Idiot's Guide to Regulation Circuits

[Joe Talmadge]

Idiot\'s Guide to Regulation Circuits

Hi folks. I'm almost embarassed to admit I've been hanging around all this time, and still am not sure I understand the implications of the various types of circuits -- constant current, voltage regulation, direct drive -- and would like an Idiot's guide. Not just what the circuit does, but what the implications are.

Constant current, I gather, tries to keep current constant even as voltage of the power source drop. I assume at some point voltage drops below some critical level and at that point the batteries are dead for that light. My guess based on this is that constant current lights keep the steadiest light output, until such time as the voltage hits the critical point, when it poops out.

Constant voltage I imagine tries to keep the voltage constant, even as the battery runs out of electrons. I assume based on this the light output is not as flat as constant current.. you've got the constant voltage to push the electrons around, but current diminishes eventually and the light dims.

One question from the above: Current, voltage, and resistance are directly related according to a mathematical formula (holy crap, I have a BSEE and I can't remember it now ... V=IR, right?). So some of what I said above doesn't make sense ... if resistance stays constant, and the voltage regulation circuit keeps either V constantt, doesn't the I have to stay current toot? Likewise, if the resistance is constant and we have a constant current regulation cirtcuit, doesn't V have to stay constant? Maybe the circuit works by somehow varying R or otherwise complicating the picture?

Step-up circuit: This sounds like taking the voltage from a particular battery source, and somehow raising it (at the cost of current capacity?) to meet a particular light source's Vf or whatever. Or maybe it's just another name for voltage regulation?

Direct drive: Is there even a regulation circuit involved here? Sounds like just driving the lighting element directly off the power source, with no intermediate tricks (e.g., R'ing away some power).

 

[Double_A]

Re: Idiot\'s Guide to Regulation Circuits

Joe-

I'm no EE, but in a situation where you have a LED got into thermal runaway, doesn't the resistance change?


GregR

 

[dat2zip]

Re: Idiot\'s Guide to Regulation Circuits

Constant current real advantage is that the LED Vf varies from device to device. So, one 1W Luxeon might be 3.0V and draw 350mA while anohter might be 3.9V @ 350mA.

Driving constant voltage with these two Luxeons and say we had a constant voltage of 3.0V would drive the one just perfect at 350mA. while the second one would be underdriven significantly. LEDs do not model like a resistor and more like a diode with varying I/V curves.

So, a constant current driver of 350mA would drive both the above diodes and everything in between with a constant 350mA which is what the LED really wants. Some amount of current, not voltage.

A step up circuit DC to DC converts from one DC voltage (Battery) up to a higher voltage (LED). While this is a generic term the name is more often refered to as a boost circuit.

There is also the Buck which steps down from one voltage to a lower voltage and a buck/boost which will operate in both domains.

Similar to constant voltage and constant current there is also constant power which is similar to constant current but different in some subtle ways.

The power equation I think is more applicable in terms of converters and that is Power = Voltage * Current.

A simply way to look at voltage, current and LEDs are as follows.

Take a dam full of water. Consider this capacity a battery.

The pressure the water exerts on the dam wall is voltage. The water is the electrons or current available. The bigger the dam the bigger the battery.

Letting water out of the dam is current flow.

Assume you connect different rivers to the dam. Each river depth and height is different (ie similar to LEDs). They all require the same amount of water flowing to keep the little fish happy.

Since the shape of the rivers are different the water flowing out of the dam requires different pressures to maintain the same water flow. A river that is tall and narrow requires more pressure to push the water downstream than a wide and flat river.

At some point in time the dam will start running out of water. At the point where the dams water pressure is less than the required pressure to maintain flow rate to the river the water volume will start to drop until there is just a trickle of water coming out of the dam.

 

[jeff1500]

Re: Idiot\'s Guide to Regulation Circuits

I'll try as long as you know that what I say might be wrong.

LEDs are diodes, current flows in one direction as soon as voltage gets high enough to jump the gap.

V=IR, but LEDs don't have any resistance, but batteries do have some R so round and round the current goes.

When an LED warms up it takes less voltage to make current jump the gap.

A purist will put a resistor in series with an led just to give the circuit some resistance, but they seem to work okay without a series resistor.

Two batteries are very convenient, but LEDs want more voltage than 2 x 1.5v or 2 x 1.2v.

There are lots of voltage step up chips out there that will draw as much current as needed from the battery to hold Vout constant. They have coils in them (inductors). They work like ignition coils on a car that step up 12 volts to what is needed to jump spark plug gaps.

If you give an led a constant voltage that's not too high, it'll work just fine. There won't be any current run away.

Constant current integrated circuits put a small value resistor after the led and then provide a constant voltage to the inlet side of that small (sense) resistor. Constant voltage to the sense resistor makes constant current through the sense resistor. Current is constant around the circuit so the LED gets constant current.

Transistor circuits are magic. I forgot how they work. They switch at a frequency that causes the coil voltage to rise until it jumps across the led. The output current stays constant as long as the battery voltage stays constant, but the battery voltage is always changing.

Nimh batteries have a pretty stable output voltage, more so than alkalines.

