I have a power source that gives 5V DC and 700mA. I need to power a ventilator that requires 5V DC and 500mA ± 10%. What is the best way to lower the amperage, would a 10Ω resistor do the trick or should I use a transistor or something else?
1228
Power supplies are available in a wide range of voltage and current ratings.  If I have a device that has specific voltage and current ratings, how do those relate to the power ratings I need to specify?  What if I don't know the device's specs, but am replacing a previous power supply with particular ratings?  Is it OK to go lower voltage, or should it always be higher?  What about current?  I don't want a 10 A supply to damage my 1 A device.
83002
Having discovered just how awful the AC ripple is on some of my 5VDC wall-adaptors, I decided to make an Arduino program to calculate the RMS AC component and DC component of an adaptor under varying current loads.  My current issue is that the Arduino (and it's ADC's and DAC's) run on 5V, so 0-5V is it's analog input and output range. Many "5V" adaptors output a little more than that (and I want to be able to test 9V and 12V adaptors as well). So, I want a way for the Arduino to attenuate the input down to something within the 0-5V range, and the only way I can think of to do this is to use the Arduino's analog voltage output (which, I realize, is just PWM digital). Hence, I need a voltage-controlled attenuator.  The application requirements are:   It must be able to attenuate the input anywhere from a gain of 1 down to a gain of about 0.25 (to get, say, a 15V input well within the 0-5V ADC range) The range of controlling voltages must also be within 0-5V. Furthermore, the range cannot be too small, since the Arduino can only select 256 values between 0-5V. In other words, I can't have a controlling voltage of 2.000V give me a gain of 1 and a controlling voltage of 2.001V give me a gain of 0.25, as the Arduino doesn't have that kind of control over its analog output.  Any attenuation needs to affect the AC and DC portions equally, or else my ac_component/dc_component ratios will be erroneous.  I would very much like not having to supply additional voltage rails. For example, I don't want to have to supply a +12V and -12V for a certain op-amp. If it can all be done with a 5V rail, that's optimal. Or, +12V will probably also be fine. Attenuation does not need to be linear with voltage. All I'm after is being able to calculate (RMS ac_component / dc_component), and those ratios will stay the same without my knowing the exact attentuation. I understand that JFETs (see the schematics below) aren't all that well-suited to DC operation for thermal issues or something. Gradual drift in the attenuation is okay. My Arduino program only samples the voltages from the wall-adaptor for about 1/10 of a second. As long as the gain doesn't change appreciably over that time span, things are fine.   The first thing I tried was this sample which I found. It kinda did what I wanted, but it's inverting, so the output ends up being negative (and out of the 0-5V range of the Arduino).         &ndash; Schematic created using   Then, I realized that the op-amp was probably just lowering the output impedance of the what's really just a voltage-divider with a JFET in place of one of the resistors, and I didn't need that because the Arduino has a much higher input impedance than the power-supplies I'll be testing, so I tried this ...          but I'm getting something strange. It looks like the AC component is getting affected by the gain-control input voltage, but not the DC component (see requirement 3). I clearly don't understand FETs like I should. Can someone either suggest some fixes to this approach, or suggest an alternative?
61037
After always being interested in learning more about electronics, I figured LED-strips would be a great start.  I'd like to power a 4 meter WS2812B-LED-strip (60 LEDs/m). These run on 5V and draw about 60mA per LED. That's 14.5A, or 75W. However, I've yet to find any power supplies for these figures, only imported industrial ones - which feels a bit scary with my limited knowledge.  I've understand that powering it through the Arduino is a bad idea, but what's the simplest and cheapest solution for powering it? I do have a bunch of old supplies for laptops and whatnot lying around.
86692
When I plug A 12V DC 8" LED strip into my car, it gets too hot to touch, but when I plug it into A 110V AC to 12V DC 120mA transformer, it works great; "cool to the touch".   So do I go buy a resister and put it in line? If so, what resister do I ask for and which band goes to the LED side?
63074
One concept of electronics I have a hard time understanding is if things like motors, actuators, solenoids etc. use as much power as they need or what you give them.   If a motor needs 12 volts and 500ma and I supply it with 12 volts and 3000ma will it only consume 500ma? Also, if I supply it with 15volts and 500ma what will happen?   It seems logical that an LED and a DC motor are way different when it comes to requiring/using electric where as an led has to be completely regulated and (I assume) a DC motor doesn't.   Is my understanding wrong?
47215
I have the following ammeter:   The seller recommends 75mV 100A shunt be placed with it. I want to know what will happen if I change the voltage going through the shunt. Should I be changing the shunt as well to read amps correctly?   I want to build a 1-40V (or maybe 50V) adjustable DC power supply and use this ammeter to measure current. What the right value of shunt that I should use to get the right measurement? Changing shunts in a lab bench power supply is impractical, of course.  EDIT: I am planning to measure only up to 5A or so. Sorry I forgot to mention this previously.
