Back before basic (voltage or current)? - Page 1
Forum section: electronics / beginners

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 01, 2019, 06:38:20 pm
content: I think I need some totally basic teaching here about what voltage vs current, I know that Voltage is the potential and current is the electrons but here is my lack of knowledge:Lets take a standard LED, I have seen some dimmer's change the voltage and other changing the current to dim the Led. So is it the resulting just the wattage that decides it's brightness or how do you know what to adjust to dim?When do you adjust the one instead of the other in other circumstances?

author: magic
author_rank: Super Contributor
timestamp: October 01, 2019, 07:14:31 pm
content: Well, I can imagine there being two separate questions.1. Which quantity is tighter correlated with brightness. I'm no semiconductor physics wizard but I suspect it's the current and for a given diode there is some more or less constant probability that each passing electron ends up emitting light rather than dissipating its energy as heat. Voltage OTOH seems more closely related to energy of each emitted photon and therefore wavelength/color. Of course that's a difference between various LED types, I don't think changing voltage applied to one diode has any effect on color.2. Which quantity is easier to control. For LEDs it certainly is current. If you set constant current, voltage will also remain almost constant and so will wattage. And experience tells that constant current drive is effective at achieving reasonably stable brightness.On the other hand, minor variation of voltage causes significant change of current, power and brightness. Voltage drive is harder to adjust and harder to maintain over time.Therefore pragmatism suggests current regulation for LEDs.

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 01, 2019, 08:15:41 pm
content: Quote from: magic on October 01, 2019, 07:14:31 pmReally great reply, thanks! :-)1) So current adjust the amount of brightness while to little volt just vont make the LED turn on?2) Always aim for constant Voltage and then make the Current change, because it's easier and more stable?Is that a fair conclusion from your answers?

author: TimFox
author_rank: Super Contributor
timestamp: October 01, 2019, 10:51:29 pm
content: For LEDs, it’s better to control the current (required to get the brightness), then make sure that the current source can supply enough voltage (technically the compliance voltage) for the LED at that current.

author: MosherIV
author_rank: Super Contributor
timestamp: October 01, 2019, 11:03:03 pm
content: Quote?Ohm's law applies. V = I × RIf the voltage is fixed and you want to change the current, then you HAVE to change the resistance.

author: rstofer
author_rank: Super Contributor
timestamp: October 01, 2019, 11:06:46 pm
content: Quote from: FriedMule on October 01, 2019, 08:15:41 pmNo..,An LED is a non-linear device, the relationship between the current through an LED is only vaguely related to the voltage across the LED. Look at Figure 4 herehttps://www.vishay.com/docs/83171/tlur640.pdfAs the voltage across the diode increases from 0V, at some point around 1.5V the current starts to flow. By the time we're at 1.7V (more or less), we're at 10 mA through the diode.So, from Figure 6 we figure out how much luminous intensity we want and how much current it takes to get that amount. Let's say 10 mA.We need 10 mA flowing through the diode and from Figure 4 we guess at 1.7V drop. Now, if we have a 12V power source, we need to drop 12V - 1.7V or 10.3V across the resistor with 10 mA through it. E = I * R or R = E / I = 10.3V / 0.01A or 1030 Ohms, call it 1k OhmsYes, we control brightness with current but that current is not independent of voltage. We need to do all of the math to figure out what size ballast resistor to choose.

author: radiolistener
author_rank: Super Contributor
timestamp: October 02, 2019, 05:38:02 am
content: Quote from: FriedMule on October 01, 2019, 06:38:20 pmJust imagine water pipe. Voltage is water pressure. Current is how many water volume flows through pipe per second. Resistance is pipe diameter.

author: magic
author_rank: Super Contributor
timestamp: October 02, 2019, 05:55:21 am
content: Just imagine damn electrons. Voltage is the difference between energy of 6241509074460762608 electrons in place A and place B, current is how often 6241509074460762608 electrons move from place A to place B. Nothing simpler that that

author: ledtester
author_rank: Super Contributor
timestamp: October 02, 2019, 06:10:36 am
content: QuoteAn LED has a V-I curve -- a relationship between the voltage across its terminals and the current through it. Different color LEDs will have different curves, e.g.:So if you control the voltage across the LED you are also controlling the current running through it, and vice-versa. You are free to adopt whichever perspective is the most convenient.There is even a third way to control the (apparent) brightness of an LED -- use PWM control to alter how long the LED is energized. If an LED is ON only 50% of the time it will appear dimmer than if it is ON continuously.