Something like that.

 

[LEDependent]

Re: Idiot\'s Guide to Regulation Circuits

[ QUOTE ]
if resistance stays constant, and the voltage regulation circuit keeps either V constantt, doesn't the I have to stay current toot? Likewise, if the resistance is constant and we have a constant current regulation cirtcuit, doesn't V have to stay constant?

[/ QUOTE ]As the voltage decreases in the batteries, the circuit uses more amperage to drive up the voltage. The resistance, calculated from the voltage output of the regulator, remains the same. A car battery, with lots of amps to spare, would need its current limited by this resistor. Smaller dry-cell batteries might not, but it's still good for posterity's sake.

 

[Joe Talmadge]

Re: Idiot\'s Guide to Regulation Circuits

Thanks to all.

LEDependent, so as the battery's voltage drops, the current drops as well. A constant-current circuit would somehow suck more current out (the method jeff1500 desscribed is very clever), which forces the battery's voltage to rise again (using V=IR) until such time as the battery just can't keep up w/ the current anymore. A voltage regulating circuit does the reverse: forces the voltage to stay high so the current stays high.

So constant current and constant voltage circuits theoretically accomlish the same thing, the real-world advantage being that lighting elements have some variance in Vf so constant current is the winner (paraphrasing dat2zip). Now I see why most people gave an approving mumble when Arc went from voltage to current regulation.

Oh wait, direct drive! Is there a circuit involved, or does it just mean hooking the lighting element to the power source with no intervening electronics and letting things work themselves out as they will?

Thanks again!

 

[IlluminatingBikr]

Re: Idiot\'s Guide to Regulation Circuits

[ QUOTE ]
Joe Talmadge said:
Oh wait, direct drive! Is there a circuit involved, or does it just mean hooking the lighting element to the power source with no intervening electronics and letting things work themselves out as they will?

[/ QUOTE ]

It means exactly what you said, and it couldn't have been said any better; "hooking the lighting element to the power source with no intervening electronics and letting things work themselves out as they will."

 

[INRETECH]

Re: Idiot\'s Guide to Regulation Circuits

The nicest and easiest circuit is to use a LM317 in constant current mode

Connect up the input to your voltage source, connect up the output to a Resistor - its value is 1.25/Desired current

Connect up the sense to the LED

As long as the input voltage is greater than the LED+2v, the circuit will work and you don't have to worry about the voltage going up too high

Remember: "A current meter is your friend", LEDs are current devices, not voltage devices

 

[Doug S]

Re: Idiot\'s Guide to Regulation Circuits

While the LM317 used in a constant current configuration is useful for some applications and is certainly easy and cheap, contrary to what Mike says above, it will require the power source to be at least 3V above the required LED voltage. This large voltage "headroom" requirement is very unsatisfactory for most portable battery powered applications. There are some 2 transistor/2 resistor solutions around that only require 0.6V headroom [0.4V if you make one of the transistors a Mosfet], and there are some 5 component solutions based on an LM334 that only require 0.1V headroom.

 

[Burnt_Retinas]

Re: Idiot\'s Guide to Regulation Circuits

Please listen to Inretech's words of wisdom: "LEDs are current devices, not voltage devices".

Chris

 

[danno]

Re: Idiot\'s Guide to Regulation Circuits

Don't try to use Ohm's law and apply it to diodes - Ohm's law works for resistors, not diodes. (As Dat2zip notes in his post, LEDs don't model like resistors). In diodes, the relationship between voltage and current is more complex than the linear relationship implied in Ohm's law. (I guess another way to look at it is to say that diode's resistance varies with current and voltage, but that's just another way of saying that Ohm's law isn't very useful with diodes).

 

[Canuke]

Re: Idiot\'s Guide to Regulation Circuits

Ohm's Law "works" for all devices, it's just that diodes have a nonlinear response over delta V (voltage change). With linear devices, if you double V, you double I for a gven value of R. Diodes, however, don't work this way... if you double V, you might get a gigantic increase in I (if you've exceeded Vf) or no increase (if you haven't).

So, Ohm's law is not being violated; the relationship between V, I and R at any given point remain valid. It's delta I versus delta V that goes haywire, *as if* the diode's R value drops from the mega-ohm to milliohms as V increases... just as the total water flow over the dam increases in a non-linear fashion as the water rises.

So while Ohm's Law still applies, the nonlinearity just complicates the math like crazy.

My nitpicky technical $.02

 

[Al]

Re: Idiot\'s Guide to Regulation Circuits

Just posted this at a new topic ... should have put it here

"Took another look at Roy's runtime graphs on Quickbeam's site: Opalec, Eternalight, Bad Boy, Palight show straight across the board 'till battery depletion ... Best type of regulation?"

 

[Bullzeyebill]

Re: Idiot\'s Guide to Regulation Circuits

Look at the MM and BB. The MM is efficient at 0.8 volts or so. The BB looses effeciency below 1.2 volts or so. Which is better, if using two batteries? Run time is good with the MM, and not so good with BB, assuming same value 1 watt leds? Sort of confusing. I know that answer for 1 watt driven off of one battery is the MM. Why is the BB touted so much over the MM?

