Understanding this load/lack of - Page 1
Forum section: electronics / beginners

author: webtop
author_rank: Contributor
timestamp: March 15, 2018, 10:20:54 pm
content: Hey all,I have a circuit which runs off a wall wart @~12vAC. I have rectifier(DB107S), and a few other components to produce a value @~5vDC. This circuit is used to drive an ESP8266 on a regulated 3.3v board, leaving the 5v to drive an LED strip.Now, when I don't have the ESP plugged in, and the VCC/GND for the socket the ESP plugs into connected to my o-scope, I see the following values:Vamp: 400mVVmin: 4.80VVmax: 5.20VWhen I plug the ESP module in, I see:Vamp: 4.80VVmin: 400mVVmax: 5.20VDoing some testing, if I supply the DC side of the equation directly, everything works as expected. Also, if I supply ~12vDC to the entry point on the board, everything also works.Can someone explain why the switch between the Vamp and the Vmin?My head says thank youPaul

author: hamster_nz
author_rank: Super Contributor
timestamp: March 15, 2018, 10:25:32 pm
content: Vamp -> Amplitude (in Volts)Between 4.8V and 5.2V is 0.4V.Between 0.4V and 5.2V is 4.8VSo the question has to be, why does plugging in your ESP module cause Vin to drop to 0.4V?

author: webtop
author_rank: Contributor
timestamp: March 15, 2018, 11:23:08 pm
content: That's what I said!The thing is: if I supply the same in DC instead of AC, so the output after the rectifier is the same, everything works perfectly.I guess I should test my assumption there, that the stage after rectification is the same, but not sure that will give me any insights.

author: hamster_nz
author_rank: Super Contributor
timestamp: March 16, 2018, 12:24:35 am
content: Any schematic? Seems to be insufficient filter capacitor between the rectifier and the 5V regulator. The module should only place about 160mA worst case.... replace the module with a 100 ohm 1/4 watt resistor (should draw 50mA, but might get a little warm).No load it should have much less ripple than 400mV

author: Shock
author_rank: Super Contributor
timestamp: March 16, 2018, 12:32:54 am
content: I'm not sure exactly what you are describing. Photos of what you see on the scope and your circuit or a schematic can really help.Are you trying to say the supply voltage is being fully loaded down when you connect your circuit, but runs fine on the bench supply?That normally means that you are exceeding the current draw of the supply or you don't have large enough capacitance or components correctly spec'd for the current draw.Are you measuring while inserting the ESP? Are you triggering off a signal? Because it also sounds like you are asking an oscilloscope triggering question.Have you tried a multimeter? Whats the actual problem, or are you just wanting to make sense of what you see?

author: webtop
author_rank: Contributor
timestamp: March 16, 2018, 05:02:38 pm
content: There is an issue with the load, that seems to be clear to me, but I'm not one of those "fix this for me" type of people. I want to know why, not just how.Capacitance was what I thought it might be. Do I need caps on both the input and output side of the rectifier, or just the output side?

author: Shock
author_rank: Super Contributor
timestamp: March 16, 2018, 08:23:33 pm
content: Trap for new players is measuring ripple on their supply unloaded and thinking they have plenty of capacitance to have smooth DC.Capacitors required on the DC output of the rectifier, you can add them in parallel as well (cumulative effect). Voltage regulators may show recommended capacitance values on their input and output in datasheets.This is what popped up googlehttps://www.electroschematics.com/7048/capacitor-input-filter-calculation/Quick and dirty way is touse a circuit simulator such as this(simple to use and has a scope function) to make a rectifier and observe the effects of different capacitance. Look at schematics of power supplies get a rough capacitance. Use ohms law with your circuits peak current draw when on your bench supply to calculate a rough dummy load resistance. Test it and observe the ripple across the load.You also need to consider the AC adapters current rating (especially running a pile of LEDs). If it can't supply enough current it won't matter how many capacitors you add.