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# led voltage calculator

Question:Hi, I have a bunch of LEDs and resistors that I bought a long time ago, found them today. I want to build a circuit and put them in my car (DC battery), am I able to test the circuit on AC voltage in my home with a 12v AC transformer? I don't want to blow the circuit trying it on AC.

Answers:Not really, make the 12V AC into DC by making a simple PSU with a few diodes and a capacitor then you can experiment

Question:Can anyone advise how you calculate how long a capacitor takes to discharge for a nominated current draw from it? With the advent of super-capacitors, I was wondering whether a 1 or 2 Farad cap would drive a string of 3-4 LEDs at 18m/A per device. Clearly they can, but will it be a flash from the LED as the cap discharges, or will it hold a 18mA on for a few seconds, minutes or hours? I will use a string of LEDs so the 18mA goes further. But how do I work out the time a LED will stay lit? Thanks Billruss. Just FYI, the cap I intend using is rated at 16V so I would probably charge to 13-14V. As regulators are wonderful things, I would use a low-dropout device. This would allow the LEDs to run at (say) 3.7V while the higher battery voltage drops. Thanks anyway - I was pretty sure it was a logarithmic function.

Answers:4 x 18 mA is 72 mA A cap starts losing voltage immediately. if you charge it up to 5 volts and allow it to fall to 3 volts, that is about: CV= it t = CV/i = 2F * 2v / 0.072 = 55 seconds you should use the exponential equation to get a better answer, but this is close. .

Question:i saw a good web site with a free voltage calculator. you just input the numbers and it will give you estimations. it had ac and dc and many different wire guages.i can't find it now.i just got lucky to stumble upon it a couple weeks ago. does anyone know of a good online calculator.

Answers:As an electrician, you upgrade to a thicker wire (12 to 10)when it goes beyond 100'. Generally you never went beyond 200' from your runs from the panel. We sometimes used stranded from solid as there is more surface to Handel amperage and less voltage drop, less resistance. A current copy of the National Electric Code has all that info. Maybe Google that as a start.

Question:I have a 12v, 5w solar panel that I used for multiple purposes. For example, powering a string of LEDs and charging batteries. These aren't done at the same time. To prevent battery overcharging and damaging the LEDs, I hooked up a LM317T voltage regulator to limit the current. Here is how I have things setup: (a) Solar panel + lead going to the Vin of the regulator (b) Regulator Vout to one side of a 100 ohm resistor (c) The other end of the resistor going to the + battery connector for charging. Or to the + side of 1 LED (d) Regulator Vadj going to the junction between the resistor lead in (c). In words, it connects between the opposite end of the resistor and the + of whatever is being powered or charged. The "right" side of the resistor. The left being the side connected to the regulator Vout. Now with a target voltage of 1.2, which is what would be needed to charge AA batteries connected in series, shouldn't I be getting 12ma off the regulator ( 1.2v needed / 100 ohm resistor)? When I connect the - of the solar panel to the - of my ammeter. And the + coming from the junction of the resistor and Vout of the regulator to the + of my ammeter, I'm getting current readings that flucturate between about 3.5 to 80+ma. So it's fluctuating with the amount of sunlight. Why are my current readings fluctuating? Isn't the regulator supposed to be adjusting the voltage level so the current flow is constant? I'm trying to determine if there's a problem with the regulator, resistor, or perhaps my own setup. So any help you can provide would be greatly appreciated!! FYI, My ultimate goal here is to limit the current to 200-250ma. This would be C/10 for the AA batteries and would allow for trickle charging to prevent overcharging. And allow me to leave the batteries charging all day (indefinitely). At 200ma, I would need a resistor of 6 ohms. And for 250ma, I would need a resistor with a value of 4.8. So I would ultimately either pick a 4.7 or 5.6 ohm resistor. Most likely the 5.6 ohm because it would give me a current of 214ma. The 4.7 ohm would give me 255ma, which is above the C/10 of 250ma. And lastly, I tried hooking the regulator up to a 9v battery to see what happens with something that doesn't change with sun intensity. Same problem, varying current levels! So my regulator isn't maintaining the current and I'm lost as to why. Once again, thanks a bunch for all your help!! One other question I have is this: For calculating the resistor needed when powering a string of LEDs connected in parallel, do you use the actual source voltage? For example, if you're using a 12v battery that puts out say 15v, would you calculate the resistor off the 12v source voltage? Or the 15v, it's actually outputting? Thanks again!!

Answers:The selection of resistor value has nothing to do with the output voltage but only with the 1.2 volts the LM317 regulator tries to maintain between its output pin and its reference pin. So, 100 ohms between these pins, with the load connected to the end of the resistor connected to the reference pin should produce a current of 1.2/100=12mA. But the regulator includes a very high gain voltage amplifier that amplifies the difference between the voltage on the output pin and the internal 1.2 volt reference and drives the output pass transistor. This can cause unstable operation if the input pin is fed from something other than a stiff voltage source that is at least 2 volts more positive than the output pin. So the first thing to check is that the solar panel is producing at least 2 volts more than the voltage at the output pin (which must be 1.2 volts more positive than the required load voltage at the other end of the current sensing resistor). If that requirement is met, you may need to add a capacitor across the input pin to the negative side of the panel, to help hold the input voltage more stable (to change more slowly as the panel current changes) as the regulator changes its conduction, so that the internal amplifier is not driven crazy by an input voltage (panel voltage) that bounces around every time the regulator tries to adjust the output voltage (and current). The data sheet for the LM317 advises what size capacitors are recommended, but extra capacitance won't hurt, if you have something larger handy. Make sure this capacitor has a voltage rating higher than what the panel can produce. -- Regards, John Popelish