Power Supply Project (total newbie to this) - Page 1
Forum section: electronics / projects

author: blewisjr
author_rank: Frequent Contributor
timestamp: March 23, 2013, 12:45:42 pm
content: I want to design a power supply Project and I don't even know where to get started but I think it would be a great project to learn a lot.Here is a basic feature list I am looking at being that 98% of my projects will be with uC's if I ever venture off to something bigger I can always buy a supply.2 channelFixed 1 Amp currentAdjustable Voltage: 3.3V, 5V, 9V, 12V, 15VLoad Switches on/offMain Power off switch as wellI am in the US so everything needs to get scaled from a 110V main.I think my requirements are simple enough but I don't even know where to get started.I would probably just use protoboard and jumper wires for the circuit placed in a project box with a simple cpu fan for venting.The switches would be just simple toggle switches and the pot would have to be a hard click pot with 5 stages.The one thing I don't know is how to limit the current and stage the voltages. I know there are linear voltage regulators but I am not sure if that is what I need.I am not looking for the answer I am looking more for guiding suggestions because I think I can learn a lot by coming up with my own schematic etc...

author: Rerouter
author_rank: Super Contributor
timestamp: March 23, 2013, 01:22:08 pm
content: it should be quite a simple project, to start you off in the right direction look up "317 power supply"but for now i think a bit of clarification will help refine how to implement it,1. do both of the channels need to be isolated from one another ?2. not too difficult,3. though its not too critical, as you will need a 6 position switch for those functions anyway, what about a pot for a variable setting? (3,6 and 12 are common, not so much 5)4. easy as long as your not too critical on contact bounce5. every and i mean every project should have one of these,6. this should just be a case of a different model transformer,now to be clear i do not like telling beginners how to do the mains side of it for legal issues, so from this point on I'm just going to assume your secondary magically generates a 14V AC signal, (about 18V dc when rectified)if you want both outputs isolated from one another that will mean each channel will need a separate wining on the secondary,the current limit should be very easy for you, it will be a single op amp, as the signal for 1A across most sense resistors should be well within the realms of very cheap op ams without rail to rail inputs, and using low side measurement will make it even easier to implement, but more on that later, it generally works by hijacking the regulators feedback network, its trying to get its set pin to 1.25V, so by pushing a higher voltage through say a diode on the op amps output you can make the voltage drop to the magical 1.25V minimum output,the bigger issue you will find will be 1A at 3.3V, as that will be the largest voltage drop across the regulator, somewhere in the ball park of 16W, and as the regulator ignoring the actual heatsink increases 5 degrees in temp per watt, this becomes an issue, now if you where willing to pay a bit more for the transformer you may find one with multiple taps say 14-10.5-6-0 which would limit that maximum to about 2.5W for any rail, and simply get a dual pole 6 position switch, which are equally quite common, to switch taps with your set voltage,

author: blewisjr
author_rank: Frequent Contributor
timestamp: March 23, 2013, 01:36:02 pm
content: I would like the 2 channels isolated but maybe it would be better to make the second channel a straight 5V fixed and have the first channel adjustable. A 6 position switch would be fine as well can always find something to use the extra position for like maybe a 1.5v or something. Would there be a way to auto drop the amps for the lower voltages to reduce heat build up?I do not know anything really about transformers but I was browsing through DigiKey and saw this not sure if it would work for this project or not.http://www.digikey.com/product-detail/en/70050K/TE70050-ND/154909