author: MosherIV
author_rank: Super Contributor
timestamp: October 02, 2019, 07:01:39 am
content: Hi. While Ohm's law still applies.Not sure if you have realised or figured out from the graphs, semiconductor have a strange property : they have a (relatively) fixed voltage drop once conducting.So the LED will always have ~1.2V across it regardless of the voltage across it or the current through it.Therefore to control the current through it, you can add a current limiting (controlling) resistor in serise with the LED.Then use Ohm's law to work out the resistor value to set the current.Love the picture Radiolistener

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 02, 2019, 11:42:31 pm
content: Thanks for all your fantastic answers! :-)Just to confirm what I do understand you say abut LED and Voltage and Current:The LED do not use any Current without Voltage and even with, lets say 0.5V the LED still do not use any current, it is first at it's minimum on-Voltage that the LED starts to use Current. To not burning the LED off, it's the famous Ohm's law that tell what resistance that is needed.It is now wary clear and I think I do know about what I need to power a LED. :-)Just one last question, I have seen dimmer's that regulated the LED by voltage, not current. It was something about current was a failior point because the LED heats up and the resistance changes, which again heightens the amount of current, that again heats the LED up even more... a cycle that fast can shorten the LED's life.

author: james_s
author_rank: Super Contributor
timestamp: October 03, 2019, 12:36:49 am
content: Again ohms law applies. If the resistance in series with the LED is constant, then varying the voltage will cause the current to vary.

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 03, 2019, 03:07:24 am
content: Quote from: james_s on October 03, 2019, 12:36:49 amYes you are completely correct, that's also why I did not understand that about adjusting Voltage instead of Current, to avoid the "evil cycle"

author: Rick Law
author_rank: Super Contributor
timestamp: October 03, 2019, 04:55:44 am
content: LED has resistance, ohm law applies, and V=IR.Think of it as LED asa resister with resistance that changes based on voltage.LED is a semi conductor, it kind of conducts but do it in a different way unlike a full conductor. With LED, V=IR, but you have to tell me what is V before I can tell you what R would be.... And, in the case of LED, it conducts one way (R=infinite when V<0) and it turns the energy (from voltage dropping across it) into light. The light comes on visibly only when voltage is at a certain level.To imagine such magic "turn on the light voltage", think of it like boiling water. If you have a pot of water that is not at boiling point, water is evaporating whenever temperature is > absolute zero, so, some steam (a few water molecule that turned into gas form) is coming out, but at such low amount that you wont see the steam - you call that evaporation. When the pot is at boiling point temperature (100C at sea-level and standard atmospheric pressure), well, being unevenly heated, the water closer to the flame probably get to boiling first. Every water molecule in that pot that reached boiling point turns into gas. Once you reach that magic "turn on" point, enough water in the pot is turning into gas that you see the steam, and it can blow a whistle or drive a turbine.So, LED being a resister with voltage-dependent resistance, you apply low voltage, very few photons come out and you have no visible light. I=V/R, but since resistance is not constant, you don't know the resistance to calculate current directly. You have to first find out at that specific voltage what R is, then I=V/R (or V=IR, same thing) can be evaluated.As you increase the voltage to near "turn on", more and more photon are coming so you see the light. Again, resistance changed by now since voltage is increased. V=IR applies. At around that range, you can indeed control brightness a bit by adjusting the voltage but that adjustment range is very small so controlling brightness that way is ineffective. It is easier to control how much current goes in by using a current limiting (ballast) resister - well, the LED is at (around) 3.15V, I am applying X volt, so at 100mA, my ballast resister should be Y. etc, etc. Easy to calculate. The LED may "settle" at 3.145 volt or 3.151 volt or whatever. With your 1% ballast resister, you are not getting better than 1% precision with voltage anyhow, so you just evaluate it as 3.15V.When you increase the voltage beyond LED's operational range, well, the thing burns out.Now, had you started with negative voltage, LED being a resister with voltage-dependent resistance, and at negative voltage, the resistance is near infinite so you have nearly no current going through - just a few electrons leaked across.

