I want to power 3 x 12v 0.2A PC fans from the Molex power supply on my PC PSU, the only issue is that I don't want them to run at full speed. So I am trying to half the voltage to 6v. I have built this, and with no load it reads 6v across R2 however when I plug the fans in it reads approx 1.5v and my bench PSU reads no current draw. What am i missing?
46886
How would one go about using a 12 V DC power source to power something which needs 4.5 V DC using resistors? Is there a way to determine how much adding a resistor would drop the voltage?
26704
I have a laptop that says it needs a 19V 3.42A power supply. I have a Dell 20V 4.51A power supply.  Can I use this power supply with my laptop, or will it destroy the machine?
95773
I have a quick questions: can I safely power a LED with 3.3v without a resistor? I have some LEDs that I typically power with 5v and a 220Ω resistor, but I can only supply 3.3v at the moment.  Additionally I have push-buttons that I typically pull-down with 10kΩ resistors when using 5v. What resistor should I use in this case? Is there a general rule on what resistor should be used given the voltage?  Thank you in advanced for your help!
106672
I am attempting to run a Molybdenum heat coil. The best I can tell it is 0.3-0.4 ohms per rack and I would like to run say two racks at 230V at 3.9KW (so it doesn't trip my 230V 20 amp breaker).  Does that mean, assuming a value for two racks is a total of 0.8 ohms, can I add a ~12.7641 ohm resistor for a total resistance of 13.5641 to run it at 3900 watts at 230 volts?  If not, how is it possible to run these? Do I need a lower voltage supply with more current?
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 ?
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?
43420
My computer's motherboard uses all four pin fan connectors with no support for speed controlling the older three pin fan connector. The problem is, all the PC fans that I have use a three pin connector. As fan as I know there are no premade solutions available, so I was thinking of building an adapter that sits in an unused drive bay.   I can't seem to find much information on how the four pin fan connecter works. Apparently, it looks something like this:   12 V 0V Tachometer PWM   The three pin header is numbered similarly:   Vcc 0V Tachometer   Could I simply use a logic level MOSFET to do the following?
121975
Note that this is a theoretical question - there is no schematic I can show. I will show some schematic, but it will be a very simplified version of an actual circuit, only for illustration purposes.  Assume I have a voltage converter that takes as an input my main voltage (from a power supply) and outputs a certain voltage, for example 1.8V. It would look something like this:         &ndash; Schematic created using   When connecting my circuit to the P.S, I notice it draws too much current (the P.S shows that).  Since I have multiple voltage converters in my circuit (not shown here), I check resistance between each output of each converter to ground. I see that the resistance between 1.8V to ground is almost 0 Ohms. Now I know the fault is either in the voltage converter or one (or more) of the other components drawing power from that 1.8V.  I desolder the resistor shown in the image to disconnect the converter from the other components and see that the converter is fine, but checking resistance from the point connected to all those components still shows 0 Ohms.  My question is - how would you check which component is the faulty one, without desoldering each suspicious component? As you can see in the image, the 1.8V supply is connected directly to the components, without a resistor/bead.  For the sake of this question, assume I have access to whatever equipment needed (no matter how pricey). I wouldn't want solutions to be limited due to availability of equipment.  Thank you!
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?
