How to divide up 4.7 amps ??  To make a bench type power supply - Page 1
Forum section: electronics / projects

author: ez24
author_rank: Super Contributor
timestamp: May 01, 2015, 06:51:40 pm
content: HiI am going to use a 90 watt 19v 4.7 amp laptop power supply to supply several voltages and amps to be used for arduino work along with prototyping, experiments etc. Using a laptop power supply is my project, no options on this.I am thinking of 3.3, 5, and 12 volts as set voltages and a variable voltage. I will have to set the amps so if all the supplies are used at the same time at max values the total amps will have to be 4.7 or less.I will be do testing and will de-rate accordingly. But to start with what amp suggestions for the following volts with the max adding up to 4.7: ( I am thinking of 500ma for the 3.3v ??)3.3512Variablethanks

author: suicidaleggroll
author_rank: Super Contributor
timestamp: May 01, 2015, 07:03:33 pm
content: It depends entirely on what you plan on using this to power. I know for me and my projects, the 12v and the variable would likely never see the light of day. It would be primarily 3.3 and some 5. Your usage might be (probably is) entirely different. Also, will you be using switching converters, linear, or a combination?

author: ez24
author_rank: Super Contributor
timestamp: May 02, 2015, 03:46:01 am
content: I should have said I will be using a 4.7 amp 90w laptop power supply and I now think with losses I will be down to 2.5 ampsThis will be a bench type of power supply for children so no AC rails at desk level, ie no bench power supply. I will be usinglow cost I and V adjustable modules (one for each voltage) from ebayIt will be used for electronic lab work by children, so for 2.5 amps how about3.3 0.5 amp5.0 1.5 amp12v 0.5 ampI want 12v so they can play with motors, servos, relays, etcSince all modules are adjustable, the instructor could re-set the pots to suit the experiment. I just need a starting point.thanks

author: suicidaleggroll
author_rank: Super Contributor
timestamp: May 02, 2015, 04:15:42 am
content: So then you're using linear regs off of the 19v directly? Are you prepared for the 3.3v reg to dissipate 8W and the 5v reg to dissipate 21W? That is a LOT of heatsinking. And if you're using linear regs, why are you derating the source's current capability? Unless you don't trust the laptop charger's specs?

author: sleemanj
author_rank: Super Contributor
timestamp: May 02, 2015, 04:31:45 am
content: Quote from: suicidaleggroll on May 02, 2015, 04:15:42 amI think the poster is going to use adjustable CC/CV switching power supply modules (LM2596 or similar based), not linear regs.

author: Rerouter
author_rank: Super Contributor
timestamp: May 02, 2015, 05:25:17 am
content: If he is, then hi is hardly down to 2.2A, using switching regs he is around 1A consumed off the 19V rail assuming 80% efficient switchers, atleast for the specs he posted,To the op, why limit yourself to each part of the supply having a slice of the pie, if your not likely to be using everything at once, just scaling up to be safe under a 4.7A headroom would be,3.3V. 2.5A5V 7.5A12V 2.5AWhen you could just as easily scale each rail for a 60% demand, so you have some nice head room for when your just focusing on one voltage, or a second only is pulling a trickle,3.3V 15A (60W @ 80% Eff)5V 10A (60W @ 80% Eff)12V 4A (60W @ 80% eff)

author: void_error
author_rank: Frequent Contributor
timestamp: May 02, 2015, 07:15:57 am
content: Quote from: ez24 on May 02, 2015, 03:46:01 amWhich ones?

author: suicidaleggroll
author_rank: Super Contributor
timestamp: May 02, 2015, 12:22:44 pm
content: Quote from: sleemanj on May 02, 2015, 04:31:45 amThen it makes absolutely no sense to split up the current as he's done. Linear regs maintain current, switching regs maintain power (minus inefficiencies). Either he's using linear regs or he doesn't understand how voltage conversion works. That's why I asked what he was using in my first post, which he didn't answer, just more of this "current splitting" business.

author: Ian.M
author_rank: Super Contributor
timestamp: May 02, 2015, 12:38:55 pm
content: For kid-safe bench work, I'd limit all the rails to 2A max. Even that is enough to burn up a breadboard if they have any low-ohm resistors or get a chip in backwards. A separate higher current 12V supply for motor work might be a better idea as then the logic/small signal analog supplies can be limited to 1A or even 500mA.For personal use, I certainly wouldn't prefer a fixed 12V output to an adjustable voltage one, which should go up to a minimum of 15V.Also, what about a negative supply for OPAMPs etc. It should track the positive supply, but doesn't need anywhere near as much current capability as it wont get used for relays or motors.