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 05, 2019, 04:06:10 am
content: Quote from: Rick Law on October 03, 2019, 04:55:44 amWOW THANKS!!! This is without doubt the best answer I could ever hope fore!!So a LED with 0.5V would use a tiny amount of current that you may be able to measure with a pA meter or something like that and until the "boiling pint not much will be visible, while at the "on pint" the LED starts to need a "lot" of current, enough to se it on a cheap pocket meter:-)

author: Rick Law
author_rank: Super Contributor
timestamp: October 07, 2019, 03:30:43 am
content: Good you found the explanation useful, and thanks, that is a nice complement you gave me.What you described is consistent with my explanation - except that I would not quantify it by using picoAmp or any unit at all but instead I would use general terms like "a very small current." The actual current (at low voltage) is entirely dependent on the design of the actual LED so it could be in picoAmp or milliAmp or whatever. To get even close to actual numbers, you will have to get it from the datasheet andit will vary a lot between different LEDs. But,what you would see consistentlyis that the operational range in Voltage as a fraction ((Max-Min)/Max) is far smaller than the operational range in Current as a fraction. So, a small deviation of voltage results in a far greater change than a small deviation of current, making current a more precise method of control.But, when it comes to a product design, which one is cheaper to make may be the choice over which one is more precise.That said, note that twice the current doesn't mean twice the brightness - that is another "check the datasheet" things. Also, the other common way of LED brightness control is PWM - control by power the LED and adjustable percent of the power-on time. This one could be the cheapest if they are already using an MCU with a PWM available.Appendix: An actual numbers example, numbers from Cree XM-L2 datasheet's Current-Voltage graph (reading number from graph is hard, so they are all approx):Voltage min (2.7V) to max (3.3V) range is 0.6v = 18% of max Volt.Current min (200mA) to max (3000mA) range is 2800mA = 93% of max Current.Just imagine adjusting with just 18% of your dial vs 93% of your dial, you can see why it would be easier to get precision with current adjustment than with voltage adjustment.

author: rstofer
author_rank: Super Contributor
timestamp: October 07, 2019, 05:42:12 am
content: Everything that needs to be understood about the Forward Voltage, Forward Current and Relative Luminous Flux is shown on 2 graphs on page 6 of the datasheet.100% Relative Luminous Flux occurs with a Forward Current of 700 mA (bottom graph) and at such a current, the Forward Voltage will be around 2.85 Volts (top graph).https://www.cree.com/led-components/media/documents/XLampXML2.pdfSo, we have found the two required design parameters by simply looking at the two graphs. Other LED datasheets are similar.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 07, 2019, 08:24:29 am
content: Boiling water analogy is a good one.Water always boils over absolute zero temperature. But very, very little boiling happens until you start approaching some arbitrarily defined threshold, say, 90 degC. Then, rate of evaporation starts going up really, really quickly. And to regulate the amount of evaporation, you can't do it by regulating temperature. There will be a massive difference between 99 and 100 degC. To control the rate of evaporation, you do it by limiting the input power - a 1kW kettle and a 10kW kettle both keep the water at approx. 100 degC, but the latter evaporates ten times as much water in the same time.LEDs have a similar curve, but it's even more tricky. Say, you have an LED that produces just the right amount of light (taking appropriate current) when you feed it 3.0V. It's very dim at 2.8V, and blows up at 3.2V. So you need to control the voltage carefully. This wouldn't be too difficult with precision regulators, but with LEDs, the "right" voltage isn't fixed, it changes! Specifically, it changes when the temperature changes, and worst, it changes in a bad direction.So the LED is happy with 3.0V, but it slowly heats up, and now the optimal voltage shifts to, say, 2.9V. You are still applying 3.0V, i.e., too much voltage, and the LED will be brighter - and consuming more current, producing more heat, heating up even more, and the optimal voltage shifts even lower, and you have even more excess, causing more and more heating, and a thermal runaway.Another issue is unit variation. One LED is happy with 3.0V, while the other is at 3.2V. So you'd need to manually adjust every unit, a nightmare for manufacturing.This is why you need tocontrol current, not voltage.Voltage and current are parameters which both can be controlled. Depending on a circuit, you typically choose either of them to be controlled, and the other will be whatever it will be. One parameter is controlled, another isa resultof the control.For a microprocessor core power supply, you would regulate the voltage, say, to 1.2V, and then the CPU takes whatever current it needs, depending on the computation it's doing.But because the LED behaves like I explained, it makes much more sense to choose the controlled parameter to be the current. When you control the current, the voltage is whatever it is. And once the LED heats up, you can measure the voltage dropping, but the current keeps the same, because it's controlled.Now, how to control current is a different topic altogether. It's not too different from controlling voltage. There are exact methods using active feedback and comparison to known reference, called regulation - the difference between voltage regulation, and current regulation, is in small details, namely, how the feedback signal is taken.Then, there are "good enough" approximating solutions that don't offer full control, but approximate it. A series resistor from a fixed voltage source does not regulate current, but makes the changes less steep.