

Don't bother with the 741, it doesn't work well with a single supply voltage and it's ancient with poor specs. Specifically its output can't go lower than Vss + 2V, so with one supply it can only output 2V minimum. Similarly it can't handle inputs within 2V of ground.

You're looking for an op-amp that can see and output to ground. The LM358 and LM324 are the standard ones but they're also old.
You can find a 'rail-to-rail' op-amp with the specs and price you want on your favourite website like Mouser or Digikey.
On Mouser you'd specify 'Negative rail' as the minimum input voltage.
Alternatively they're marketed as 'single supply' op amps, though be careful they can't output all the way to your supply.
(For example max output Vcc - 1.5V, so with 3.3V supply you can only have 1.8V output. A rail-to-rail amp goes within millivolts of either ground or supply)

 

Would it work with a LM386N-1 amp? I know I have one of those laying around to test it with and see how well it works in my situation. If not, I can get a 741 on Tuesday, (everything is close here tomorrow because of Labour Day, but I'd like to play on the breadboard tomorrow if possible. lol.)

The LM386's gain of 20 could be padded down.

My favorite op amps have always been the TL08x series. You can get a TL082 at Radio Shack but I'm unsure what the minimum power supply it will operate off of is. If the power supply isn't a problem they're great for use with 12V circuits.

If the power supply isn't a problem you could certainly try the LM386 and put the output through a divider network such that your end gain results as 2. It certainly wouldn't be my ideal choice for anything unless you just happened to be sitting on a barrel full of them already.

 

Hmm, this is getting more complicated than I wanted. lol. Its from a PWM style fan but they run too slow by default so I was looking for some way to double or at least increase the voltage a little bit and still allow it to ramp up the fans when needed by increasing the source voltage. Anything simple that can give me a 30% increase in voltage or anything? lol. The 741 circuit seemed promising but if it doesn't work at anything less than 2v, I'm not sure if thats too much or not.

 

PWM circuits don't usually increase a voltage, but instead change the duty cycle of a waveform. You should be able to substitute a slightly smaller capacitor into the pwm freq setting network(usually a cap and a resistor) and that would increase the PWM switching freq, giving you a faster fan speed. Do you have a schematic, or even a picture of the circuit board to post?

 

When it really gets down to the truth, it's hard to operate most anything at very low voltages unless it involves an amplifier that can be sourced with at least 5V.

 

Actually this guys circuit seems to be exactly what I need.

http://forums.afterdawn.com/t.cfm/f-153/new_fan_fix-865649/#5250406

I repair PS3's and I always like to boost up the fan to prevent the system from getting too hot and that is what my whole thread was about. I have a 555 around, so I'm going to give it a shot in a day or so and see how it works but he seems to have done some research on this and tested it as I've seen him post on other threads of the same problem.

 

One way you could do this without requiring an external power supply at the required voltage would be a charge pump.

You could build a simple doubling circuit with an oscillator and a few discrete parts (diodes, capacitors.)

 

KillerBug's circuit that you linked to will not provide adjustable PWM; they might as well have used a fixed resistor instead of a pot.

See the attached; it is Killerbug's circuit modified to work properly on 3.3v with a standard bjt (transistorized) 555 timer.

Note the addition of the 100 Ohm resistor between pin 3 and the pot. This is necessary to prevent burning out the pot if you set the duty cycle up against one end or the other.

Note also the addition of the two diodes. This provides different current paths for the charge and discharge portions of the cycle.

Note the replacement of the 3rd cap with a 4.7k resistor. This is necessary due to operating a bjt 555 timer from such a low voltage. Without this resistor, the timer will probably get "stuck".

 

KillerBug's circuit that you linked to will not provide adjustable PWM; they might as well have used a fixed resistor instead of a pot.

See the attached; it is Killerbug's circuit modified to work properly on 3.3v with a standard bjt (transistorized) 555 timer.

Note the addition of the 100 Ohm resistor between pin 3 and the pot. This is necessary to prevent burning out the pot if you set the duty cycle up against one end or the other.

Note also the addition of the two diodes. This provides different current paths for the charge and discharge portions of the cycle.

Note the replacement of the 3rd cap with a 4.7k resistor. This is necessary due to operating a bjt 555 timer from such a low voltage. Without this resistor, the timer will probably get "stuck".

