Question  Is it safe to "bypass" (as in circumvent) a linear voltage regulator by applying a voltage to its output (equal to the specified regulator output voltage), while the regulator input remains unconnected?  Details  I have a circuit which is powered by a 6V adapter (a "wall-wart"). The 6V adapter supplies the input of an  (through a diode), which delivers 5V regulated output to the rest of the circuit.  Now I am interested to see how the circuit would respond to a different 5V supply. That is, I would like to power the circuit from an external 5V supply, bypassing the ADP3338 (without otherwise tampering with the circuit).  My idea was to simply connect an external 5V supply to the output of the ADP3338, while leaving the input (6V from adapter) unconnected. This is illustrated below:    However, I am not sure if this is safe, or if it will destroy the regulator, or if it will mess with the rest of my circuit in some other way.  The functional block diagram from the ADP3338 specs, depicted below, shows a resistive voltage divider between GND and OUT. I measured 0.7M Ohms between these two pins, so I guess that should not be an issue. But how will the rest of the regulator circuitry react?  Thanks for your help.
121080
I'm designing a system that can be powered both at 12V and 5V. The circuit works at 5V, so a voltage regulator is used when the power is at 12V. A jumper (or several jumpers, given your answers) will route the power lines accordingly. My idea is to use a single jumper that:   in position A (12V input) connects the power supply to the voltage regulator's input; voltage regulator's output goes to the circuit in position B (5V input) connects the power supply to the voltage regulator's output, so the main power goes to the circuit; voltage regulator's input is floating   Here is a reference schematic:    Is it safe to do so? Will the voltage regulator be harmed if I drive its output when the input is floating? (in that case, I need 2 jumpers, so I can disconnect the output line)  Thanks!
25415
I am trying to build a DC-DC Buck-Boost Converter using  basic schematic and an Arduino with the analogWrite() function as my PWM generator. I only want to step up 2.4-3V to ~5V. If it's just the capacitor and no load, the converter works so good that I have to switch it off after a few seconds since the voltage I measure on my capacitor is exceeding the capacitor limit. However, when I connect any load to the capacitor(for example a motor or a resistor) my voltage drops to a really low value(0.1-2v) and stays that way no matter what frequency PWM signal I send to the transistor using Arduino. I really tried but couldn't find the cause of the problem. Is it because I have two grounds, the battery pack one and the Arduino one? Is it because of the inductor(I tried using a smaller, 45uH, torroidal one but couldn't get any better results)? So what am I doing wrong?   I only expect to draw 500mA of current at 5 volts at the output side, and according to  calculator my inductor and transistor should be fine.  Components: 100uF 35V electrolytic capacitor, 200uH inductor, BYW29 rectifier diode, TIP 120 power transistor, 1k resistor    A schematic of the circuit or at least how it is supposed to look. Maybe I miswired something? The resistor is to represent  the load, the value doesn't matter since my problem persists at any value but the multimeters "infinite" resistance.   This is how my circuit looks   I used one coil of an old transformer as my inductor    However I also tried this inductor
33129
I have two laptops (same manufacture), with the same type of power connector.  However, the power supplies/transformers are slightly different.    The output on the first laptop's power supply is 15.6&nbsp;V at 8.0&nbsp;A. The output on the second laptop's power supply is 15.6&nbsp;V at 5&nbsp;A.  Clearly the voltages are the same, but the currents are different. I assume the second laptop's power supply can not be used on the first, because it can't supply enough power to the laptop.   However, can the first laptop's power supply be safely used on the second laptop?
23953
Is it safe to connect a battery in series to wall via adapter?  I have an AC adapter which gives me 10 vDC from the wall (220 volts). I've measured the adapter and it outputs 10 vDC. I have a 3 volt battery. I would like to connect in series to get 13 vDC.   Is this possible? Is this safe? If not, what can go wrong?
