Psu Without Knob - Page 1
Forum section: electronics / beginners

author: theironlefty
author_rank: Contributor
timestamp: January 05, 2014, 02:23:25 pm
content: Here is my psu i design it it much uses Voltage Regulators Wich gonna get heated very quickly also includes DisplayCurrently Output is Low than it says Display takes all power i am gonna fix it later

author: mariush
author_rank: Super Contributor
timestamp: January 05, 2014, 02:34:25 pm
content: Why would you do something as silly as this? Didn't you hear about ADJUSTABLE regulators like LM317?If you insist on using FIXED voltage regulators, and you're only going to use one of those at a time at high current or all of those but at low currents, chain them so that the input of one comes from the output of the other, like 7812 - 7815-7812-7809 .. etcThis way each regulator is only going to dissipate the difference between input voltage and output voltage or about 3v times the current, instead of each regulator dissipating (20v+ - output voltage) x current ...saying 20v because you need 20v or more to get 18v from the 7818 regulator.

author: theironlefty
author_rank: Contributor
timestamp: January 05, 2014, 02:43:13 pm
content: Quote from: mariush on January 05, 2014, 02:34:25 pmDude i am 11 child i don't know entire electricity+i use circuit wizard theres no adjustable voltage regulators

author: theironlefty
author_rank: Contributor
timestamp: January 05, 2014, 02:55:49 pm
content: update

author: BravoV
author_rank: Super Contributor
timestamp: January 05, 2014, 02:57:45 pm
content: Even you're 11 years old, just simple question that I think you understand, what is the purpose of this thread ? Asking for feedback ? brag on your design ? or what ?

author: theironlefty
author_rank: Contributor
timestamp: January 05, 2014, 03:03:04 pm
content: Quote from: BravoV on January 05, 2014, 02:57:45 pmi am asking because of Some help more ideas to add

author: BravoV
author_rank: Super Contributor
timestamp: January 05, 2014, 03:08:31 pm
content: I did you a favor, reported this thread to mods to move it into Beginner section, before the mob eats you alive there.

author: c4757p
author_rank: Super Contributor
timestamp: January 05, 2014, 03:11:30 pm
content: OK, then. Is there something wrong with it that needs to be fixed? Like this:Quote from: theironlefty on January 05, 2014, 02:23:25 pmIs it overheating? You'll want to use heatsinks - the regulators should be bolted onto metal blocks to help move heat off of them. Remember that the regulators' tabs are conductive, so if you bolt them to thesameblock you will need something insulating to go between them. Something likethis- I don't know where you get your parts, though, so I don't know where you can find some.Also, I just noticed that you chained them like mariush said. It's a good idea, but you have to remember something: each regulator will only work right if the input is at least about 2V higher than the output. This means you should not chain the 7805 after the 7806, because that only gives it one volt.Quote from: BravoV on January 05, 2014, 03:08:31 pm

author: theironlefty
author_rank: Contributor
timestamp: January 05, 2014, 03:15:50 pm
content: well like i dont know how to make 7 segment display work to add into this psu

author: Jon86
author_rank: Frequent Contributor
timestamp: January 05, 2014, 03:16:39 pm
content: What software are you using for this?

author: theironlefty
author_rank: Contributor
timestamp: January 05, 2014, 03:18:58 pm
content: Circuit wizard

author: Jon86
author_rank: Frequent Contributor
timestamp: January 05, 2014, 03:19:23 pm
content: Quote from: theironlefty on January 05, 2014, 03:15:50 pmIf you don't know, then I doubt you're going to be able to. If you're planning to build this up, use a voltmeter module off ebay.

author: c4757p
author_rank: Super Contributor
timestamp: January 05, 2014, 03:20:17 pm
content: Quote from: theironlefty on January 05, 2014, 03:15:50 pmJust a plain old 7-segment display, that looks like this:is going to be quite difficult for you. These don't measure anything, they're just LEDs in a box. You will need a controller that can measure voltage or current and then turn on the right LEDs.However, you can buy these:Digital panel meterThey have all that integrated. You'll have to find one that measures what you want to display. What do you want to display? The current?