author: ez24
author_rank: Super Contributor
timestamp: May 02, 2015, 06:23:53 pm
content: Quoteyes these (sorry for not giving enough details at the beginning)-http://www.ebay.com/itm/400714751657?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT(notice this one has 50v caps, I ended up getting some with 35v caps)surprisingly these are hard to find on ebay. Most have the little inductor. I learned about this module from Ian.Mand/or this one:[url=http://www.ebay.com/itm/111299051606?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT]http://www.ebay.com/itm/111299051606?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT- What I like about this one are the LEDs. If the module is pre-set correctly and at the start of the class, I assume (I do not have this yet) the V led will be on. If the student shorts out something, the instructor could see the C led is on. So it seems to me to be a fast way to troubleshoot (just by walking by). But it adds a lot of cost. So I am going to develop two versions, one with the cheaper modules and one with this one.One module for each voltage.I plan to use UL listed laptop supplies and this has really increased the cost, from about $10 to $30.QuoteI am afraid I forgot Ohms Law. Someone told me to do the calculations based on watts. I was using watts but based all my calcs on 19v, not the watts based on each different voltage. Now I get it. (voltage conversion)The cheap module says the limit is 5 a and the expensive one with heat sinks says 2a. I believe the 2a one. I guess the cheap one will be much much less, maybe 1 amp. What I learned is the laptop supply is not going to be my limit, it will be the cheap module. Once I get them in, I will look up the datasheet for temps and use a Harbor Freight infrared thermometer to check the temps on the chips and set the currents accordingly.QuoteCorrectToday's goal.Some good advice:QuoteI will change the 12v to adjustable that can go to at least 15v. So I will have 3.3 and 5.0 set and one adjustableThe adj will be the larger module.I thought about a neg supply for OPAMPS but I do not know how to begin using the ebay modules. I was keeping my eyes open and I ordered Art of Electronics from the library. Any ideas on how to start on this?Good advice on the 2a max for any voltage to protect the breadboards. I will do this. So the max will be either 2a or what is safe from the module, which ever is less.Now I am starting to nail down what I want and once I get the modules in and start testing, I will start a new topic.Any ideas on a name? Maybe "Laptop bench supply - safe for children"Want to stress that no AC power lines are to be on the desk. In other words no AC powered power supply. At the library they use laptop power supplies that are kept on the floor in the children's section. I have the same idea, the AC part will be on the floor. I think laptop power supplies are so common, everyone will feel comfortable with them.I have two goals now:1. come up with a tracking (not sure what this is) OPAMP supply2. Some sort of protection on the 19v line from the brick. I would like a solid state protection with an indicator LED, but since I have no idea on how to do this, I will start with a fuse. After reading some topics, even using a fuse is not all that easy (picking one - like slow, fast ). Soooo Since I will be using a 19v 4.7 amp supply - what about a 4 amp slow blow fuse (slow because the brick has some protection in it)? I want to add as much protection as I can. The brick will have protection, the modules are suppose to have protection (I will test them), and I still want more protection. Cannot have enough protection since this will be designed for children (I am guessing high school level).FYI I am trying to develop a bench type power supply for 3rd world countries (poor countries) schools. One option will I will allow them to furnish their own laptop power supply, so they can save money. My idea is to come up with some prototypes and then try and raise funds to build 20 units. Along the way I will need to find volunteers to assemble them and a school somewhere to use and test them. Part of my idea is the students could assemble their own units using my units and give those to another school and they could do the same thing, etc. This is all Part 1 of my project, I have several other ideas but I need to finish the power supply first.thanks everyone

author: rickselectricalprojects
author_rank: Regular Contributor
timestamp: May 16, 2015, 11:27:09 am
content: if you want 3.3v, 5v, 12v and a variable voltage than you should get or find an atx power supply. they put out a much higher current and the put out 3.3v, 5v and 12v. for the variable part i would purchase a cheap step down module of ebay and connect it to the 12v rail. hope that helpshttp://www.ebay.com.au/itm/DC-DC-CC-CV-Buck-Converter-Step-down-Power-Supply-Module-7-32V-to-0-8-28V-12A-GA-/181617142420?pt=LH_DefaultDomain_15&hash=item2a4939aa94

