Replacing the Pot in a power supply with a differnt value/ohms law vs variable  - Page 1
Forum section: electronics / beginners

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 15, 2017, 07:58:49 am
content: So I have a little power supply that is great for little projects. itsSpecifications :QuoteI have been feeding it 12VDC from a computer power supply and it seems to work. Is this bad for it?What I like most is the variable voltage so I'm putting it in a case with a heat sink.Questions:1. I have the following parts to improve it:100k pot 50k pot and 10k pot (comes with 4.7k pot)Two 2.2k 10watt power resistors,10ohm 10 watt resistor, a whole bunch of 1/4 watt resistors. I want to ideally make a fine and coarse voltage adjustment or at least use one pot if thats not possible with these parts.2. Two 3300 uf eletrolytic 25V rating. Eight 220uf 50V rating, eight 100uf 35V. Can I parallel this with the output to give it more instantaneous current? It already has a 2200 uf 35v and a 1uf 50V on the board. Or would adding a capacitor make a trivial amount of difference?3. What does ohms law do with one fixed and one variable cap across the wiper or the ends of the pot?

author: danadak
author_rank: Super Contributor
timestamp: February 15, 2017, 12:29:21 pm
content: The transient response of the LM317 is on the order of 5 - 10 uS. A 100 uFcap, with its ESR, may result in an even longer transient response than theLM317. A combination of electrolytic and MLC caps might result in better results.You could spice this, but make sure your spice model is complete enough fortransient analysis. Same for caps used.The LM317 has a ref leg current of 100 uA. And if you want output to go to0 than its nominal 1.25V you will have to take ref leg below ground by thatamount.http://electronics.stackexchange.com/questions/244354/making-lm317-output-voltage-adjustable-down-to-0-vRegards, Dana.

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 15, 2017, 11:33:17 pm
content: It goes down to about 1.25 volts and thats fine with me. So the LM317 is not a type of mosfet? I need to stop assuming everything in that shape package is one.What does the transient response do? And why do I need it short?Spice like Pspice? Last time I heard of that it was a computer program my father had on the computer in the 80's. Think 5"1/4 inch floppies. Guessing this is some sort of emulation software.I think people have different levels of "beginner".Basically I want to replace the little tiny pot it came with, with some thing easy to adjust. And make the circuit more robulst if I can. I actually shorted it for almost 10 minutes by accident and it was saved by a huge Al heat sink I put on the LM. The heat sink was probably around 100'C or more; I noticed the smell of burning or more and it survived. Accidental shorts are invitable with my poor eye sight so I need something that can handle that.Also I took the LM off the board and attached ~1.5 inch 20ga solid copper wires so I could mount it on the heat sink. Are these wires going to cause loss or heat that will make a difference? I havent turned it on since I have done this.

author: danadak
author_rank: Super Contributor
timestamp: February 16, 2017, 12:17:10 am
content: It goes down to about 1.25 volts and thats fine with me. So the LM317 is not a type of mosfet? I need to stop assuming everything in that shape package is one.QuoteWhat does the transient response do? And why do I need it short?QuoteSpice like Pspice? Last time I heard of that it was a computer program my father had on the computer in the 80's. Think 5"1/4 inch floppies. Guessing this is some sort of emulation software.QuoteRegards, Dana.

author: james_s
author_rank: Super Contributor
timestamp: February 16, 2017, 01:28:03 am
content: Quote from: raspberrypi on February 15, 2017, 11:33:17 pmNo, it's not a MOSFET, and yes, stop making that assumption. There is absolutely no relation between the package style and what silicon it contains. That is called a TO-220 package and it is used for many types of transistors, including Darlington types, MOSFETs, thyristors like SCRs and Triacs, simple diodes, even power resistors and thermal switches. You cannot make assumptions about semiconductors just by looking at the package, look at the number on it and find the datasheet.

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 16, 2017, 01:50:41 am
content: Is there anyway to make this work with 10k pots and a combination of fixed resistors? I measured the original and it feeds 0-4.7k. Why does it give more voltage with more resistance?I found this video but still don't get the relationship of the ohms adjustment to voltage. Is it because its going through active to saturation? Is the energy being lost as heat when you are running it at less then full output voltage? Would it give off heat under no load? It doesn't seem like it.https://youtu.be/I1h-HZmIJ4E

author: james_s
author_rank: Super Contributor
timestamp: February 16, 2017, 02:30:36 am
content: Why are you asking about mosfets? The LM317 is not a mosfet, it does not contain a mosfet, it does not act anything like a mosfet.The LM317 maintains a voltage difference of 1.2V between the output and set pin. The two resistors used to set the voltage, one fixed and one usually adjustable form a voltage divider with one end referenced to ground and the other connected to the output. The higher the resistance of the one between set and output the higher the output voltage.