author: mariush
author_rank: Super Contributor
timestamp: January 05, 2014, 03:21:43 pm
content: Ok, since you're a child I'm going to try to explain some things to you.In order to give you a stable output voltage, these voltage regulators need at least 2v above the output voltage. If you have a 7818, which outputs 18v, you need 20v at input to make it work.These voltage regulators 78xx manage to output a stable voltage by dissipating the excess as heat. If the input voltage is 10 v and the output voltage is 5v (when you use a 7805) and you have a lightbulb or something on the output using 200 mA (0.2A), then the regulator will produce heat in the amount of (10v - 5v ) x 0.2 = 1 watt.The regulator by itself is only capable of dissipating about 0.5-1 watts. Anything more, it must be put on a heatsink.So if all regulators in your design use 20v as input voltage like you had in the first picture, the smaller regulator 7805 would dissipate 1 watt of heat with less than 100 mA of current (basically a few leds), because you would have (20v input - 5v output ) x 0.1 = 15 x 0.1 = 1.5 wattsNow if you connect them like in the second picture, the difference between input and output voltage is about 3v everywhere except with 7805 and 7806 and 7808 and 7809: you can't power the 7805 with the output from 7806 because there's only 1v difference between them.: you can't power the 7808 with the output from 7809 because there's only 1v difference between themIf you really want to go ahead with this design, rearrange to be like this :7812 -> 78097812 -> 78087809 -> 78067808-> 7805So let's take the case where you want 0.1A from 5v regulator (a few leds) ... the chain would be 7805 <- 7808 <- 7812 <- 7815 <-78187805 takes power from 7808, so it outputs 5v 0.1A and dissipates (8v-5v) x 0.1A = 0.3w7808 needs to produce 0.1A for 7805, so it outputs 8v 0.1A and dissipates (12v-8v) x 0.1A = 0.4w7812 needs to produce 0.1A for 7808, so it outputs 12v 0.1A and dissipates (15v - 12v) x 0.1A = 0.3w7815 needs to produce 0.1A for 7812, so it outputs 15v 0.1A and dissipates (18v - 15v) x 0.1A = 0.3w7818 needs to produce 0.1A for 7815 so it outputs 18v 0.1A and dissipates (20v - 18v) x 0.1A = 0.2wSo all these regulators 7818, 7818, 7815, 7808, 7805 produce 0.1A out of the maximum of 1-1.5A they can produce, and to get 5v 0.1A or 0.5 watts, you're dissipating 1.5 watts of energy. Your power supply is 25 % efficient.But compare it with your previous design. 7805 doing 5v 0.1A from 20v input, would dissipate (20v - 5v ) x 0.1 = 15 x 0.1 = 1.5 watts. Same energy wasted as heat but all energy is now concentrated on a very tiny chip instead of being spread on 5 chips.If you however needed to 0.3A or 0.5A , for a fan or something for example, you'd produce several watts of heat on a tiny cheap and burn it.The smart way to do such a power supply is to use a transformer with two taps, one which would output about 12v DC , one which would output 20-24v DC. Connect the 7805 up to 7809 to first winding of transformer (after it's rectified using a bridge rectifier and smoothed out with a capacitor), connect the others to the larger input voltage.

author: RJSC
author_rank: Regular Contributor
timestamp: January 05, 2014, 03:22:00 pm
content: Quote from: theironlefty on January 05, 2014, 03:03:04 pmWell, I congratulate you for getting started in electronics at your age.When I was about your age (10~11) I built my first "circuit": a full bridge rectifier out of 4 diodes salvaged from scrap TV and radios at my grandmother's house in order to make a DC motor run from my bicycle Bottle Dynamo which outputted 6V AC, as I was fed up of disposable batteries and had no AC/DC transformer.Only thing I had at the time was a single old electronics book at the town library to learn from. Internet had reached my country 2 years earlier but just for universities and research institutions.Now you have the web and a vibrant community to get you started.Are you playing with hardware or just simulations?

