Could someone help me understand this schematic? - Page 1
Forum section: electronics / beginners

author: mangekyo
author_rank: Contributor
timestamp: November 07, 2022, 11:33:01 pm
content: Why is VCC connected to GND of 0505S?

author: wasedadoc
author_rank: Super Contributor
timestamp: November 07, 2022, 11:40:18 pm
content: Quote from: mangekyo on November 07, 2022, 11:33:01 pmIt isn't.

author: golden_labels
author_rank: Super Contributor
timestamp: November 07, 2022, 11:58:02 pm
content: Consider, what happens when the OFF line is low or high. In particular, what is the input voltage. The name of that line is a hint already.The forum allows adding attachments: using that feature will protect the post from rotting.

author: coromonadalix
author_rank: Super Contributor
timestamp: November 08, 2022, 01:46:44 am
content: its an isolated dc-dc converter perfectly normal the iso ground and iso vcc are the isolated voltage output,when measuring the voltage you have to take that into consideration to avoid being fooled

author: pcprogrammer
author_rank: Super Contributor
timestamp: November 08, 2022, 06:49:49 am
content: To further explain golden_labels post, the N channel MOSFET is used to switch the ground to the dc-dc converter and thus turn it on or off.When the OFF input is connected to ground the dc-dc converter is switched off, and when connected to VCC or left open the dc-dc converter is switched on.The comment about rotting is to indicate that the picture you show is on some other server on the internet, and when it disappears your post will look rather silly, so in the future please use the attachment option to include a picture with the post, making them inseparable.

author: coromonadalix
author_rank: Super Contributor
timestamp: November 08, 2022, 08:46:40 am
content: well i would say it act as a switch .... but personally i would not use it like this, you have many models with an on/off pinthe problem would be : you need an low rds impedance, because the mosfet will have to take all the dc-dc converter output current ... it's nothing if its stay in the milliamps rangeBut personally i don't like this design, i would never play with the ground "level"

author: golden_labels
author_rank: Super Contributor
timestamp: November 08, 2022, 09:08:43 pm
content: I wanted mangekyo to ponder on circuit’s operation to understand the concept behind it. But since that part is already spoiled, let’s deal with the details.Quote from: coromonadalix on November 08, 2022, 08:46:40 amOFF pin on a chip makes sense, if it has to stay in the circuit and be powered when not in use. This is not the case here: RFM-0505S has no connections other than the power input, which is getting disconnected.Quote from: coromonadalix on November 08, 2022, 08:46:40 amIt’s a 1W converter with 5V output. So the output current is below 200mA. The primary side current will is within specs of a jellybean BC547, not to mention MOSFETs. And that’s for the worst case scenario. I doubt the unspecified EZO sensor chip will actually eat 1W.Quote from: coromonadalix on November 08, 2022, 08:46:40 amThe ground level is never played with here. If the MOSFET is turned off, there is no ground level for RFM-0505S.

author: mangekyo
author_rank: Contributor
timestamp: November 10, 2022, 01:52:07 am
content: Thanks for the explanation everyone. I honestly am never going to use the "off" function. Could I just remove everything from off to GND-IN on 0505 and just connect GND-IN to GND instead?

author: golden_labels
author_rank: Super Contributor
timestamp: November 10, 2022, 05:42:51 am
content: Short answer: yes, go ahead.Long answer: yes, with a small “but”.The “but” is: I am not sure about the role of R1 pull-up. It certainly allows OFF to be driven by an open-collector/drain output or OFF to be floating. I strongly believe this is why it was placed there.However, it may also serve a secondary function. In absence of any OFF signal, the MOSFET will not turn on until the GS voltage reaches 1–2V (depending on the transistor model), together with GS capacitance and 1MΩ resistor providing a tiny turn-on delay. So it may be shifting powering RFM-0505 until VCC reaches some sensible value. RFM-0505 itself will not be damaged if the voltage is too low (page 1, Note 3), but the output may be out of spec. If you have an oscilloscope, see if there is no weirdness while you power on the entire circuit. I doubt there is, but may be worth spotting the problem now instead of chasing a transient issue later.

author: mangekyo
author_rank: Contributor
timestamp: November 12, 2022, 04:30:25 pm
content: For the sake of learning, I am going to make two circuits. One with the mosfet and one without. Would any n-fet with a 5v gate voltage work? The microcontroller runs on 5v. Both vcc and the OFF signal is 5v.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 12, 2022, 07:09:49 pm
content: I think I've already seen this diagram somewhere.

author: pcprogrammer
author_rank: Super Contributor
timestamp: November 12, 2022, 07:14:17 pm
content: Quote from: mangekyo on November 12, 2022, 04:30:25 pmA IRFZ44 will do the trick:https://www.vishay.com/docs/91291/91291.pdf

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 12, 2022, 07:45:07 pm
content: There're crazy modern fets out there!Check theDMG2301L, threshold -0.4V, 1.5A @ -1.5VGS , 4A @ -2VGS

author: golden_labels
author_rank: Super Contributor
timestamp: November 13, 2022, 12:07:28 pm
content: Learning would happen if the version without MOSFET was likely to experience issues, so you could easily observe the difference. It is not, which is why I gave a short answer: go ahead. The longer answer only highlighs a possibility of this MOSFET having a secondary function. Something you may wish to guard against, by checking output on an oscilloscope.

author: mangekyo
author_rank: Contributor
timestamp: November 14, 2022, 02:11:50 am
content: Quote from: pcprogrammer on November 12, 2022, 07:14:17 pmWill this work?https://datasheet.lcsc.com/lcsc/1912111437_Yangzhou-Yangjie-Elec-Tech-2N7002_C389059.pdf

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 14, 2022, 03:02:10 am
content: That's a pretty weak fet, but could be ok.Normally those isolated converters only feed few parts, but you must ensure beforehand or it might release the magic smoke!Leave a reasonable margin, ex. don't use 300mA parts for 250mA, the circuit might consume more power later on.I'd try to find a 500mA fet, but a good thing is you can parallel them just fine, doubling the current capability.