author: IanB
author_rank: Super Contributor
timestamp: February 16, 2017, 03:09:15 am
content: Quote from: raspberrypi on February 16, 2017, 01:50:41 amIf you are feeding the supply with 12 V DC, then the maximum regulated output voltage will be about 9 V. Given that, you could replace the 4.7 k pot with a 1 k pot. A smaller value pot like that will give finer control over the adjustment. For even better control, try to find a nice big knob to stick on the pot. There's nothing worse than trying to adjust a pot by turning the spindle directly with your fingers.

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 16, 2017, 04:58:17 am
content: Quote from: IanB on February 16, 2017, 03:09:15 amSmallest I have is a 10k pot and a bunch of fixed resistors. Is there anyway to make the 10k pot work?

author: james_s
author_rank: Super Contributor
timestamp: February 16, 2017, 05:19:36 am
content: A 10k pot should work fine, just size the fixed resistor accordingly, the equation to calculate it is in the datasheet.Vo = Vref(1 + R2 / R1) Vo is the desired output voltage, Vref is 1.2, R1 is the fixed resistor and R2 is your 10k pot.

author: Nerull
author_rank: Frequent Contributor
timestamp: February 16, 2017, 05:51:28 am
content: Quote from: raspberrypi on February 16, 2017, 01:50:41 amThe LM317 is not a single component, it is an integrated circuit. You can't think of it like a dumb part, because it isn't one - it is an active voltage regulator. It adjusts it's output voltage to maintain a voltage of 1.25V on it's ADJ pin. The resistors form a voltage divider, and the adjustable pot allows you to change the output by changing the voltage divider so that the desired output voltage results in a voltage of 1.25V to the adjust pin.When you are outputting low voltages, the difference between the input and output is created via a voltage drop. It will dissipate power, just like a resistor dropping the same voltage at the same current. P=VI, so heat increases with both increased current and increased voltage drop.

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 16, 2017, 04:59:46 pm
content: Quote from: james_s on February 16, 2017, 05:19:36 amSo what am I looking for in the data sheet? Will a 1/4 watt resistor work? The desired output voltage is 1.25-9 or 12 volts or so.

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 16, 2017, 06:12:04 pm
content: This is whats in the lm 317:So its not just a simple transistor.Why does it have what looks like variable resistors in it? How are they adjusted? By temperature?

author: james_s
author_rank: Super Contributor
timestamp: February 16, 2017, 06:44:20 pm
content: The variable resistor in it is a thermistor, temperature sensitive resistor used for the thermal protection.I gave you the equation from the datasheet, just plug in the numbers and refer to the schematic to see where the two resistors are located. The resistor values are not particularly critical, you won't hurt anything by experimenting. 1/4W is plenty, the resistors should not dissipate a significant amount of power, they're only a voltage divider feeding the sense pin.

author: SkyMaster
author_rank: Frequent Contributor
timestamp: February 16, 2017, 11:49:43 pm
content: Quote from: raspberrypi on February 15, 2017, 07:58:49 amQuote from: raspberrypi on February 15, 2017, 11:33:17 pmraspberrypi,Powering your power supply from 12 VDC coming from a computer power supply is not a problem at all. The schematic you shown also show that 12 VDC is acceptable.Download the TI LM317 datasheet from herewww.ti.com/lit/ds/symlink/lm317.pdfOn the TI LM317 datasheet, the circuit on page 10, section 8.2, figure 9, is very similar to the circuit you have.The formula james_s made reference to is shown at section 8.2.2.I understand that you want a fine and coarse adjustment. This is done using two potentimeters in series. You could add a 470 ohm to the 4.7k potentiometer you already have the 470 ohm pot would be the fine adjustment. You want Linear potentiometer, not logarithmic.In theory, you could replace R1 with 1.2k and then add a 50k potentiometer in series with 4.7k potentiometer, and then the 4.7k pot would be the fine adjustment. I say in theory because all the examples in the datasheet are using a fairly low value for R1.C3 on your circuit, is the same a Cadj on figure 9 on the TI datasheet. The datasheet reads "Cadj is recommended to improve ripple rejection. It prevents amplification of the ripple as the output voltage is adjusted higher."IanB is right, if your kit didn't came with a knob for the 4.7k potentiometer, you should get one as this will help you adjust the potentiometer.Mounting the LM317 remotely from the pcb using wires should not cause any problem.Be careful when you desolder components from a pcb, too much heat can lift a pad.