99043
I have some doubts about how to select a voltage regulator.  I am planning to use a Texas Instruments TPS7A05 LDO regulator to regulate my 5V (250 mA) to 3.3V (200mA) to power my STM32 microcontroller. I am using a battery as source.  My input to LDO regulator:   input voltage :5v input current : 250 mA   Expected output:   output voltage : 3.3v output current : 200 mA ( to power my STM32 microcontroller)  My doubts:   Dropout voltage :   I understood that dropout voltage is the difference between input and output voltage . The difference between my input voltage and my expected output voltage is 5V-3.3V =1.75V, but in the datasheet it is given like   So I am getting my voltage drop of 1750 mV. It is more than 230 mV so to get 200 mA of output current my voltage drop should be 230 mV or what then why they have given the maximum input voltage as 5V and maximum current as 200 mA. So based on my doubt I cannot use this regulator or ehat or my understanding is wrong and we can use it.  Ground Current : I understood that ground current is the difference between input current and output current. So the difference between my input current and my expected output current is 250 mA - 200 mA = 50 mA but in the data sheet it is given like:    It is given like as 6 micro amperes, but based on my input and output current difference is 50 mA it is lot more than the given.So can I use this regulator based on my input and output or not or can I use a resistor connected in series so that I am able to get the ground current as given in the data sheet.  If my understanding is wrong please correct me.  Can I use this regulator for my application or not?  Please find the attached link of Texas instruments
102243
I'm wondering, is it possible to provide for example 5V/500mA from a single 1.2V battery and consuming only 50mAh from the battery by using a low input voltage boost converter and a power amplifier ? (L272 and a TPS61200 for example)  I found that it's possile to go from 1.2v to 5v with the TPS61200 and go from 50mA to 500mA with the L272 but what if they're used together ? Will it preserve my battery lifespan ?
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I have a Canon SX210 IS, which doesn't have the possibility to plug-in a external power source.  Because of that, I want to make a connection to the pins where the battery usually is. I'm a little confused about the voltage requirement because the battery says 3.7V and the base of the camera says 4.2V.  Since I have a car battery and a 7805 voltage regulator I thought I could power the camera directly with the 5V the regulator would give me. But I don't know if it's safe to do that.  You may ask why would I want to power the camera with 5V instead of using 4.3 from a DC regulator. Well, I'm outside the city right now so there is no way to get a DC power supply like that.  (don't ask where I took the 7805 from).  I've found out that my helicopter charger has a voltage of 4.2V and a maximum  current of 850 mA, but it's a typical switched power supply (like the cell phone ones). Is it dangerous to feed my camera with that? What about a little paralell capacitor to filter noises?
99737
What is / how can I get a student-safe power supply for electronics projects?  I'm looking for a student-safe power supply for solder less breadboard.  I'm developing a course to teach electrical engineering to grade school students as an after-school program.  I'm wrapping up the first course and I've learned a lot, including that I need to find a safer power supply.  We're mostly using 9V batteries and connecting them to the breadboard via alligator clips.  After lots of searching, I'm realizing that I need to turn towards the expert community for guidance.  I hope that with the rise of the Makers  this question will become increasingingly relevant with time as more kids get into electronics.  Inextricably linked to this question is what is the minimum power source to use.  I've liked using 9V batteries since we can connect to them using alligator clips; but I worry that they are just a little too powerful.  I can imagine them somehow starting a fire with them.  My #1 priority is to keep the students safe.  Kids just have no concept that they can cause damage to themselves and property.  When I ask "where is that smoke coming from?" I'll get a gleeful response from the student that is seeing how many batteries he could hook together and short out.   I really need to limit the damage they can do.  I mainly don't want anyone electrocuted or anything to catch fire.  Kids will routinely attach LEDs backwards, leave them like that and then act surprised when they explode.   After safety, I want to protect the equipment.  Kids will hook up a LED backwards all the time.  After protection, I want to give the kids feedback as to what is going on.  A light that indicates power is on would be awesome.  A light or siren or circuit break that indicated a short would be awesome too.  Battery powered supplies are slightly preferred, just to keep upfront costs low and to avoid having to run outlets to each table.  Boundaries:   We are only using simple LEDs, BJT transistors, resistors and maybe a timer IC.    Similiar Questions:       References:  Course details are here:   We currently sometimes use  from the Squishy Circuits Store and/or a 9V battery with alligator clips.  Epilogue  I found it tougher than expected to find a suitable wall wart, so I'm listing what I finally found for future reference.  I'm about to test the following combination:         Epilogue 2  A year-plus in, I've decided to drop the in-line power switch since it wasn't obvious when it was 'on' vs. 'off'.  I'm instead relying upon the students to unplug the rat-tail from the supply, which is quick and easy.  Also, since I'm primarily using this for a squishy-dough project, I'm experimenting with with using two button batteries instead of a wall power.
