How to create an isolated circuit to power a bluetooth module - Page 1
Forum section: electronics / projects

author: milad1234
author_rank: Newbie
timestamp: September 28, 2016, 11:41:14 pm
content: I am trying to come up with a circuit to isolate a 5 V coming up from USB port to power a bluetooth module.The 5 V coming from USB port needs to power a PIC MCU and a bluetooth module.The 5V feeding the bluetooth has to be isolated from the 5V feeding the PIC MCU.I am thinking a low pass filter would do the job! Any suggestions?

author: milad1234
author_rank: Newbie
timestamp: September 28, 2016, 11:58:09 pm
content: Basically I am trying to make sure no noise from common 5 V caused by PIC circuit doesn't effect RF.

author: wine+dine
author_rank: Contributor
timestamp: September 29, 2016, 07:05:34 am
content: That Bluetooth module is designed to work off a normal quality D.C. supply in digital logic surroundings. Keep in mind it works at 2.4 GHz where there won't be much disturbance anymore from your PIC's 10-20 MHz or whatever it is.I would simply hook it up and use your typical 100n bypass caps on the PIC, the module etc. Only if you can /really/ identify supply related problems would you need to go further, e.g. add a small RF choke between the circuits. But only if you can measure before/after, not add it like sprinkling magic fairy dust while your problem might be, let's say, antenna related.Nothing to do with isolation.

author: pix3l
author_rank: Regular Contributor
timestamp: September 29, 2016, 08:27:20 am
content: Bluetooth modules usually have a good shield which makes sure there is no interference from outside. Of course a ferrite and capacitor in the power supply line of the module never hurts..

author: milad1234
author_rank: Newbie
timestamp: October 05, 2016, 10:05:35 pm
content: I am actually using this bluetooth board:https://www.sparkfun.com/products/12576It has a LDO voltage regulator.Does it do filtering as well,right?Adding a ferrite bead to TX trace would that do the trick? My pic is running at 24 MHz and UART Baud rate is 115200.The ferrite bead with lowest frequency i can find is 25 MHz.So would that possibly work or has to test it to see if necessary?

author: Stephen37
author_rank: Contributor
timestamp: October 06, 2016, 09:51:26 am
content: I don't understand why would you isolate them. You can always use an isolated dc converter + optocoupler for data pins but they are pretty expensive (10$ for a 2W dc converter and ~2$ for the opto), also if the noise would be significant it would most likely still pass through the layers of pcb or travel on its surface if we are talking about something over 1Ghz.For the noise you don't need isolation, you need good quality bypass capacitors on both the pic and the bluetooth module as close as possible to the power pins.

author: batteksystem
author_rank: Regular Contributor
timestamp: October 06, 2016, 10:01:17 am
content: Quote from: milad1234 on September 28, 2016, 11:41:14 pmA low pass filter will not be able to isolate the 5V feeding the PIC MCU.

author: wine+dine
author_rank: Contributor
timestamp: October 07, 2016, 06:35:01 am
content: Quote from: batteksystem on October 06, 2016, 10:01:17 amThat statement is irrelevant. Define "isolate" and against what? Galvanic (DC)? PIC clock/switching noise on the supply?The OP assumed he needs to "isolate" the bluetooth module's supply from the PIC's to avoid RFI. As explained above, there is unlikely to be any interference at 2.4 GHz from the 10-20 MHz PIC, especially if standard bypassing practice is followed. It follows that no "isolation" is needed except if concrete proof of RFI is present, and even then galvanic isolation will not help; the RF would need to be kept out. That is what LPFs do (including those called bypass caps).