voltage and current regulation

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unknown_entity
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voltage and current regulation

Post by unknown_entity »

OK... the big picture of what i want to do is regulate the voltage and current independantly to a LED. This is a special purpose application and i already know i have to regulate both voltage and current.<p>I already know how to work with linear fixed and adjustable voltage regulators like the LM317. So the voltage regulation is not a problem for me.<p>I have never worked with linear current regulators before. Im hoping that a lm317 or similar part can be used as an adjustable current reg. I have only seen one example of such a circuit in application notes on a spec sheet. It was not very helpful to me.<p>
So my question is Can an lm317 be used as an adjustable current reg? If not what part can be used in conjunction with a lm317 output wired as an adjustable voltage reg to adjustably limit the max current throught the voltage reg. <p>Any simple linear adj current reg circuits would really help me out here. Especially if they use the lm317. <p>Im sure the two regs will have to be in series.
EX.
(The lm317 V reg giving the regulated +v to the pos terminal and the current reg in series with the pos(+v)out. the load would be placed across the pos(+v) and gnd terminals)<p>
Thanks in advance,
James T.
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Chris Smith
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Re: voltage and current regulation

Post by Chris Smith »

I have a diagram to show you how its done if you like. <p>Sent you a personal message.
unknown_entity
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Re: voltage and current regulation

Post by unknown_entity »

chris, i sent you an email to the addy in your pm
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Chris Smith
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Re: voltage and current regulation

Post by Chris Smith »

It hasn’t come through yet, 10:42 Am 17th?<p>Oops, It just came in.<p>Let me know if they went out, [arrived] Hot mail shows no record of anything sent? I sent TWO.<p>[ October 17, 2005: Message edited by: Chris Smith ]</p>
unknown_entity
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Re: voltage and current regulation

Post by unknown_entity »

yea i got it... Thanks again Chris. It's pretty much the same as the circuit i saw before but it lookd odd to me at first because of the resistor in series with the output.<p>What im thinking does all the output current pass thru this resistor, if so i'll have to use a fixed one for a proper power rating.<p>
But i'll just play with it and see how it goes.
Enzo
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Re: voltage and current regulation

Post by Enzo »

If you are experimenting with LED brightness or something, think about it this way. The voltage across the LED will stay about the same. We put a resistor in series with the LED for the purpose of limiting current through it. We calculate the value of the resistance by using the target current through the LED and the difference between the power supply voltage and the LED voltage drop.<p>Different types of LED have their own inherent characteristic voltage drop, but I will use 1.2 volts for the examples. And LED types also have a maximum current rating. Plain old vanilla LEDs don't like to see more than about 20ma. SO let's use that as our target.<p>Since the LED will drop the 1.2v across it no matter what, I then calculate the resistor I need in series to handle the power supplpy voltage at the current desired. If I have 5 or 12 volts, it means a different resistor for each.<p>For a 5v supply, I subtract the 1.2v to get 3.8v. Since current is the same through a series circuit, the 20ma and 3.8v yields 190 ohms by using Ohm's Law. (3.8v/0.020A = 190 ohm) That is an odd value, so 180 ohms is close enough or go to 220 ohms.<p>If you were using a 12v supply, it becomes 12-1.2 or 10.8v. At 20ma, we need a 540 ohm resistor in series with the LED. (10.8v/0.020A = 540 ohm) 560 ohms is the closest standard value.<p>If I use a resistor to limit current, and you can limit it to whatever you want by the above calculations, I can add a variable resistor in series additionally to vary the current. In the 12v example I came up with a 540 ohm resistor to limit current to about 20ma. If I doubled the resistor to 1080 ohms, the LED current would drop to 10ma. So if I replaced the resistor with a 5k pot, I could vary the current through the LED from only 2ma (at the 5k setting) all the way up to whatewver the supply could put out with an LED dead short across it.<p>In practice I would put the pot in series with a resistor so as to limit how high I could allow the current to go. A pot can be turned to zero. In the 12v circuit, if I put a 360 ohm resistor in series with my pot, then if the pot is turned to zero, the resistor remains in the circuit to limit current to 30ma.<p>If you have a fixed resistor, you can also vary the voltage across the LED and resistor. That will also vary the current through the LED. For any given supply voltage, you can calculate teh LED current by using the resistor value. Or simply put a current meter in series with the LED.<p>To adjust the current through an LED then, you can either vary the supply voltage or the series resistor.<p>To control current through an LED, you don't need a sophisticated circuit. The LED will have a stable voltage drop across itself. It may vary a little as current goes up, but whatever level of current you set, it will remain pretty darn stable. So a simple variable series resistor will control LED current easily. The current will be as regulated as your power source. If the power supply is regulated, then so will be the current through the LED.<p>A regulated voltage power supply will prevent the AC mains voltage variation - and there is typically a lot - from getting into your LED circuit. Once that voltage source is regulated all you need is a resistor to regulate the LED current.
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haklesup
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Re: voltage and current regulation

