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I am trying to build a DC-DC Buck-Boost Converter using this basic schematic and an Arduino with the analogWrite() function as my PWM generator. I only want to step up 2.4-3V to ~5V. If it's just the capacitor and no load, the converter works so good that I have to switch it off after a few seconds since the voltage I measure on my capacitor is exceeding the capacitor limit. However, when I connect any load to the capacitor(for example a motor or a resistor) my voltage drops to a really low value(0.1-2v) and stays that way no matter what frequency PWM signal I send to the transistor using Arduino. I really tried but couldn't find the cause of the problem. Is it because I have two grounds, the battery pack one and the Arduino one? Is it because of the inductor(I tried using a smaller, 45uH, torroidal one but couldn't get any better results)? So what am I doing wrong?

I only expect to draw 500mA of current at 5 volts at the output side, and according to this calculator my inductor and transistor should be fine.

Components: 100uF 35V electrolytic capacitor, 200uH inductor, BYW29 rectifier diode, TIP 120 power transistor, 1k resistor

enter image description here

A schematic of the circuit or at least how it is supposed to look. Maybe I miswired something? The resistor is to represent the load, the value doesn't matter since my problem persists at any value but the multimeters "infinite" resistance. This is how my circuit looks

This is how my circuit looks I used one coil of an old transformer as my inductor

I used one coil of an old transformer as my inductor enter image description here However I also tried this inductor

However I also tried this inductor

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  • \$\begingroup\$ Your circuit diagram doesn't show where the load would go; also, there doesn't appear to be any control loop feedback to stabilise the output voltage. \$\endgroup\$ – pjc50 Sep 4 '13 at 9:35
  • \$\begingroup\$ Don't mind the control loop, I'm not stabilizing the votage in any way right now, I am just testing with different PWM values. I modified the diagram. \$\endgroup\$ – Calin Sep 4 '13 at 9:37
  • \$\begingroup\$ perhaps a bit more research on buck regulators would help, try nlvocables.com/blog/?p=894 to start with. \$\endgroup\$ – JIm Dearden Sep 4 '13 at 9:49
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    \$\begingroup\$ My gut feeling says: an 1N400x is too slow in this application (consider a (power) schottky); 100uF is probably a bit low for this frequency; Arduino's default PWM frequency is probably a bit low (490Hz about two order of magnitude too low for the transformer); use a base resistor and an NPN transistor in common emitter. Double check the capacitor and the diode polarity. \$\endgroup\$ – jippie Sep 4 '13 at 11:13
  • \$\begingroup\$ Everything works now. Using pin 6 on the Arduino and raising its PWM frequency to 61kHz helped a lot. Now I can get even 20 Volts with a load attached. \$\endgroup\$ – Calin Sep 4 '13 at 13:31
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The diagram you're building from is basically wrong in a number of critical respects.

Firstly, the positive output of the boost is on the lower rail. This makes it much harder to follow. The return path from the boost is then not to battery ground but to battery positive! What you've actually built is a strange sort of resonant tank circuit; that's consistent with the observation of a large no-load voltage but no ability to drive actual loads.

You've also used a PNP symbol for an NPN transistor. There's no base resistor so you're potentially sinking a large current from the PWM pin.

Here is a more conventional boost design: http://reibot.org/2011/08/07/intro-to-boost-converter/ which uses a MOSFET, which is probably going to be required to achieve worthwhile efficiency.

Try this instead:

schematic

simulate this circuit – Schematic created using CircuitLab

That works in the simulator (try "time domain", time step 0.00001s for 0.01s or 0.1s). Ignore the part numbers, they're circuitlab defaults.

Edit for the record: the TIP120 was another problem, Calin switched to MOSFETs which worked properly. I'm still skeptical about achieving 5V @ 500ma from this as that implies >1A input from 2.1V (NiMH?) batteries through a breadboard, but Calin now has it basically working.

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  • \$\begingroup\$ (V3 represents the Arduino PWM output. Start with a 1k-10k load just to verify basic operation) \$\endgroup\$ – pjc50 Sep 4 '13 at 10:07
  • \$\begingroup\$ @Calin Where's the Arduino<->circuit ground connection? \$\endgroup\$ – AndrejaKo Sep 4 '13 at 10:18
  • \$\begingroup\$ Forgot to put it, it's there: i.imgur.com/BL3iT22.jpg \$\endgroup\$ – Calin Sep 4 '13 at 10:20
  • \$\begingroup\$ @pjc50 The band on the capacitor is connected to ground. It looks right to me. \$\endgroup\$ – AndrejaKo Sep 4 '13 at 10:23
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    \$\begingroup\$ let us continue this discussion in chat \$\endgroup\$ – Calin Sep 4 '13 at 13:31

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