I am working on a project that involves powering a 3.3V microcontroller off a doorbell transformer. The design uses a full-wave bridge rectifier (1) to convert the AC voltage from the doorbell transformer to DC, which is smoothed by a capacitor. A switching regulator (2) then converts the DC voltage from the bridge rectifier to 3.3V to power the MCU.    Since the doorbell still needs to function, a switch is used to trigger the doorbell chime. However, I am running into an issue: when the doorbell button is pressed, shorting the AC inputs to the bridge rectifier, it destroys the switching regulator.  One initial thought provided by a  was that the doorbell solenoid created a high voltage inductive kick that was killing the circuit. To prevent this, a bidirectional TVS diode was installed across the doorbell solenoid (3) for ESD supression. The bridge rectifier does not appear to have any issues and continues to function normally even after the switching regulator fails.    I am working to find a way to allow the doorbell to be triggered without damaging the voltage regulator and I would greatly appreciate any insight into why the regulator might be failing.  All of the datasheets for the components are listed below:  1) : Bridge rectifier  2) : Switching regulator  3) : TVS diode  edit: In a previous question I thought that the bridge rectifier was failing. It turns out that it is actually the regulator failing.
120518
I am working on a design that involves powering a 3.3V microcontroller off a doorbell transformer but I am running into an issue with the bridge rectifier failing when the AC inputs to the rectifier are shorted.    There are two basic parts of the design. First, a full-wave bridge rectifier is used to convert the 21V AC supply from the doorbell transformer to roughly 26V DC. A switching regulator is used to deliver 3.3V for powering the microcontroller. A smoothing capacitor reduces the ripple created by the bridge rectifier. That part of the circuit is working as expected and the microntroller can be powered from the doorbell transformer.  The second part of the circuit is simple. To allow the doorbell to be used normally while powering the microcontroller, a switch shorts the AC inputs. This closes the circuit and powers the solenoid in the doorbell that sounds the chime. The issue I am running into is the bridge rectifier failing when the switch is pressed.  I would really appriciate any insight into why the bridge rectifier fails and how the circuit might be modified to prevent the failure.  Bridge rectifier datasheet:
12988
Sorry if this is a stupid question, but I am mainly working with 5V PIC circuits and now want to integrate a component (nRF24l01+) that needs a 3.3V supply.  As I have a box full of ~7V wall-warts, I normally use a 7805 or similar LDO regulator to power the circuit. Now I want to also get 3.3V without using a separate external supply.  I'm looking for guidance on the "right" way to do this. I think the options are:   Use a voltage divider on the output of my 5V regulator to get 3.3V Use a 3.3V LDO regulator fed directly from the 7V wall-wart   Use the output of the 5V regulator as input to a 3.3V regulator   I can see that all of these would work (would they?) and that option 3 seems to waste least power, but I don't know what gotchas might arise by chaining two regulators like this.  What things do I need to consider when making this choice?
29388
I've been experimenting on a lot of half-bridge-based PC PSU schematics. There's one thing I cannot understand:    As can be seen from above, there's an extra winding in series with the high side driving winding (marked with red ?) and one end of it goes directly to the primary of power transformer.  What is its purpose?  Thanks.
32554
When I measure a 9V DC wall adapter power supply with a multimeter it shows 11.8V. When connected to my board this voltage lowers to 10V. Is this normal? Should I be afraid of toasting my  supplied board when using this adapter?  Edit: This regulator accepts up to 12V inputs. So that will be no problem. A second "9V" adapter yields 15V to the multimeter. Will this one burn my circuit?