author: Kalvin
author_rank: Super Contributor
timestamp: May 16, 2015, 03:46:36 pm
content: I would use those cheap, adjustable switch mode regulators as a preregulators for the LM317s. If theses cheap smps fail for any reason, the LM317s will still keep the output regulated. Maybe the LM317 will heat heavily and then shut down. Also, I would build a simple crowbar for 3V and 5V outputs, just in case. And of course use resettable fuse for each output.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2015, 05:29:47 pm
content: The typical switching regulators are not good for a lab supply: they are relatively slow in load regulation and may get upset with load capacity (though some are very good in respect to stability). Also residual ripple and noise is rather high. So even for the 3 V and 5 V its more suitable to use switcher to get something like 4.5 and 6 V and than a linear regulator for the final output. There are special low drop regulators (e.g. MIC5190) using a low power, higher voltage supply in addition - these are usually better than simple 3 pin LDO regulators.

author: Kalvin
author_rank: Super Contributor
timestamp: May 16, 2015, 05:52:32 pm
content: I was not thinking efficiency that much, but instead tried to come up with something that is very inexpensive, well known, easy to get, and can take abuse. For example, if the cheap smps buck preregulator should fail, the linear regulator will have to survive from the 19V input voltage and still work as a regulator in order to protect the circuitry connected to the power supply. If someone connects +12V to +5V or +5V to +3V by accident, the power supply should not fail. It may blow a fuse, though. And if the fuses are resettable, no harm done. For extra protection for the circuit-under-test, I thought that crowbars for +5V and +3V might be a simple way to provide protection for situations if someone connects the +12V to either +5V or +3V accidentally. At least the voltage would be clamped to +5 and +3V giving at least some protection for the circuit-under-test.About negative output voltage: Maybe somebody can figure out how to modify these adjustable buck or buck-boost converters to inverting topology.Edit: The LM555 along with two power transistors, two 1N4001 diodes and few passive components and output filtering would provide an inverting charge pump with at least 100mA of negative voltage suitable for simple op amp circuits. LM555 will run max. 18V, but using a LM317 as current limiter, the 19V input voltage should drop well below 18V, suitable for LM555. If the LM555 is powered by the adjustable +12V ... +15V voltage rail, one may get almost a decent tracking negative voltage, although the voltage regulation and ripple attenuation may not be the best.

author: ez24
author_rank: Super Contributor
timestamp: May 16, 2015, 06:35:43 pm
content: Quote from: rickselectricalprojects on May 16, 2015, 11:27:09 amI do not want to use a ATX because I do not want any mains on the desktop. I went to the children's section at the library and there were a lot of laptop ps laying all around, even some next to the computer (ie within reach of a child). ATX is too dangerous for my application. Also my limit on any voltage will be 2 amps, based on someone's suggestion in this topic, because I am using breadboards. It is breadboards that are setting my limits.In regard to the module you linked, I ordered some yesterday. I got some of the smaller ones in and so far they seem to work (under 1 amp) but they just do not look right to me (too small). So I am sure I will use the one you linked to.thanks

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2015, 07:02:57 pm
content: At just about 1 A maximum, the LM317 after a SM regulatory to something like 8 V should be enough, even for the 3.3 V or similar.This is still only about 12-15 W for the regulators at a short. So plenty of power left for a variable and possible a negative.Just the variable output might want a regulator that can be adjusted all the way down to 0 and possible with adjustable current limit. This might even be just a linear regulator (possibly 723, or OP based) if heat is not so critical. There should be enough power to get 1 A @ 3.3 V, 2 A @ 5 V and up to 2 A variable (e.g. 0-16 V) this way.Some regulators like LT1072 or MC34063 can also be used to make a negative (or possibly isolated) voltage. There may even be ready made modules, but this type is less common. I would not recommend a charge pump for the negative supply, as the tend to be very noisy, due to current spikes, even at moderate power.