author: mariush
author_rank: Super Contributor
timestamp: March 23, 2013, 02:42:51 pm
content: That can only do 7 vac 1A.You should think about starting from an existing design and trying to understand it.For example, have a look at this one, it's a 1x18v 2A power supply :http://savedonthe.net/download/476/Schematic-Adjustable-Linear-PSU-1x18v-2A-Circuit-Specialists-1802x.htmlThey're using a single transformer with multiple secondaries, 2 for the output , 2 for the +/-8b and +/- 5v for opams and leds and meters and another secondary for 12v for the relay.They're switching between one secondary and two secondaries on the output using a relay - the reason for that is to keep the power dissipated on the tip3055 low - ex. without relay, the transistor would only see the 20v or so from transformer, and if you want 3.3v at 1A on output, you'd waste [20v-3.3v] x 1a = ~ 17w as heat. With relay, let's say when you go lower than 8v, relay triggeres and moves to the 10v winding so now you only waste [10v-3.3v]x1a = ~7wIf you change it to use 2 transistors in parallel mounted on a larger heatsink (see dave's video about the 450$ psu and how they used two transistors on heatsink), or you increase the size of your heatsink and/or maybe make your power supply with a fan, you could drop the switching with relay and always keep the transformer on the biggest output on secondary, and just let the transistor dissipate the voltage difference as heat, for the sake of making your power supply cheaper/simpler..They're also using two ICL7106 chips which are basically multimeter chips with the capability to drive lcd segments - if you'd want you could use ICL7107 which can drive seven segment digits directly but they're kinda expensive at around 5-7$ a piece, in volume they're cheaper (and require quite a few capacitors and resistors around).But you could just skip those and use a microcontroller's adc to get the voltage and current, as you don't need super accuracy. They're using them because they make several models of power supplies and just reuse the lcd screens and meter boards.I would basically standardize on 2x 9-10v 1-3 A (depends on how much you need/want) transformers or something around that value. This will be good for both purposes, either to make +/-5v or +/-8v for opams and microcontroller (if any) or to use it as actual raw ac for the channel.For example on a basic search on digikey, found this which would suit the purpose :http://www.digikey.com/product-detail/en/VPS20-2200/237-1265-ND/666151You've got 20v AC 2.2A with a tap at 10v so if you want you can use a relay to switch between 10v and 20v... after it's rectified, moved through pass transistors and all that you still have about 16v at around 1.5-1.8A , so it will satisfy your need for up to 15v.It can equally be used to get +/-5v using some plain 7805 - they will heat a bit as the drop is big from 10v down to 5 but it's nothing major.The rest of the schematic is quite simple, just some plain opamps, nothing complex ... basically you're only interested in the bottom part of the page, the top side is the scary meters and lcd segment traces which you can ignore.

author: blewisjr
author_rank: Frequent Contributor
timestamp: March 23, 2013, 03:12:18 pm
content: Ok thanks for the info I will look into the schematic when I have more time.Being that it is 2 channels I can actually run say a PIC16F1829 Hooked into a 2x16 segment LCD. The chip has 2 ADC's so I can read both channels and list the Current and Voltage on the 1 LCD. The PIC16 can easily be driven at 3.3V or 5V. I would for sure use a CPU fan to help dissipate the heat out of the unit not reason not to they are simple and cheap. Then the 2 transistors and Heatsink I should easily be able to avoid the relay and stay really cool.

author: mariush
author_rank: Super Contributor
timestamp: March 23, 2013, 03:27:33 pm
content: In regard to heatsinks, I was thinking more of something like this :http://www.digikey.com/product-detail/en/530002B02500G/HS380-ND/1216384http://www.digikey.com/product-detail/en/RA-T2X-64E/RA-T2X-64E-ND/2416490One for each pass transistor, with a 120mm fan moving air over the heatsinks, you would probably get both of both worlds, cool heatsinks and silent operation.There are more expensive heatsinks, like this for example, which would probably be just good enough to run without fans... but i don't think it's worth it, it's a bit expensive and large:http://www.digikey.com/product-detail/en/421K/345-1049-ND/340345For comparison, my Tenma 30v 3A psu used a single heatsink mounted on the back of the case for the two transistors (one for each channel) :https://www.eevblog.com/forum/reviews/tenma-72-8695-2-x-32v-3a-5v-2a-mini-review-teardown/?action=dlattach;attach=30760;imagehttps://www.eevblog.com/forum/reviews/tenma-72-8695-2-x-32v-3a-5v-2a-mini-review-teardown/?action=dlattach;attach=30774;imageBut they're cheating a bit : the transistors are isolated from the heatsink with those pads, as the heatsink is also tied to the metal case so some heat goes through the case.

author: blewisjr
author_rank: Frequent Contributor
timestamp: March 23, 2013, 03:47:55 pm
content: Would it actually be possible to use the full range of the 2 transformers? That would allow for adjustable current as well correct?

author: rdl
author_rank: Super Contributor
timestamp: March 23, 2013, 03:48:45 pm
content: I built a similar power supply several years ago. I used an Avel-Lindberg Y236103 toroidal transformer. It has 2 independent secondaries and each is rated 15 volts at 1 amp. I bought it from parts-express.com, it's kind of expensive at almost $30, but is a very nice transformer.