91558
I have a circuit which uses an ATtiny85 output to a transistor (S9013), which will trigger a relay that controls a motor.  The reason for the transistor to relay is because I do not have a transistor that can take the current of the motor, so I am isolating it with a relay.  While I can purchase all these, it will take time to ship.  Here is the circuit:   SCHEMATIC NOT UPDATED, I HAVE A DIODE GOING ACROSS THE MOTOR, AND A 100uF CAPACITOR GOING ACROSS THE VCC AND GND TO SMOOTH OUT THE VOLTAGE (have also tried 100nF).  Everything works if I don't connect the motor OR if I isolate the motor with its own power source.  The issue now is that when the motor is switched on, there is a massive voltage drop and the ATtiny85 will reset. Here is a waveform of the motor trying to turn on three times:    Is there a way to prevent this drop or at least reduce it to a point where the ATtiny85 won't reset? I'm thinking wire up a simple voltage regulator but not sure if the added complexity will introduce other issues. I also know I can go out and buy a motor or voltage regulators but let's avoid that option.  EDIT: so as per the answer, a diode (cathode at +ve, anode at VCC) was added between +ve and VCC, assuming that there is a capacitor connected in parallel with the ATtiny85 (+ve end at VCC, -ve end at GND). Theory is that the cap gets charged while the motor is off to VCC. When the motor turns on, it's going to drop that voltage down to below VCC and the diode is going to be reverse-biased and prevent any conflict from the supply to going into the ATtiny85. The capacitor can now freely discharge into the ATtiny85 without the supply affecting it (without the diode, supply will average capacitor down faster than if it would just discharge alone). For reference, the bulk capacitance value I used was 100uF.
32000
I am replacing a power receptacle at my apartment. It has the standard four wires, with one of the two reds going to a nearby toggle switch.  I am concerned because when the toggle switch is off, I measure 16V between one of the red wires and a white. (The other red wire always measures 0 V, so I assume it doesn't go to anything.)  I don't want to wire up the red if it's going to sit at 16 V when it's supposed to be off. Any ideas why this is happening or what I can do? Or is 16 sufficiently close to 0?
56075
I will be turning a relay on from my circuit by sending a signal 500ms to 1000ms long, and i am trying to protect against poor wiring or faulty relay from the user that causes a short circuit on the relay which could blow the fuse on the module  The power supply is rated for 2A and the fuse on the module is rated for 1A. I would like to create a protection that even if there is some short circuit on the relay output the fuse cannot blow,  My solution was to add a resistor on series of the relay, but this would mean that resistor should have atleast 5watt rating for it to survive, or a high value resistance for it to survive. Having a high resistance also might mean that the relay might not turn on because there wont be enough voltage that will cross it.    Paralleling a resistor to the relay seem kinda useless also since the current will not flow to the resistor but to the short instead.  How should i approach this? i possible how can this be done with basic components (resistor,capacitor,diodes etc)
77177
I am trying to design a circuit that runs an esp32 with a dh11 sensor but i am unsure if it would work.  On the top left is a mcp73831-5-OT li ion battery charging chip (the random wire on the left is 5v in from the solar panel) On the top right is the AMS1117 3.3v variant regulator (picked it for the tiny package and availabilty) On the bottom left is a esp32 module and its accompanying circuitry (im really suspicious abou that circuit though) On the bottom right is a dht11 connected to power and data with its accompanying pull up  So,if i build this,would it work? If not,what should i change?
4105
Following an earlier question ,  I would love to implement the  as suggested by Russell McMahon.   That way I could switch between 4AA batteries (1.2V*4 or 1.5V*4) and USB power. The battery voltage would nicely be regulated to 5V too. And the idea was to use the shutdown pin of the chip IF USB power is detected. The device also has to work when no batteries are present.  I am not too experienced with this, but I thought to connect the 5V from the USB to the SHUTDOWN pin from the LDO chip. Pitywise, the pin function is inverted, if SHUTDOWN is low, Vout from the LDO chip is low.  I am now considering using an XOR gate to make the thing work:         &ndash; Schematic created using   I would love to limit the amount of components, but the number of diodes is already high, and the whole thing feels a bit crafted   My questions:    Is there a better suitable chip (with an inverted shutdown pin) is there a better circuit (without voltage drop) to get this (I would prefer to get rid of D1, but then, the XOR has no desired operation)
67477
According to various sources, the ripple voltage of a full-wave bridge rectifier is   $$ V_r = \frac{V_s}{2}fCR $$  where \$V_s\$ is the peak of the input voltage source, \$f\$ is the frequency, \$C\$ is the capacitor and \$R\$ the resistor.  I have simulated this circuit on PSPICE, circuitlab, and built it in real life:   \$R\$ is \$4.7K\$ and \$C\$ is \$10 uF\$. Voltage source is sine wave with \$0 V\$ offset, \$5V\$ amplitude and \$40 Hz\$ frequency.  All 3 experiments give me a ripple voltage of approximately \$0.65V\$, but the formula gives me \$1.330 V\$ (twice as much).  What am I missing?
