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14Vf LED question

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krenshala

Tue Sept 22 2015, 12:22AM

Registered Member #143 Joined: Sat Feb 11 2006, 04:25PM
Location: Austin TX, NorAm, Sol III
Posts: 28

I'm working on a Halloween costume for my son that will incorporate some 14Vf @ 10mA blue LEDs (Linrose B4302H6) being run in parallel off a single 9V battery, and wanted to double check my logic with you folks and your collective electronics expertise.

I understand that normally you want an appropriately sized resistor in the circuit before the LED to avoid having too much voltage across it. In this setup, is that still needed/recommended? I don't think it is, but wanted to verify I wasn't showing my electronics noobness again.

Dr. Slack

Tue Sept 22 2015, 05:31AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

You don't have a resistor to 'avoid too much voltage', you have a resistor to control the amount of current through the LED.

With a 9v battery, and a 14v LED, no current will flow. A LED (an LED?) isn't like a filament lamp that will draw current at any voltage, it needs to exceed a threshold. However, a quick look at a supplier shows they are quoted as 12v, which I guess is the threshhold. That's still well above the 9v battery voltage. I presume 14v is the voltage at the nominal 10mA current.

You will need two 9v batts in series to give you 18v, then the 4v excess above the LED vf appearing across the resisotr needs to push 10mA through its resistance.

Using Ohms Law, what resistor value do you need for 10mA with a 4v drop?

klugesmith

Tue Sept 22 2015, 01:57PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Rarely do I feel compelled to correct Dr Slack, but these LEDs have built-in resistors.
So a 12 V model wouild run at reduced brightness on 9 V.
H6 (blue) seems to be new; H5 (green) drew more search hits.

For reasons explained by Dr Slack, the LED brightness will drop substantially as the battery voltage runs down. That could happen pretty fast if there are many LEDs in parallel, so be sure to test before the big day. If there's a 5V model, you might run two in series with no more drain than one 12V or 14V model, but it would be even more sensitive to decline of battery voltage.

Us old timers are reminded of neon indicator lamps with built-in resistors for direct connection to mains. Their orange color befits Halloween, but 90 V batteries were already rare old-stock items when I got one in about 1970.

Bored Chemist

Tue Sept 22 2015, 05:06PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Dr Slack seems not to realise that an LED has a fairly well defined voltage at any given current (and, of course, vice versa).
So, if you have too much voltage you also have too much current and the distinction between the two is a bit meaningless.

krenshala

Tue Sept 22 2015, 05:19PM

Registered Member #143 Joined: Sat Feb 11 2006, 04:25PM
Location: Austin TX, NorAm, Sol III
Posts: 28

From the reading I've done over the last day or so, it looked like I needed a resistor before the LED if the source voltage was higher than that rated for the LED. The information was vague about whether one was still a good idea if the source voltage was lower, thus my question.

The LEDs themselves have an integral zener (sp?) to prevent reverse voltage causing damage (based on my reading and the packaging), but everything I was reading about "normal" voltage LEDs (less than 5V) was saying to make sure there was a resistor in the circuit between the power source and the LEDs to avoid burning out the LED.

I've tested two in parallel and they definitely light up just fine for my needs. Based on what I could find out about the battery, I should get at least a couple of hours of illumination before the (maximum) 10 LEDs kill it dead.

klugesmith, Nixie tubes for the win!

Anyway, thank you all for the input on this!

Ash Small

Tue Sept 22 2015, 06:20PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

A 'spare' battery or three might not go amiss here.

Dr. Slack

Wed Sept 23 2015, 08:11AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Bored Chemist seems to miss the point I was making. To be fair, most people seem to miss most points I make, it can be very frustrating. So, to spell it out more clearly.

Is it meaningfull to worry about whether it's current or voltage being supplied to a load, when the load has a defined current at all voltages?

It's a question of whether the concept is useful to you. Let's take the example (to be closer to Bored Chemist's field of ken) of an electrolytic cell, with dilute sulphuric acid and platinum electrodes*. This is actually very similar to a LED load, as there is a quantum mechanically defined voltage below which the intended behaviour doesn't happen. Below roughly 1.7v, you cannot split a water molecule. Below a specific voltage, depending on some horrible physics involving band-gaps, you won't excite carriers and produce photons. Below a few volts depending on the colour, a LED is a DED, a dark-emitting-device. Both of these devices may conduct some leakage current by other mechanisms below the threshhold voltage, but neither could be said to be 'working' in this state.

Both of these devices are intended to be used a little above the threshold voltage. However, both have a low slope resistance. That means as the voltage increases a little, the current increases a lot. It is therefore more stable to drive them with a constant current, rather than voltage. It is also therefore more sensible to quote the operating current, rather than the voltage. Although the voltage drop taken by the QM mechanism is constant, the device has some bulk resistance, contact resistance, polarisation etc, which increases the supply voltage needed. In a well designed device, this extra resistance is kept as small as economically practical.

The threshhold voltage may not be particularly constant, either against temperature, over aging, and certainly between batches and individual LEDs, depending as it does on the physical makeup of the semiconductor. Given the low slope resistance, small differences in voltage mean big differences in current. It is impractical to specify the forward voltage of commercial grade LEDs to sufficient accuracy such that the operating current is well defined at a given voltage.