author: theironlefty
author_rank: Contributor
timestamp: January 05, 2014, 03:30:32 pm
content: Quote from: RJSC on January 05, 2014, 03:22:00 pmboth of them

author: papo
author_rank: Regular Contributor
timestamp: January 05, 2014, 03:40:02 pm
content: HelloQuote from: theironlefty on January 05, 2014, 03:03:04 pmDo you mean in terms of improving the circuit while keeping the features that you have, or do you want to add more features?In any case, a couple on comments on the software: First, just because it works in software does not mean that it works with real parts. This is something to watch out for. That being said, simulation is a handy tool for debugging and design if used properly. But building the real circuit and probing around is much more fun.Are you planning to build the circuit up with parts and probe around? Because if that is the case, a variable regulator is worth looking into. I would not get restricted by the parts a simulator offers if your goal is to design real hardware.Some comments on the circuit: For all the parts you are designing into the circuit, go online and read the datasheet. You will learn a lot by doing so, both in terms of how to use the part as well as in understanding how the parts work internally. That way, you would for example see that the regulators should be decoupled with capacitors under some conditions.Also, because you have an inductive load (a motor), it might be wise to bypass the regulators with a diode. These regulators typically don't like it if you apply -- due to the inductive kickback -- an output voltage higher than the input. Make sure that the voltage fed by CN1 is high enough for even the 8V regulator to maintain regulation. (Just out of curiosity, what does this circuit wizard simulator do as you ramp down the input voltage from 10V to, say, 7V?)You should also decouple the display driver under some conditions, check the datasheet. I didn't fully understand what that display is doing, though. At first I thought you were monitoring the current, but that display is kind of standalone, fed by a signal source. What's the purpose of this? Can you feed this portion of the circuit from the 8V regulator you already have or do you really need a dedicated regulator for that?In case you want to add more features, I would look for current monitoring and limiting, variable voltage (if you want that), or making the supply more rugged against abuse.Also don't feel bad if people seem to be a little harsh. Doing this kind of stuff at 11 years is great!RegardsMatt

author: theironlefty
author_rank: Contributor
timestamp: January 05, 2014, 03:44:50 pm
content: Quote from: papo on January 05, 2014, 03:40:02 pmhello matt i wanted to add pontentiometer and Display for voltage

author: Jon86
author_rank: Frequent Contributor
timestamp: January 05, 2014, 03:47:29 pm
content: Quote from: theironlefty on January 05, 2014, 03:44:50 pmPotentiometer for...? Controlling the output voltage?If so, you can do that with an LM317. Google it and read the datasheet, it'll show you how to wire it up.

author: theironlefty
author_rank: Contributor
timestamp: January 05, 2014, 03:51:56 pm
content: Quote from: Jon86 on January 05, 2014, 03:47:29 pmthanks anyway i am playing game now gonna make it soon

author: papo
author_rank: Regular Contributor
timestamp: January 05, 2014, 03:53:51 pm
content: Quote from: theironlefty on January 05, 2014, 03:44:50 pmCool. Does that mean that you want to convert the "knobless" supply into something that has a variable voltage, set by the potentiometer? If so you have a couple of options. The first one is to use a variable regulator, but your simulator does not provide such a part. If you really want to simulate it, you could also build the regulator from discrete parts but I don't suggest doing that at this stage. There is another trick which allows you to boost the output voltage of a 7805 higher than 5 V:http://www.electronicsforu.com/electronicsforu/circuitarchives/view_article.asp?sno=203&article_type=1&id=387&tt=unhotThen you somehow have to pick up the output voltage and display it on the bar display. For testing, I would advise you to develop both of these things separately. That means you develop the supply with variable output as one "project" and the voltage-display as another one. For that you can use a potentiometer for testing. When both work you merge the two.Which one would you like to go for first?RegardsMatt

author: theironlefty
author_rank: Contributor
timestamp: January 05, 2014, 03:57:07 pm
content: Quote from: papo on January 05, 2014, 03:53:51 pmno my software has Variable Resistor