111912
I'm looking for feedback and improvements on my design below. Specific questions are at the end. I created this as an attempt to answer my own, . From my feedback there, it seems like I needed to be more clear in my requirements. These are screenshots from Eagle. If there's a better way to share, please let me know so I can correct it. Entire board  The overall purpose of this board is to translate between 12V logic, and 3.3V logic. The top portion of the board represents the 12V side. The LEDs are placeholders to indicate whether the 12V signal is HIGH or LOW. In practice, these would be replaced with more , requiring up to 3A. The switches are also placeholders, allowing one to manually turn the 12V signal on and off. They are only for test purposes, the &quot;real&quot; 12V signal would be coming as the output from other relays. The bottom half is the 3.3V side. This will be connected to a  (BBB). The board will connect to 2 output GPIO pins and 2 input GPIO pins of the BBB. Overall, I want to translate the 3.3V I/O from the BBB to the 12V signals on the other side (to control more relays and read their signals). I have designed 3 modules to control both directions individually, and together. 3V3_TO_12V Module  This module converts the 3.3V GPIO output from the BBB to a 12V signal on the other side. When the 3.3V signal is low, the 12V signal is low. When the 3.3V signal is high, the 12V signal is high. To do this, the 3.3V GPIO output is connect to a , which turns a  relay on and off. This switches the 12V power as output of the module to be used elsewhere. 12V_TO_3V3 Module  This module converts a 12V signal to 3.3V signal, to be read by a GPIO input from the BBB on the other side. When the 12V signal is low, the 3.3V signal is low. When the 12V signal is high, the 3.3V signal is high. To do this, the 12V signal simply turns a  relay on and off. This switches the 3.3V power to be read as input by the BBB. DUAL_12V↔3V3 Module  This module combines the previous two modules to create a bidirectional translation. If the 12V side is high, the 3.3V input to the BBB GPIO should be high. If the 3.3V output from the BBB GPIO is high, then the 12V side should be high too. It's similar to a logical OR, if either side is high, the corresponding signal on the other side should be high too. A single 12V signal has to be split into individual input and output signals on the 3.3V side for the BBB. Questions  Is the  and  relay combination overkill for having the BBB GPIO output control the 12V power line? I like the isolation and safety it provides, but it seems too much (in terms of cost and board space) Is there a better solution for the 12V to 3.3V conversion. I like the simplicity of the  relay switching a 3.3V signal, but it takes up lots of room. Does the DUAL_12V↔3V3 Module (which combines each translation direction) even make sense? This seems wasteful to use 2 relays and a high side driver, but I haven't found simpler solution that achieves the same result. Should I add pulldown resistors for the BBB GPIO inputs, or are the internal pulldown resistors good enough?  Any other feedback is welcome. I'm new to electronic design.
53459
The circuits I describe are entirely made of 7400 series logic gates (7402, 7404 and 7408 ic). I'm trying to build a rising (positive) edge pulse detector using logic gates.  The following circuit should work in theory:   (see )  I do not expect the short output to be seen through a LED so, to test it, I make it trigger an SR latch to its up state:    However, it doesn't have effect on the latch. So I added an inductor to delay the input to the NOT gates in the pulse detector circuit and it worked:   But now I can see the short flashing output through a LED which I should not because it would mean it is too long to work with a circuit like that:   Which intends to toggle the D Latch output on each clock rising edge (Note that this is a D Latch not a D Flip-flop)  And anyway there is no place for inductors in integrated circuits so there must be a way to do this only with logic gates. Can someone solve this mystery?  BTW It does not show in my schematics but I did put 10K pull-down resistors where there might be floating pins.