Bill

 

[UK Owl]

Re: Idiot\'s Guide to Regulation Circuits

To try and simplify things a bit...

An led needs a certain voltage to allow current to flow.

e.g. at 3 volts a white LED may cause 2 or 3 milli-amps to flow, will probably pull 15 milli-amps at 3.3 volts, but at 3.4 volts it to act as a diode and allow the maximum current available form the source. This is the danger of direct drive.

If your battery has 6 volts, but you require 3.4 volts for the current to flow and have a spare 2.6 volts.

You use a resistor to drop these 2.6 volts, using ohms law to calculate the resistor value for the current you wish the LED to take.

e.g. if you want 30 milli-amps i=0.03

remember v = 2.6 volts

so the resistor you need is r = v / i = 2.6 / 0.03
= 86.667 ohms

the nearest prefered value to this is 82 ohms (I think)
this will give 31.7 milli-amps.

note: this ignores the fact that the foreward voltage will alter slightly with temperature of the LED, thus the 'thermal-runaway' you will have heared of.

as your battery runs down the voltage will drop, say it drops to 5 volts....

the current through the LED will now be i = v / r
i = 1.6 / 82 = 19.5 milli-amps.

This is two third's of the current seen at a full 6 volts on the battery.

suppose the battery now falls to 4.6 volts
the LED current will be 14.6 milli-amps, this is half the original current.

What a regulation circuit will do, is the same as adjusting this resistance to compensate for a drop in supply voltage.

I have made simple regulator circuits that require only 1.2 volts more than the LED foreward voltage of 3.4 volts, so the circuit will maintain a constant current until the battery voltage falls to 1.2 + 3.4 = 4.6 volts.

A slightly less linear circuit can be made requiring only 0.6 volts more than the LED foreward voltage.

I have read on this forum about a 'magic resistor' circuit that has been built using FET transistors that performs even better.

 

[Phaserburn]

Re: Idiot\'s Guide to Regulation Circuits

UK Owl,

Thanks for that explanation. As many times as I've read about this topic here on CPF, yours is the clearest I've seen. I actually think I'm beginning to understand (and that ain't easy...!)

I am in line for one of those Magic Resistors. I understand what it's supposed to do, but don't truly understand how much better it is supposed to do it. Any additional insight on that?

 

[Doug Owen]

Re: Idiot\'s Guide to Regulation Circuits

[ QUOTE ]
Phaserburn said:


I am in line for one of those Magic Resistors. I understand what it's supposed to do, but don't truly understand how much better it is supposed to do it. Any additional insight on that?

[/ QUOTE ]

May I modestly suggest something I wrote on the subject some months back? It has to do with 'smart resistors' if you will, FWIW have a slight problem with 'magic' resistor since at least to me magic is something else entirely........

'Smart Resistor'

Doug Owen

 

[Phaserburn]

Re: Idiot\'s Guide to Regulation Circuits

Interesting. So then, what is the difference between "current regulation" (ala Arc LS/SF L4) and the Magic Resistor?

 

[Doug Owen]

Re: Idiot\'s Guide to Regulation Circuits

[ QUOTE ]
Phaserburn said:
Interesting. So then, what is the difference between "current regulation" (ala Arc LS/SF L4) and the Magic Resistor?

[/ QUOTE ]

Both the Arc and series regulators are regulators, that is the 'look' at the actual output and adjust something to try to make that match a pre set value. Both are looking at current (as I understand the *current* version of the Arc, the first was a Voltage Regulator?), and are therefore current regulators I guess, but for sure both are using current regulation as their main operating mode.

The major difference is how. The Arc *raises* the voltage, series regulators, OTOH lower it. There is also a class of 'switchers' so called 'buck', but they're not in the discussion. One has to basically make some AC from the DC in order to get the higher voltage needed. Typically, this is done by 'charging' an inductor (running current through it to build a magnetic field) and 'dumping' it (shutting down the current so the collapsing field will generate the necessary voltage), with attendant losses.

Series regulators, OTOH, use a variable conduction element (typically a transistor) to pass more or less current as needed. You really can visualize it as a correct value resistor in series with the LED, with the attendant losses here as well. Think of it as a self optimizing resistor, as things change it automatically adjusts itself to maintain the pre set current. Consider a sprinkler on a hose. The pressure (and therefore flow) to the sprinkler is controlled by the valve, which you set to exactly water the lawn. If the pressure changes, you need to adjust the valve. This is an automatically adjusting valve. Like the water system, series regulators will not run at proper levels with the base pressure (Voltage) gets too low. FWIW this can be a shortcoming or a feature.....

Doug Owen

 

[Phaserburn]

Re: Idiot\'s Guide to Regulation Circuits

Aaah. So, the current regulation of the L4/LS type doesn't change as the "pressure" from the battery changes. It must be preset to the best middle setting possible to accomodate. The Magic Resistor does change, and would therefore be superior at maintaining exact current to a LED. Is the MR also more efficient, with less loss? Why would this be a shortcoming instead of a feature? As it relates to runtime?