I see the output though of that is staying at 3.3V which is what we don't want. We want a lower voltage than that, so could that have been the reason why killerbug's version is that way? It should only be outputing about 0.75v to 2v max. If you have time, could you upload the Spice file you made in that demo there, I'd like to play around with it. I've been trying to learn spice somewhat for things like this.

Thanks to everyone for all the help so far.

 

I see the output though of that is staying at 3.3V which is what we don't want. We want a lower voltage than that, so could that have been the reason why killerbug's version is that way?

Killerbug's output also varies between 3.3v and ~0v. It's a pulse, not a DC level. However, you can adjust the ON and OFF time (thus pulse-width modulation) so that the average voltage over time will be the PW (Pulse width, or ON time) divided by the PRT (pulse repetition time), multiplied by 3.3v; or otherwise stated as:
Vout(avg) = 3.3*PW/PRT

If you really need a DC level, then an RC filter could be added to the PWM output.

It should only be outputing about 0.75v to 2v max. If you have time, could you upload the Spice file you made in that demo there, I'd like to play around with it. I've been trying to learn spice somewhat for things like this.

Thanks to everyone for all the help so far.

I created the simulation using LTSpice, available as a free download from Linear Technology. Google "LTSpice download".

You should also join the Yahoo! LTSpice Users' Group; it's free, and there is a lot of support and lots of models available in the libraries.

After you install LTSpice, you will need to download the attached files:
555 PWM pin 7.asc - the simulation file; goes in C:\Program Files\LTC\SwitcherCad
pot.zip - contains two files:
pot.sym - goes in C:\Program Files\LTC\SwitcherCad\lib\sym
pot.sub - goes in C:\Program Files\LTC\SwitcherCad\lib\sub
LM555.zip - contains two files:
LM555.sym - goes in C:\Program Files\LTC\SwitcherCad\lib\sym\misc
LM555.sub - goes in C:\Program Files\LTC\SwitcherCad\lib\sub

Then you can run LTSpice, open the 555 PWM pin 7.asc file, and run the simulation.

LTSpice comes with an idealized 555 timer; it works fast, but it doesn't simulate a real bjt 555 timer; it's optimized for speed. It's more like a supercharged CMOS version. Unfortunately, you can't actually buy a 555 timer that's as good as their model.

 

Thanks for the project. I already found your LM555 model and your Pot model from google a few mins ago and installed them and was playing around. I don't have any formal training in circuits by any means but I have a very, very, very basic idea of how things work and what they do. I've built a few circuits from schematics before so I'm enjoying this challenge. As far as I know, its a PWM style fan. I don't have an oscilloscope or anything like that to measure the fan, but if I measure the voltage when the fan is on low, its around 0.65v with my multimeter, it doesn't seem to fluctuate in anyway, just a steady voltage. If I cut that wire and wire in my own power source and apply say 1.5v, the fan runs faster. If I give it a full 3.3v, it sounds like a full jet airplane engine and runs at full speed. There isn't a lot of information on how the fan on the PS3 works, so its sort of guessing. If it was a PWM, would the multimeter show the voltage going up and down, or would it be too fast for the multimeter to really register?

 

Ok, now that I have a better understanding of PWM fans, scrape my whole initial question. lol. Since my initial source signal is already a PWM signal, is there anyway to increase the duty signal of my current signal coming in?

 

Ok, now that I have a better understanding of PWM fans, scrape my whole initial question. lol. Since my initial source signal is already a PWM signal, is there anyway to increase the duty signal of my current signal coming in?

One way would be to filter the PWM signal into an analog voltage, amplify this voltage, and feed that into a PWM generator. The generator would output a different frequency from the input, but that shouldn't be an issue.

 

I breadboarded killerbugs' circuit and it worked fine. The pot adjusts the speed of the fan down fine. Its actually a 100k pot but it controls the fan from a low speed (100k) to a highspeed (0k). I even simulated it in ltspice and it seems to perform almost exactly like yours with the output rising and peaking with varied duty cycles.

My next question is about thermistors. A 100k thermistor that is 100k at room temp, and goes down to zero as it gets hotter, approaching say 70c. I seem many different thermistors from digikey but its hard to decipher what ranges and such they work at. That would work perfect though, because in place of the 100k pot, I would use a thermistor.