5 volt to get 3.3 volts?

Comments

• msrobots Posts: 3,704

  2019-02-22 22:00

  yes it does.
  
  As higher the input voltage is as more 'stress' the regulator has. I needs to get rid of the heat and has 'more to do' with a bigger voltage difference. So the output is not as stable.
  
  And some regulators need a minimum load to stabilize, so one LED might not be enough current to get stabilize the output. I guess that with a higher V-diff you might need a higher load to stabilize. But I do not know if this is correct.
  
  Enjoy!
  
  Mike
• Phil Pilgrim (PhiPi) Posts: 23,514

  2019-02-22 22:14 edited 2019-02-22 22:18

  What kind of capacitor did you put across the output of the 3.3V regulator?
  
  -Phil
• Iguanaman Posts: 32

  2019-02-23 18:48

  I have not put any large capacitors on the output side of the regulator. Small 0.1mf capacitors are the only ones I use on the regulator output side. I put 1000mf on the input power side to absorb any power fluctuations. I am just using a 9 volt battery. If I put larger capacitors on the power output side the voltage goes up. I found a new voltage regulator from Digi-key a number 81A5VDUG3-LM1117t-3.3. I hooked this one up and it is spot on 3.3 volts. I think I will use this from now on. Thank you for your input Msrobots and Phil Pilgrim (Phipi). 23 feb 2019(sat).
• Phil Pilgrim (PhiPi) Posts: 23,514

  2019-02-23 21:13 edited 2019-02-23 21:14

  Whenever you use a regulator, it's always important to check the datasheet. For the '2937, the datasheet says:
  
  "[The output capacitor is] required for stability. COUT must be at least 10 µF (over full expected operating temperature range) and located as close as possible to the regulator. The equivalent series resistance, ESR, of this capacitor may be
  as high as 3 Ω."
  
  In your case, you measured a much higher-than 3.3V, because you did not use enough capacitance or low-enough ESR on the output, and/or did not place the cap close enough to the regulator.
  
  This also applies to the '1117, whose datasheet says:
  
  "The output capacitor is critical in maintaining regulator stability, and must meet the required conditions for both
  minimum amount of capacitance and equivalent series resistance (ESR). The minimum output capacitance
  required by the LM1117 is 10 µF, if a tantalum capacitor is used. Any increase of the output capacitance will
  merely improve the loop stability and transient response. The ESR of the output capacitor should range between
  0.3 Ω to 22 Ω."
  
  The fact that you're measuring the correct output voltage with only 0.1uF can be chalked up to luck -- that may not last, IMO.
  
  -Phil
  
  
• Mark_T Posts: 1,981

  2019-02-23 23:08

  And you have to check the right datasheet. The AP1117 for instance (as opposed to LM1117) requires 4.7uF minimum,
  ceramic X7R or tantalum, else 47uF electrolytic.
  
  Its an easy thing to forget to check, it simplifies things to stick to one regulator chip you know, and I tend
  to use the AP1117-50 and -33 a lot.
• Peter Jakacki Posts: 10,193

  2019-02-23 23:45

  A bigger capacitor is usually not a better capacitor when it comes to regulators. A 10uF or "more" that might be recommended for some also specify the ESR which is why they normally recommend a tantalum or ceramic. Electrolytics in general I find are not suitable for the regulator itself and you should avoid "overcapping" the output as you want the regulator to do its job. Larger electrolytics usually have higher inductance thereby contributing to a high ESR (it's like sticking a resistor and an inductor in series with the cap).
  
  Also be aware that besides minimum load requirements of some regulators that you can't just rely on the reading from your meter. If the regulator is improperly decoupled as in the instance of incorrect caps, then the output may actually be ringing or oscillating at a high frequency. Meters only report the average DC but if in doubt just flick the meter over to the AC range and know that if it is reading say 1VAC that you then have a problem.