93952
Is it possible to have a 5 meter wire that has 10,000 amps or more current flowing? I assume this can be done with a step-down transformer and the voltage could be 2V and lower... I assume that most of these wires have low resistance? So that heat would be considered.
77882
How can I connecting two different DC power sources together? for example 15Vdc(5Amp) with 5Vdc(2Amp). I want to make one DC source (20Vdc,7Amp) by more powerful supplies.
113680
I have two doubts, requesting you to answer my doubts separately. :)  1) I need a resistance of 'X', so is it better to use a single resistor of 'X' value or multiple resistance of r1+r2+r3='X'? What  I mean is using multiple resistors in series instead of using a single resistor has any advantage? Will it reduce the resistors getting overheated?  2) Consider a 1W 2k2 resistor and a 1/4W 2k2 resistor. Is the heat produced by resistors of different watts different? Which resistor will get more heated in the same conditions (I mean the current, voltage etc given both resistors are same)  Regards, Kiran.
30982
I found the  at Digikey.  The datasheet says the device is rated for 400A @ 25C, 1200A @ 25C for 1ms, with a voltage rating of 300V and PD of 1000W.  Really?  This TO-264 package can control 400A of current all day long?  I can short out my TIG welder with it in DC mode?  How do those leads even carry 400A of current?
69093
I am trying to install a 60 amp sub panel in my house to power my wife's saltwater tanks. I know I need 6/3 to go from main panel to sub panel. My question is I have to run it along the wall to get to the ceiling then down the wall to get to sub panel. What type of wire and do I need conduit? The sub panel will be just outside the backdoor under the porch.
40509
Designing the power supply for an radiation detector, need to supply up to 1500V at 2 mA from mains (170 Vdc).  I've largely considered a flyback converter, but cannot find a flyback transformer with a gain of N=~16 (for a gain of 8 at 50% duty cycle) rated for 1500V at the output winding.  Looked into charge pumps/voltage multipliers some but not in depth yet.  Also just recently learned about CCFL inverter transformers, but still need some time to understand them better.  This project requires a small size and weight, so using a large transformer is not preferred.  What are other ways to step up voltage which I may look into?
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
117600
I'm using a repurposed ATX power supply for my hobby projects since it's got 3.3/±5/±12 outputs, all of which are really convenient. But one thing I didn't really think about until I slipped my probes across the pins of an opamp, since I've always dealt with commercial/proper lab power supplies in my school labs, was that an ATX power supply will gladly deliver lots of current if that 12V line (or any other) is shorted to ground. The poor LM318 didn't stand a chance. My meter and supply survived, but in the interest of not killing anything in the future, myself included, I was wondering what the best option was for short/overcurrent protection?  I was thinking of sticking some high wattage resistors at the output of the supply before connecting them to the rails of my project breadboard (I use a separate breadboard with terminal blocks for power, which the ATX supply connects to). The problem is if I'm drawing a lot of current (LEDs etc) this will sag the voltage on the line. And, for example, if I use a 1W 200ohm resistor on the 12V line, it limits my current to only 60mA - if I want more, I need some really beefy wattage resistors. I can probably work around the voltage sagging by using a voltage regulator (eg at 10V), but this all doesn't seem like the best way to go about doing things.   I'd appreciate some input from someone more experienced than I am.
22457
To power my microcontroller (ATmega8), I am using a ~5.4V voltage source. I want to ensure that I don't accidentally connect the voltage source in reverse, and figured a diode would be a nice way to accomplish this as from what I have learned so far, a diode allows current to flow in one direction, and blocks it in the other.  But what I also learned is that diodes create drops in voltage. I have a few of the typical diodes (1N4001, 1N4148, etc.), and would like to use them to achieve the aforementioned result without dropping the voltage as it would be too low to power the IC.  My question is, is there any way to do this with a diode? Or do I need some other component (if so, what would you recommend)?
122477
I have 12V, Use these 12V to power a device (that has a 12V output signal) and to power a 7805. After the 7805 there is a pic microcontroller where I would have to connect the output from my device. I was thinking of having 2 resistors in a 3/1 ratio in order to have about 4V at the pin of the pic. Can this be done, given that the ground is common, or will it give some sort of problem with the lm7805?
121757
I’m building a battery powered data logger with a microcontroller and several sensors. I’d like to measure the supply current of individual parts with idle currents in the μA range and peak currents in the low mA range. The current is not constant which rules out a simple multimeter.  Usually I’d (temporarily) replace ferrite beads on the PCB with a 10Ω resistor and measure the voltage drop with my DS1104Z oscilloscope. However, the voltage drop of some μV for 1 or 2 μA of current is simply drowning in the noise and peak currents of ~10mA already result in a noticeable supply drop (so increasing the resistance is out of the question).  I was thinking about building a small board with a 5Ω resistor and OPAMP set to something like 100x amplification, but first wanted to know if this is a good idea or if you know of any affordable, ready-made solutions. Obviously the bandwidth doesn’t have to be high, but some accuracy would be desirable.  Thanks in advance.  Michael