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 17, 2017, 03:32:43 am
content: Quote from: SkyMaster on February 16, 2017, 11:49:43 pmThanks most helpful so far. The pot it come with is one of those tiny screw driver adjustable ones. I want to replace it with one that fits a knob. Maybe I'll put in the 50k and then mount the tiny 4.7k on the outside of the case I'm building so I would still have the option if I really needed it. When its in series its wiper to wiper outside pins to out side pins? The original circuit board had one outside and wiper pin connected together.The power supply is a vellman kit that is only 1"X2" pcb with a tiny little adjustment pot. So it was a pain to hook up and adjust. I'm also going to add two LEDs on on the input side with a resistor and an extra diode for AC in (I just found a good 15VAC 1100mA wall linear wall power supply). Sort of a half wave rectified light circuit, Then add a LED on the output side that will change in brightness or go out when I turn it all the way down.

author: SkyMaster
author_rank: Frequent Contributor
timestamp: February 17, 2017, 04:59:10 am
content: Quote from: raspberrypi on February 17, 2017, 03:32:43 amYour screwdriver adjustable 4.7k pot is probably a pcb mounted trimmer potentiometer. Which is not very useful for what you want to do. I suggest that you replace this 4.7k trimmer with your 10k potentiometer. But you will then probably realise that only half of the range of the potentiometer is being used, you would need to increase the value of R1; probably doubling its value.Shorting the unused potentiometer terminal to the wiper is a common practice, but you do not need to do it. If you connect two potentimeter is series, it should be from the wiper of the first, to the outside terminal of the second potentiometer. You want them in series, not in parallel.The circuit you have shown on your first post already has a diode bridge (D1 to D4), you can feed AC there; you should not need an additional diode.If you try to use a LED as some kind of variable intensity indicator, you will probably realise that it will not work very well. LED brightness can only be varied on a very narrow range of voltage. In order to get a LED brighness to vary from dim to fully bright, you need PWM (Pulse Width Modulation). It would be simpler to just buy a $7 Voltage Display Module from ebay/amazon/aliexpress.

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 17, 2017, 07:10:38 am
content: So that does work using a 50k pot and a 4.7k pot for fine tune, and a 1.2k fixed R1 but the problem is the range acts strange, sort of.With the 4.7k pot turned in one direction you get the full range of 1.2V to 22V but turned the other way the range is 6v- 22v. But the fine tuning doesn't always give you ~5v of adjustment. If this was a commercial product you wouldn't do this to make a coarse/fine knob because you have to adjust the fine to get below 6 volts.I guess I'm trying to understand why the LM part acts like this in the first way. How do fine/coarse knobs work on a commercial power supply? I'm trying to learn what I'm doing before just guessing random parts to play with; the circuit board is becoming a mess as I attach more and more wires to it.

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 18, 2017, 06:37:52 am
content: So now that this thing is all apart I thought of this:IT has a 2200uf cap (35v) in it. What if I upgrade that to a 3300uf cap (25v)? The supply only goes to 20 volts and im feeding it 15VAC. What does this big cap do? Is it just smoothing or would it provide more current? And would it be of use to put another 3300 on the output in parallel? I'm still trying to figure out what the LM317 does, but I went from thinking it was a mosfet and not knowing that you could fit all those parts in a non "IC dip" package two days ago to now modifying it. Thanks!

author: IanB
author_rank: Super Contributor
timestamp: February 18, 2017, 06:47:07 am
content: Quote from: raspberrypi on February 18, 2017, 06:37:52 amWhy do you think this might be an upgrade? You have a device which was designed with appropriate components and values to work as it should. If you go in and start changing things around without understanding what they do, you could just as easily make it work worse than before. There is a really good rule in engineering: "If it ain't broke, don't fix it!"QuoteWhat you have is a regulated power supply: it takes a varying input voltage and produces a regulated output voltage. The LM317 is what does the regulation.As to the capacitors it is to be assumed they are designed of the right size for the job. Making something too big is potentially just as bad as making it too small. You ask if it would be of use to put another capacitor on the output. But it would only be of use if the power supply is not useful now, don't you think? What problem does it have that you would be trying to fix? If it has no problems, why not use it as it is and not try to mess with it?