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My question is the following: We have two 220V 20A generators connected in parallel to total power 8.8 kw 220 v 40 A load  is 5.5 ohms I'm interested how each generator sees this load since every generator gives half the power. I put this question because one generator can not see this load like 5.5 ohms because it gives half power to not the full power I would like to have somebody explain it a little bit better because it makes me confused .     simplified question   my question is that since every generator in the parallel circuit gives half power in the ideal case it would mean that the generator 1 that gives 220v 20A that is half the power can not "see" the resistance of 5.5 ohms because if "seen" this resistance he had to give complete current in that case we have two problem generator 1 burned because it is not made for such a current problem two if generator 2 gives the same current then the law on energy conservation is not worth what is absolutely not possible  thanks    The last attempt to explain what I asked is very simple  we have two 220v 20A generators and a load of 5.5 ohms if we connect this resistance to just one generator it exceeds its power because it is 220v and 20 A 4.4 kw and the current at that resistor is 40A which would mean that it exceeds the maximum power of one generator ok if we add another generator then together have enough power to supply load it if we know that every generator gives half power ie 220v and 20A  it is impossible to "see "a resistance of 5.5 ohms
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I recently got a desktop LED lamp, and I feel a "strange" slight vibration when touching the metal case. I found that other devices also have high voltage on their case.  What is the reason/trade-off to design the PSU that way? Are "real" transformers inefficient? Even if it's harmless, why is the high voltage connected to the metal case, and not just through insulated wires?    When I turn the lamp on, the setup in the picture measures the power line (220V) 214Vpp (59Vrms). When I also touch the metal case, the voltage drops down to around 30V.  AC adaptor:
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
77814
I'm attempting to build an LED bed lamp that will be powered by 240V mains. The LEDs are  which if I use with a 12V supply and have calculated correctly will draw 0.24W each. I plan to use 5 or 6 of them in the lamp so will consume a maximum of 1.5W.  Initially I thought I'd use a transformer and bridge rectifier to get the required 12V DC but the transformer weighs a ton and is too bulky. The next option was to use a  but I'm wondering is there is a better way?  I think it would be possible to power the LEDs directly from mains using only a rectifier without mains and stepping down the voltage but then it's not safe. Are there any smallish components that can both isolate the mains and supply a DC voltage?
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?
107643
I asked a similar question  about this, but I still don't  quite understand.  Say I grab onto two wires from a standard 120V 20A wall socket. I would fry. I would have 20A at 120 forced on my body.  But, when speaking in terms of circuits, as in my linked question, if I have a power supply outputting 100mA maximum at 5v, for some reason my device only gets exactly what it needs? Why is this?  Most things flow from an area of high concentration to low concentration, taking the path of least resistence. If you connect two wires to a power supply, why don't the maximum number of electrons want to go through your circuit?  If current behaves this way...why do you have to regulate voltage?
39949
I want to supply constant voltage to my LED's but the supply varies from 3.7V to 4.2V (Li-ion battery). I cannot use 3.3V voltage regulator as it will need at least 4.3V voltage at the input but my battery can supply only 4.2V max. so is there any other way I can supply regulated 3.3V to my LED.   RGB LED = 3.3V, 60mA Battery = 3.7V - 4.2V, 3000mA Number of LEDs = 24 The LED's are connected in parallel
98189
This might be a bit long to explain but actually a power supply noise is affecting a system's measurements. I will briefly try to explain the system and the noise issue with several pics. This is directly related to my previous question:   I'm using this [system][1] together with this [module][2](SECTION 4 shows ELECTRONIC COMPONENTS). This is a pressure scanner system. Basically the ZOC modules amplifies and multiplexes 64 pressure channels(500Hz sampling freq. for each channel) and sends the particular channel's analog voltage output(chosen by a decoder code sent from the system) to the RAD_unit which then converts this voltage to a digital by its ADCs and sends to a PC.  Here is the basic illustration of the system setup:  [![enter image description here][3]][3]  All system is fed from one AC mains outlet using a power strip.  Depending on the day, time of the day or powering the system through different AC mains outlets I was getting different magnitude of spiky noises in my measurements. Below shows this difference. Both voltage is on vertical axis represents sensor data and scaled with factor 100.(For example 10 in vertical pressure axis is 0.1 Volt). Here is a comparison of two measurements where the bottom is when there is spiky noise:  [![enter image description here][4]][4]  Later on I checked the +-15VDC power supply outputs(RPM100 in above illustartion) which powers the module and sensors, and I found out there is noise similar to the one in measurements.   Here is how the power supply connects to the module:  [![enter image description here][5]][5]  And here are some screenshots of the power supply's +15V to GND measurements from scope in AC coupling:  A very noisy situation:  In FFT view:  A less spiky/noisy situation:  [![enter image description here][8]][8]  And here when there is almost no significant noise(the gap is scope's artifact):  Question:  It seems to me there is a big correlation between the measurement noise and the power supply noise here. Power supply is very old at least a decade old but a special one and powers all the units with its special connector.  1-) What kind of power supply noise can it be? It is obviously effected by the AC lines)  2-) For a quick solution, Im planning to cut the cable and place 100nF caps between +15V to GND and -15V to GND. Do you think it can be a solution? And if so would 100nF be okay?