5 volt to get 3.3 volts?
Iguanaman
Posts: 32
in Propeller 1
I measured the volts coming off the 4BAR7XUG3-LM2937ET-3.3 voltage regulator supplied with the Propeller 40 pin starter kit. When I hook up the 9 volt battery to the voltage regulator I get some ware around 5.5 to 6 volts coming out of the regulator output. When I copy the circuit in the Propeller 40 pin starter kit where the 3.3 volt regulator gets its input power from the output of the 5 volt regulator. Then the power output from the 3.3 volt regulator drops to about 4.6 volts. I put the 3.3 volt power regulator under load of (5) LED with (5) 220 resisters and the power goes to 4.3 volts? The power coming out of the 5 volt regulator is spot on 5 volts + or - .03 volts. I was expecting the 3.3 volt regulator to output real close to 3.3 volts but it is not even close. MY QUESTION: Douse the 3.3 volt regulator first have to get its power input from the 5 volt regulator power output to even come close to the rack-amended 3.3 volt for the Propeller? Putting raw power into the 3.3 volt regulator directly from the 9 volt gives me to much voltage. Voltage that goes beyond the rack-amended input voltage for the Propeller 40 pin chip!? Thank you for any input. 22 feb 2019(fri).
Comments
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
"[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
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.
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.