26449
I'm designing a USB device which can either be powered by USB or an external 5V power supply.  When an external 5V power supply is present, the device should be powered solely by this external supply. Otherwise, the device should be powered by the USB bus. In other words, the external supply should always be prioritized, regardless of voltage level compared to Vbus. This rules out solutions based on diode-ORing.  Furthermore, the device will potentially consume quite a bit of power (several amps), so a solution based on MOSFETs would be preferred. When drawing a lot of power, the external supply is required.  So far I've found only one IC which seems to do what I want, the . However it's rather expensive and supports up to three supplies, which is more than I need. Does anyone know of other simpler (cheaper?) ICs?  I've come up with  which appears to work in simulation, but I'm not sure it will work in practice. For instance, is it possible to get an inrush current from Vbus to Vext during the transition period? Does anyone know of a simpler circuit to safely switch between the two power supplies?
39052
Possible Duplicate:        I have a Dell Vostro 1500 laptop, but unfortunately I lost the power supply in Indianapolis last month. I have a power supply for an old Sony laptop lying around, that looks like it fits in the same port.  Is it safe to use the old power supply in the new laptop? If they're not compatible will it simply not work, or will it reverse the polarity and open a gate to Zuul or something similar?  (A new PSU is on order, but UPS failed to deliver it yesterday and I need it today, hence this stopgap solution.)  Thanks!
45108
If I use the internal 1.1V reference for the ADC, and my Analog input exceeds 1.1V, lets say 2.5V, is that harmful to my microcontroller? Or will ADC value simply clip (to 0x3FF) at 1.1V?  For convenience, I am including an excerpt block diagram from the ATMega328 datasheet.
76547
I know that for a linear regulator to go into regulation, it is necessary to maintain (at least) a specific dropout voltage between its input and output.  But what happens if the input voltage drops so much that it will be closer to the intended output value than the dropout? What will be the "new" output voltage?
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!
43954
I need to amplify a voltage, which comes from a DAC and is 0-2.5V and scale it 0-25V in an operational amplifier. After amplified, I need to offset it -1.25V, so the final voltage is -1.25 to 23.75V. In other words, being an operational amplifier, I want to do the y=ax+b operation. with a=10, and b=-1.25V  I thought of 2 different ways of doing this.   The first idea I had is to use a second amplifier as a summing amplifier, with a fixed -1.25V voltage.  The second idea is to use a 1.2V zener in series with the output, along with a biasing resistor to -V.   Why do I want this? I want to set the ADJ pin in an LM338 voltage regulator from an MCU. LM338 out is ADJ+1.25V. If ADJ is -1.25V then the output is 0V.
103555
As with other ICs, it's standard practice to place bypass capacitors near the supply voltage pins of op amps. But I've seen conflicting opinions on how to properly bypass an op amp (, for example). Some people suggest putting a single capacitor between the V+ and V- pins. Others suggest using two capacitors, one from V+ to ground and one from V- to ground. Which of these methods gives the best result? I'm using  as unity-gain voltage followers for audio signals, but I'd like to know whether the answer's the same for other situations as well.         &ndash; Schematic created using
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)
101840
Have set up a voltage divider inputting 1 volt into a LM741 op amp to act as a voltage follower. For some reason I get 2 volts at the output. Have checked everything I can think of and can’t figure out why.