I'm trying to figure out how to make a nice power supply to the radio module HM-TRP.

I heard that running certain voltage regulators with capacitors having high ESR values is a bad idea and will cause instability.

For this reason I made the voltage supply setup to my circuit as follows:

The 7.2V battery powers everything and gives power to the 5V regulator LM2940.

Due to specs in the datasheet, I placed a 22uF electrolytic cap as close to the input and output of the LM2940 as possible.

then on the 5V side, I sprinkled a few 47nF ceramic capacitors throughout the board to reduce ESR and to give each section of the board better chances of a solid voltage. I have about a total of maybe 10 ceramics on-board (Its a large project).

Next, I have an LM1117 which the 5V feeds into in order to get a clean 3V output. It's ok if the output is between 2 and 3.9V. The output of that regulator currently has a 220uF electrolytic capacitor attached to it along with another 220uF (not shown) capacitor in parallel.

I then test my entire circuit out. Every single thing works beautifully, except for the radio module. I try running the thing with a speed of 38400bps and with the two wireless devices no farther apart than a meter.

When I run the test, for continuous wireless data communication, the data flow works nicely for about a few minutes, then the radio on my board stops working for roughly a minute, then it starts working again. and the process repeats. I did not create code in my microcontroller on-board that makes these delays happen. In fact, when the remote unit sends data to the on-board radio, the light must blink green. This light doesn't go on when the on-board radio stops working for the short time.

So I thought maybe its an issue with the ESR of the capacitors? So I tried replacing the second 220uF electrolytic capacitor with a 100nF ceramic capacitor. The results were maybe 1% improved but I still get that hiccup where the on-board radio module stops working for a period of time then works again.

Question is, is my voltage regulator setup? or could I use a better model number for either regulator?

And what about my capacitors? is my ESR out of valid range? or should I just go with a new kind of capacitors?


  • \$\begingroup\$ ESR specs are in regulator and capacitor data sheets. ESR of Caps farish from regulators is liable to have less affect as track inductance and resistance then dominate. Monitor power rails - idally with a scope. If voltages are in spec and there is no oscillation then behaviour you describe is unlikely to be caused by VRs. \$\endgroup\$
    – Russell McMahon
    Commented Jul 29, 2018 at 13:42
  • \$\begingroup\$ Check dropout V for LM1117 (you should give us links to datasheets). You are allowing only 5-3.3 = 1.7V headroom. Probably OK with an LDO, but depending on loads oin 5V and 3V3 rail this arrangement may overheat IC2 or dropoutIC1. \$\endgroup\$
    – Russell McMahon
    Commented Jul 29, 2018 at 13:44
  • \$\begingroup\$ I already did that and one datasheet said maximum 1.3V dropout for 800mA but the only parts directly attached to the LM1117 is the radio module itself, a 74HCT04 inverter (for voltage conversion) and the decoupling capacitors. Worst case scenario for me should be 5-1.3=3.7V max voltage so I basically have 0.4V to play with since the regulator makes output to 3.3V max. Also with those circuits, I'd think the worse case scenario is that 120mA will be used from the HM-TRP radio. \$\endgroup\$ Commented Jul 29, 2018 at 15:25
  • \$\begingroup\$ Where are the requested datasheet links ? :-) | HM-TRP data sheet . || What did the voltmeter and/or scope observations show? \$\endgroup\$
    – Russell McMahon
    Commented Jul 30, 2018 at 2:04
  • \$\begingroup\$ I was trying to see if the power supply voltage would show any instability and the worst I got for instability was sometimes a 0.01V drop but that's nothing. Could matching the air link speed to the UART speed be the problem? Maybe I should double the air link speed. I tried upping each radio to 56K speed and data goes faster but the module seem to go off for a period of time then on again. \$\endgroup\$ Commented Jul 30, 2018 at 3:16

1 Answer 1


Booth of the regulators have thermal shutdown. Did you considered the regulators overheat? first touch the regulators to see if it is true. anyway you can normally see the regulator oscillations using a scope, so you don't have to guess.


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