If you supply way above the threshold voltage, and your power supply is beefy enough to supply the current the device takes at that voltage, then the device will fail. Depending on the failure mechanism, bulk overheating, melting a connecting wire, bubbling so violently the electrolyte spits out of the vessel, it might happen in micro-seconds or minutes. Here the current is being limited by the bulk resistance, which we already know to be small. The overheating of components in series is most easily described by considering the current flowing. It is far more practical to talk about a maximum current, rather than a maximum voltage, for a device with low slope resistance.

So, in summary, for LEDs and electrochemical cells, and a few other things besides like zener diodes, it makes most sense to talk about a threshold voltage, an operating current, and a maximum current.

Does a resistor drop volts?

Well, yes, but having that concept doesn't help you to figure out what, why, where or how much. You might notice that the weight of a car compresses the road surface slightly. If that's the concept of a vehicle you use to choose your next one, you might end up with a humvee, a 40 ton truck, or a road roller, which would probably be a mistake. Similarly, when you choose a resistor, you really need to know whether it's one of a pair of resistors establishing a ratio (gain setting for an amplifier for instance), or defining a current at a specific voltage drop (current sourcing for a small battery charger or LED). The only application I can think of where the concept 'avoiding too many volts on the load' is remotely applicable, is when you use a mains filament bulb in series with a piece of mains equipment of unknown provenance when you power it up for the first time 'just in case'. In all other circumstances, 'avoiding too many volts' is just kindergarten electronics. I remember when I was about 10 or something, and had just read about resistors. I wanted to know what resistor would drop the voltage from my 12v Scaletrix transformer to the 9v needed for a tranny radio. I also remember, to my shame, becoming quite aggressive to an uncle who worked for an electronics company (in the HR department!) when he wouldn't tell me what value to use, but he couldn't explain my conceptual error either. You are correct if you've spotted there would be other problems as well! If your concept means that you ask an inappropriate question, it is not a useful concept. I guess I am still, well not haunted, but mindful of that experience to this day, and try to correct conceptual errors before they lead people off track.

Internal resistor?

OK, mea culpa, Having searched for the data sheet for L4302, I found only a brief description on Fry's giving a 12v figure for forward volts. I didn't find out whether it had an internal resistor. Can I suggest that all posters, if they mention a specific part, that they include a working link to a data sheet, or attach a pdf if it's small enough. It's impolite to post such a question without a quick way for the responders to find the data sheet. If that's too strong, then let's just say that it's polite to make the data sheet available easily to people who are going to take the trouble to help you out. What I say still stands for a bare LED. Note that a LED with an internal series resistor has a *high* slope resistance, and so can be voltage driven.

* I always used to use carbon rod electrodes recovered from zinc-carbon batteries for this job (cheaper than platinum, esp for a 10 year old). I have posted on another forum my method of removing these intact from spent cells. I won't go into full details here, but it involved a 12v car battery, croc-clips, and sand-bags in an open space. See my remarks above about threshold voltage, excess current and failure modes.

Ash Small

Wed Sept 23 2015, 09:22AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Dr. Slack wrote ...

Let's take the example (to be closer to Bored Chemist's field of ken) of an electrolytic cell, with dilute sulphuric acid and platinum electrodes*. This is actually very similar to a LED load, as there is a quantum mechanically defined voltage below which the intended behaviour doesn't happen. Below roughly 1.7v, you cannot split a water molecule. Below a specific voltage, depending on some horrible physics involving band-gaps, you won't excite carriers and produce photons. Below a few volts depending on the colour, a LED is a DED, a dark-emitting-device. Both of these devices may conduct some leakage current by other mechanisms below the threshhold voltage, but neither could be said to be 'working' in this state.

You are the first person I've heard admit that electrolytic cells have a 'threshold voltage' Neil.

I've tried to discuss this with 'people who should know better' on other forums, but been told unanimously that electrolytic cells follow 'Ohm's law'.

My own 'field of study' regards electro-polishing tanks, where, after the first threshhold voltage is reached, electro-etching commences, then, at a second threshold voltage (dependant on conditions like electrolyte composition, etc), electro-polishing commences.

I've been working on a hypothesis for well over a decade now. I should discuss it with you sometime.

(Sorry to go 'off topic', slightly)

EDIT: there is actually a third threshold, after which electro-chemical machining commences, but I've only achieved this state by accident.

Dr. Slack

Wed Sept 23 2015, 10:00AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

It depends on the chemistry. Two copper electrodes in copper sulphate has a zero threshold, and it follows ohm's law (to the extent that the solution temperature allows). The reactions at the two electrodes have equal and opposite voltages. If the algebraic sum of the two electrode reactions is not zero, for instance different electrode materials, or like platinum where they don't take part in the reaction, then there will be a threshold voltage.

krenshala

Wed Sept 23 2015, 12:14PM

Registered Member #143 Joined: Sat Feb 11 2006, 04:25PM
Location: Austin TX, NorAm, Sol III
Posts: 28

Ash Small wrote ...

A 'spare' battery or three might not go amiss here.

Yeah, I've got a couple extras and plan to use a 9v battery holder to make swapping then trivial.

--

Dr.Slack, I appreciate your effort to explain why the resistor is/may be needed, and will definitely look into things more. I am always looking to know 'why' something works the way it does, though in this case I only asked 'does it work' in my post.

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