author: ez24
author_rank: Super Contributor
timestamp: May 16, 2015, 07:16:13 pm
content: Quote from: Kalvin on May 16, 2015, 03:46:36 pmI am using a good quality (UL listed Toshiba) laptop power supply that puts out 19v at 4.7 amps. I was not going to use a LM317 because of the power loss going from 19v to 3.3 and 5.0 BUT I will look into using a buck module (like this one -->http://www.ebay.com/itm/141647982239) and use it to go from say 19v to 4.0 (whatever works on the 317).And use the 317 (like this one -->http://www.ebay.com/itm/New-LM317-DC-DC-Converters-Circuit-Boards-Module-ZF-Hot-Linear-Regulator-/281665088997?pt=LH_DefaultDomain_0&hash=item41948c2de5) to get to the final voltage and see what this does for ripple, safety, and reliability. Seems like a good idea.I will have to look into crowbar and resettable fuses (I do not know what these are - can someone give me a hint on what specs to start with). I like the ideas of lots of safety backups. I have shorted the Toshiba and the small modules I have with no problems so far, but more the better.Quote from: Kleinstein on May 16, 2015, 05:29:47 pmI have to learn --> load regulation, load capacity, stability, ripple, and noise. I got a Rigol DZ1054 but after looking at the test waves, I am not sure it will help with noise readings. But I am starting to search the forum, look at YT on these subjects to test the modules."So even for the 3 V and 5 V its more suitable to use switcher to get something like 4.5 and 6 V and than a linear regulator for the final output." -- this is where I got the idea ha ha ha thanks"There are special low drop regulators (e.g. MIC5190) using a low power, higher voltage supply in addition - these are usually better than simple 3 pin LDO regulators." -- too advance for me right nowQuoteReally good ideas and test methods - I will try to implement all of them - thanksQuoteMore good stuff to look into - again thanks

author: Kalvin
author_rank: Super Contributor
timestamp: May 17, 2015, 04:29:15 pm
content: Just for a reference and memo: An example of a simple, very inexpensive adjustable power supply using very inexpensive parts that are easily available and are well known:A typical LM317 circuit for an adjustable benchtop power supply (See fig. 22):http://www.onsemi.com/pub_link/Collateral/LM317-D.PDF- The circuit can be connected after a switching preregulator in order to reduce heat dissipation.- If the switching preregulator is connected as a tracking preregulator tracking the output of the LM317, the efficiency can be inreaced.- The LM317 output voltage will not go 0V unless a negative supply voltage is available. This can be derived from the switching preregulator (see fig. 3):http://www.maximintegrated.com/cn/app-notes/index.mvp/id/660- The fets are used as constant current sink, and they could be replaced by a current sink created by 2N3904s or 2N3904 and LM431.- Fot the adjustable output voltages, there should be an adjustable limit for the maximum output voltage. A trimpot or something at the front panel that can be set by a screwdriver. For a quick&dirty solution, and adjustable crowbar will do the trick. As the power supply is well designed, the crowbar shouldn't do any damage.- The outputs must behave well when the 19V power supply is connected and disconnected. No voltage spikes allowed. Crowbar will help also on this.http://www.ti.com/lit/ds/symlink/lm431.pdfFor a simple crowbar, see fig. 20.For a current sink, see fig. 25.Here are some other usable parts used in low-budget power supply projects:- LM358 / LM324 Op amp.- LM393 / LM339 Comparator.- LM431 Voltage reference, current sink, crowbar.

author: ez24
author_rank: Super Contributor
timestamp: May 17, 2015, 07:38:39 pm
content: Thanks KalvinLots to study.I am starting with a laptop power supply and chinese buck modules. Yesterday I saw a ripple voltage for the first time (and first time using a scope) which was 70 mVpp at 3.3 v at 0.5 amps. I just don't know if 70 is good or bad.I ordered some 317s to see if I can reduce the ripple.Later today I plan to start a new topic just on the modules with pictures of the modules and waveforms.Someone earlier said that I should limit the currents to 2 amps because of the breadboard limitations so that is what I am going to do ie 2 amps max on all voltages. So far I think I will have 3.3, 5, 12 volts and one adjustable. I want to try and come up with a dual supply for op amps but so far no idea on how by using buck modules.

author: Kalvin
author_rank: Super Contributor
timestamp: May 17, 2015, 07:59:52 pm
content: Good start ez24!The 70mV ripple at 3.3V is 2%, which is not too bad. Nice to hear that you have access to oscilloscope so that you can actually see what's going on at the output voltage.I do agree, that 2A is sufficient upper limit for any output for safety reasons.Edit: Those cheap chinese switching regulators should be good for 1A - maybe up to 2A - as their heat sinks are quite modest. I haven't played with them extensively so I cannot tell how well those cheap modules perform, how well they regulate, how well they respond to load changes and how robust they are. That's why is I mentioned that it might be a good idea to connect a LM317 after the switching regulator should the switching regulator fail and supply the 19V input voltage directly to the output. For extra benefit, the LM317 should reduce the output ripple, too.There are quite many excellent designers here at eevblog forum who can create simple, straight forward and robust power supplies that can take abuse and use very inexpensive parts. Hopefully they will contribute here too.I just tried collect some basic, very simple designs using very cheap parts available that may be useful in your nice project.