author: papo
author_rank: Regular Contributor
timestamp: January 05, 2014, 04:13:03 pm
content: Quote from: theironlefty on January 05, 2014, 03:57:07 pmYes, but a variable resistor is not the same as a variable regulator. You can use a variable regulator plus a variable resistor (a potentiometer/rheostat) to get variable voltage. You can also do this to some extent with a fixed regulator plus a variable resistor as is explained in the link I posted above.What you cannot do however is this:http://123d.circuits.io/circuits/76465-potentiometer-loadingCan you figure out why this does not work? And also, are you familiar with the concept of voltage dividers? If not that would be something you could look into next in order to understand how a potentiometer works.RegardsMatt

author: liquibyte
author_rank: Frequent Contributor
timestamp: January 05, 2014, 04:21:22 pm
content: I read this as he doesn't have a pot and wants to change voltages with a switch. Maybe something like this?

author: homebrew
author_rank: Frequent Contributor
timestamp: January 05, 2014, 05:10:05 pm
content: If you want to use those fixed voltage regulators you have to add some protection diodes (as others have already suggested) and capacitors to them to get the output stable. I have drawn you a single stage of that kind of regulator setup.D3 protects the regulator from a wrongly connected power source. You can omit that diode if you are absolutely sure that the input polarity will always be correct. Remember that you will lose approx. 0.7V of input voltage on that diode.D1 is there to protect the regulator from reverse current (which it would destroy, even if applied shortly). Leave it in there!D2 protects the regulator from negative voltages (for example if other power sources are connected to the load)C1 and C2 are required to keep the regulator from oscillating and should be placed as near as possible to there regulator. They should be of foil or ceramic type but not electrolytic.C3 and C4 stabilise the in and output voltages.Good luck with your supply!

author: papo
author_rank: Regular Contributor
timestamp: January 05, 2014, 05:34:40 pm
content: Quote from: liquibyte on January 05, 2014, 04:21:22 pmOh I see, I was not aware of that. Then again he says that he has a variable resistor... I'm confused.Anyways, I think the circuit you posted is a nice start. There's one thing, though: When you turn the switch, the adjust pin will be pulled up since the resistor network will get disconnected. This results in possible spikes of the output voltage. Just something to be aware of.RegardsMatt

author: JoeO
author_rank: Frequent Contributor
timestamp: January 05, 2014, 05:55:23 pm
content: Quote from: papo on January 05, 2014, 05:34:40 pmThey sell Make-Before-Break switches. They are called MBB switches.

author: theironlefty
author_rank: Contributor
timestamp: January 05, 2014, 06:55:18 pm
content: Quote from: homebrew on January 05, 2014, 05:10:05 pmStill over 5V

author: Simon123
author_rank: Regular Contributor
timestamp: January 05, 2014, 07:00:45 pm
content: There is no load.Add load and voltage should drop.78xx have minimum load requrements.get this data and calculate resistor value with ohms law.

author: theironlefty
author_rank: Contributor
timestamp: January 05, 2014, 07:05:56 pm
content: Quote from: Simon123 on January 05, 2014, 07:00:45 pmhow to add load idk that many

author: Simon123
author_rank: Regular Contributor
timestamp: January 05, 2014, 07:06:46 pm
content: Ok, minimum load is 5mA.Therefore you calculate 5V/5mA=1000R or 1k.Put 1k resistor over output and ground.And ohms law is VOLTAGE=CURRENT*RESISTANCE

author: Simon123
author_rank: Regular Contributor
timestamp: January 05, 2014, 07:13:12 pm
content: And instead of using digital voltmeter just get analog one. Much simpler for beginner. You just connect it over ground and output, but be aware of polarity.

author: ivan747
author_rank: Super Contributor
timestamp: January 06, 2014, 12:10:26 am
content: Circuit wizard is great for beginners like you. When I was close to 13 or 14 I also used it, that was 4 years ago.Would you prefer something that works like the first picture (the black one) or something that works like the second picture (excuse the looks of the second one)?The great thing about electronics is that you can often copy someone's circuits without knowing everything about them and get them working if the circuit really works.