Post by haklesup »

Exceuse me if I am mincing words but last time I checked Ohms law, you couldn't simultaniously regulate both voltage and current to whatever you want without changing the load. The two are always locked toghether by the ratio of the load impedance and are never independent.<p>You can regulate one (V or I) and limit the other but if the current limit kicks in, the voltage is no longer regulated (or vice versa). You can only regulate the operating point (V and I) by varying the load.<p>Again, excuse me for nit picking your verbage.
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Chris Smith
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Re: voltage and current regulation

Post by Chris Smith »

You can run a variable resistor at the out put for a variable current regulation. The way to set both the voltage and current together, is to “Two stage the system”. <p>If you voltage regulate into a large cap or battery [like] value, and then current regulate that output, you get both voltage and current regulation at a steady pace.<p>However, you cant vary the input / output voltage mid stream or on the fly, without the current being affected in some way. <p>Also there is another design that uses a transistor network in place of the simple resistor and it gives feed back to keep it even more steady. And then there are the op amp circuits that keep all the factors at a given. For a simple Current and voltage regulation together, this one works.<p>[ October 17, 2005: Message edited by: Chris Smith ]</p>
L. Daniel Rosa
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Re: voltage and current regulation

Post by L. Daniel Rosa »

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr> This is a special purpose application and i already know i have to regulate both voltage and current. <hr></blockquote><p>Really? Why's that? Most folks'll go with regulated voltage because there's a voltage regulator already in the circuit or on hand. A current regulator takes good care of an LED. Is there an application that requires the LED current to vary with temperature with an upper limit imposed by the current regulator?
unknown_entity
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Re: voltage and current regulation

Post by unknown_entity »

it's a lighting source for micro endoscopy.
Now we're just running a demo model.
I recently switched over from a high power xenon lamp to a special led for the lighting source.<p>So im just looking for a supply where i can tweak it to account for different led's or arrays. And the current limit is really just a safety feature because some of the leds im working with are some kind of high power thing... it's rated 150-200mA @ 3V and not cheap.
unknown_entity
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Re: voltage and current regulation

Post by unknown_entity »

To Chris & Hack...<p>I basicly building a scaled down benchtop supply. Because i'll be using V and I regs it run in either constant current or constant voltage mode depending on which reg is the limiting factor.
rshayes
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Re: voltage and current regulation

Post by rshayes »

You can actually operate the LED using only a current regulator. The voltage across the LED would be determined by the LED characteristics. With current regulation, you can also eliminate the resistor in series with the LED.<p>Adjustable three terminal regulators, such as the LM317, can be used as current regulators by placing a resistor between the regulator out and the adjustment terminal. The adjustment terminal is also connected to the load. The regulator tries to maintain the voltage across the resistor at 1.25 volts. A 12.5 ohm resistor would set the output to about 100 milliamps. The minimum current is about 10 milliamps for this arrangement, since the regulator needs that as a minimum load for proper operation. Different load currents can be set by using different resistors selected by a switch.
Will
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Re: voltage and current regulation

Post by Will »

Not disagreeing with anyone - simply trying to clarify by changing the terms i.e. For a given LED temperature then, once you fix/regulate the current to some specific value, you have irrevocably fixed the PD/drop/volts across the LED to some specific value. If you try to change it without changing the current then you will have a real problem.
BB
unknown_entity
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Re: voltage and current regulation

Post by unknown_entity »

thanks for the input everyone... i ended up just making and adjustable current reg using an LM7805. The whole point of having a voltage reg was just to make the whole thing more idiot proof once it is built.(since it would be used by people other that myself)
Robert Reed
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Re: voltage and current regulation

Post by Robert Reed »

Entity
I hope I'm not to late on this, but if all you really want is a constant current source that can be varied, the simplest form of this is an emitter follower transistor. Feed a voltage into the base and the transistor emitter will follow it (- 1/2 volt). Connect your device to the collector on one end and the supply on the other end. Emitter current is determined by Ohms law, i.e. ---1 volt across 20 Ohms = 50 ma. The collector current will be maintained at this level regardless of the load. Actually it will be slightly less because a very small portion of this will be diverted thru the B-E junction, but with most transistors having an HFE of 100 or so this is negligable. The only limiting factor here is the supply voltage and this would only show up with hi impedance loads
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