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 18, 2017, 07:00:35 am
content: Quote from: IanB on February 18, 2017, 06:47:07 amMy point of asking is so I know what it does so I can see if changing it will do something better. This wasn't designed for any particular application. Its being made into a experimental power adj. supply. I have changed about half the parts in it so far. Its purpose is to learn electronics, if I don't start taking things apart and just leave everything the way it is I'm not going to learn anything new and I might as well just devote my time to learning how to stick lego blocks together.I mess with everything always have. I took apart our telephone when I was four so I could see how it worked. I have been taking things apart and modifying them ever since. I'll stop the day after I die. There is always room for improvement If someone can make something I can make it better, if not at least I tried and learned n the process.

author: IanB
author_rank: Super Contributor
timestamp: February 18, 2017, 07:20:54 am
content: Quote from: raspberrypi on February 18, 2017, 07:00:35 amQuoteBut you are not learning. You have been advised what the LM317 does and pointed to the data sheet for it. The data sheet contains application examples, shows typical circuits and recommends appropriate values for resistors and capacitors in the circuit.If you are not going to read the data sheet before trying to swap out components you are not going to make any progress in your learning.

author: james_s
author_rank: Super Contributor
timestamp: February 18, 2017, 07:38:54 am
content: Quote from: raspberrypi on February 18, 2017, 07:00:35 amSure but unless you know what you're doing, you aren't really learning much and you're not going to make it better. If you want to improve an electronic circuit, you have to at least have a clue how it works in the first place. The chance of improving upon the existing design through random changes is negligible. A power supply is an excellent candidate for learning though, it's very simple, everything is well documented and straightforward, it's well within the realm of a beginner to understand how it works, but you have to try to understand, don't just guess. Read the datasheet, look up rectification so you understand how the rectifier turns AC into pulsating DC and the capacitor turns pulsating DC into smooth DC. Learn how a regulator works, don't try to understand the whole internal circuit of the LM317 at first but try a simple transistor regulator with a zener reference. That Falstad simulator I posted a link to has many excellent example circuits you can play with. Understand the underlying theory, or at least try, don't just guess, that's not doing yourself any favors.

author: SkyMaster
author_rank: Frequent Contributor
timestamp: February 18, 2017, 03:56:37 pm
content: Quote from: raspberrypi on February 17, 2017, 07:10:38 amFrom your explanation, your Coarse and Fine pots are not acting strangely.You have to leave the Fine pot at the mid-position, and adjust the voltage with the Coarse potentiometer. Then you use the Fine potentiometer for the final adjustment.I have a GW Instek GPS-3030DD on my bench, it has a Coarse and Fine adjustments. The Fine adjustment has a range of 3.0 V. With the Fine at mid-position, the Coarse potentiometer will bring the voltage down to 1.5 V; in order to lower the voltage to 0 V, the Fine potentiometer has to be turned from its mid-position to its minimum level.In your case, if you are changing the values of R1 and the potentiometer(s), you have to respect the formula from the LM317 datasheet, else you may (will) get unpredictable results.

author: Audioguru
author_rank: Super Contributor
timestamp: February 18, 2017, 04:25:53 pm
content: When you have course and fine pots then one pot is always in the wrong position.The LM317 is designed to use only 120 ohms for R1. The datasheet shows that the more expensive LM117 can use 240 ohms. If the value of R1 is increased then the datasheet says that the output voltage will rise when the load current is low so then itdoes not regulate the voltage.

author: SkyMaster
author_rank: Frequent Contributor
timestamp: February 18, 2017, 05:00:47 pm
content: Quote from: Audioguru on February 18, 2017, 04:25:53 pmThe Texas Instruments LM317 datasheet (Revised September 2016) makes not mention of using only 120 ohms for R1. In fact, most examples use 240 ohms for R1, one example use 1.2 k.www.ti.com/lit/ds/symlink/lm317.pdfWhat am I missing?

author: IanB
author_rank: Super Contributor
timestamp: February 18, 2017, 05:34:44 pm
content: Quote from: SkyMaster on February 18, 2017, 05:00:47 pmThe datasheet says the LM317 requires a minimum load current of about 10 mA for correct regulation. Setting R1 = 120 ohms provides a load current of 10 mA to satisfy this requirement.It is OK to use a larger value like 1.2 k for R1 provided that the actual load is drawing 10 mA or more.

author: SkyMaster
author_rank: Frequent Contributor
timestamp: February 18, 2017, 05:46:32 pm
content: Quote from: IanB on February 18, 2017, 05:34:44 pmParameter labeled "Minimum load current to maintain regulation"; thank you for pointing that out.

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 23, 2017, 02:26:32 pm
content: Quote from: Audioguru on February 18, 2017, 04:25:53 pmI hooked up a 1200 ohm fixed into a 50k var and the 4.7k var and seems to work with a load and without.