16853
I'm currently encountering a problem when using a relay switch with a 5v receiver. When using the same receiver with a row of LEDS it works flawlessly. I've noticed that the relay keeps making the switching noise over and over like I'm pressing the transmitter really fast apart from the fact I'm not. Also, when the transmitter is moved closer to the receiver from 20" to around 1" the problem stops. Below is some circuits to help show you my setup.  Any help appreciated thanks.  Setup Specs   USB 5v DC (2AMPS) Receiver (XY-DJM-5V - SC2272-M4) Relay (SRD-05VDC-SL-C 5V DC) Transmitter (XY-04) Diode (1N4001) Transistor (2N2222A) Resistor (1.8K Ohms)          &ndash; Schematic created using   Antenna Signal Workaround or Booster  I believe this issue is related to wireless signal on D0. Basically above 20" inches the output to the relay drops from 5.05v to 0.5v and is obviously causing the problem. I think I need to boost the signal or replace the transistor with something that triggers at low and high current but have no clue. Any help appreciated, I'm new at this.  Board Setup
24453
I built a simple bridge rectifier.         &ndash; Schematic created using   I connected the output of the function generator with CH1 of my oscilloscope and  with the input of the rectifier (using a T-Piece). The output goes into CH2.   The original wave looks like this:    After connecting CH2 the picture looks like this:   While CH2 looks good,  CH1 now shows the wave I would expect without capacitator but not the expected original wave.   Could somebody explain why? I'm using a Gwinstek GDS-1072A-U Oscilloscope and a Gwinstek SFG-1003 function generator.
33108
Hi I have a Diode Bridge as outlined in the image below. As you can see I am getting a negative voltage on one pair of diodes instead of this voltage being flipped to positive.   Can you see anything wrong with my design or does it look like the simulator program getting it wrong?    Using Every Circuit Simulator on Android.
96108
I want to know if it is possible to design a power supply for a circuit that can operate from DC and AC using the same input terminals.   Thus, the output should be xxV DC (5-24 or whatever is decided on), and on the input side the consumer can connect whatever he has available - 120V AC, 230V AC or 24V DC.  A transformerless AC to DC power supply can work for at least the AC side, but the ideal one looks to be the capacitive type which would not allow DC to get through, am I correct?  There is also an resitive transformerless design, not ideal at all since it is very ineffecient.     Any ideas? Thanks!
30731
This question stems from notations on schematics and seemingly conflicting information I'm seeing. I suspect that I'm seeing different vernacular for the same concepts--but I'm in a place where no one's ever told me that an "elevator" is a "lift". Then again, I may have the concept completely off, and need to be schooled so I don't blow up my workshop. :)  With DC: Batteries have +/- terminals. Most schematics I see show the circuit with voltage in and ground. I've heard that most schematics don't trace the return path to the negative terminal because that's understood and it doesn't need to clutter everything up. I've also heard that on a DC circuit, ground is SYNONYMOUS with the negative terminal. On schematics, I've seen V-in and ground, I've also seen V-in, ground, and a separate trace connecting to the negative terminal.   Then, we move to AC. There's a hot wire (positive), a neutral wire, and ground. I ~assume~ that in an AC circuit, positive correlates to positive, neutral to negative, and ground to ground. Transformers will correlate the +/- when changing DC.  What are the facts and what are the myths? How can I tell if I need to ground something to earth vs. "ground" to the negative terminal? When do I ground to the chassis of my device? Are there standard conventions in a schematic that would indicate GROUNDED ground vs. return-to-the-source ground? Or is that something you know from experience and doing an analysis of the circuit? Is it generally safe to assume I can connect ground to negative? Or are there cases in which that would be a Very Bad Thing, and how do I identify those cases?  Just trying to wrap my head around +/-/ground in AC vs. DC, and how that voltage is used...