author: mariush
author_rank: Super Contributor
timestamp: March 23, 2013, 04:22:31 pm
content: Quote from: blewisjr on March 23, 2013, 03:47:55 pmThat design already allows you to adjust current. You just have to short out the output and play with the potentiometers... see VR3 and VR 4, two 10k pots you can put on the front of the power supply.otherwise, I'm not quite sure I understand what you mean by full range.The transformer has a rating on it, let's say 20v 2.2A (42VA), like the one I linked above. That 20V is RMS value usually, which means when you rectify it to DC, you actually get a higher voltage, 1.41 x 20v = 28.2v peak, in average a bit less... then you have the voltage drop on the bridge rectifier.. since two diodes are conducting all the time, so you have about 0.8-1.4v drop on the diodes. Next, you have to keep in mind the current can't magically remain the same, you're still limited by the maximum rating of 42VA specified by the transformer... so you recalculate the current based on your new DC voltage.That's why I said a 20v transformer would probably give you a safe 18v or so DC maximum, but at a maximum current less than the 2.2A the transformer has specified.See page 11 on this catalog where it explains some terms and shows some formulas ... :http://www.hammondmfg.com/pdf/5C08.pdfin your case I think the "FULLWAVE BRIDGE Capacitor Input Load" applies...Also keep in mind that at low current draw, the transformer will output higher AC voltage than 20v and will gradually get closer to 20v AC as you pull more current from it. That's why it's important to use capacitors with correct voltage rating after the bridge rectifier. For example, you'd think that as you have a 20v AC transformer you could use a capacitor rated for 25c, but no, after rectification and taking out the voltage drop on the diodes, you may have 26-28v after the bridge rectifier. Even 35v capacitors aren't quite safe, as on low load, the transformer may get close to 30-32v, or if your mains power may go towards 120v ac you may get more voltage on the secondary.You need a separate transformer or two separate windings, not necessarily so big in current, to create the separate voltages for your controller and the opamps, these should be separate than the windings for your actual transformer used for power output.So if you want to save money you can get maybe a 2x9-12v 0.5A ... 500mA should be enough for controller, lcd display, opamps etc.

author: blewisjr
author_rank: Frequent Contributor
timestamp: March 23, 2013, 08:00:27 pm
content: Wow so complicated!!! This information is quite helpful but after hearing it I don't think it will be safe to circuit this with a protoboard and jumpers that is quite a bit of voltage the jumpers would have to be able to withstand at least 40+V to be safe I would think.

author: mariush
author_rank: Super Contributor
timestamp: March 23, 2013, 08:36:44 pm
content: You're safe to do it on the breadboard (but you'd probably need 2-3 breadboards), as long as you're not going to connect something requiring a lot of current to the output. The leads usually used with breadboards can't handle much current.Test the power supply on the breadboard with some computer fans, maybe a flashlight bulb, some leds (with resistors) etc etc... you'd be completely safe with that.Your only safety concern should be just to make the primary side of the transformer(s) completely isolated, so that you won't be able to touch it even by accident. Solder the mains cable wires to the proper place then use electrician tape and completely cover that side, or maybe put the cable wires through heatshrink tubing and after you solder the wire, move the heatshrink tube over the solder point and heatshrink everything (i'd still use electrician's tape over the heatshrink tubing though).

author: c4757p
author_rank: Super Contributor
timestamp: March 23, 2013, 08:41:12 pm
content: Quote from: blewisjr on March 23, 2013, 08:00:27 pmBreadboard is fine for 40V. I have done numerous ±20V (40V rail to rail) analog circuits on breadboards without incident. Hell, I've done 350V on a breadboard before, though I don't really advise doing it yourself -thatdefinitely takes special precautions. But 40V is nothing.

author: blewisjr
author_rank: Frequent Contributor
timestamp: March 23, 2013, 08:50:01 pm
content: Quote from: c4757p on March 23, 2013, 08:41:12 pmSorry did not mean breadboard was talking about the PCB protoboards where you build the circuit links by soldering jumper wires to make the connection traces.

author: c4757p
author_rank: Super Contributor
timestamp: March 23, 2013, 08:52:44 pm
content: That's even better because if you need to make some extra clearance between two points, you can just put space between them and scrape off the copper pads. (At 40V, that board has more than enough clearance as it is, so don't bother.)

author: blewisjr
author_rank: Frequent Contributor
timestamp: March 23, 2013, 09:15:16 pm
content: Quote from: c4757p on March 23, 2013, 08:52:44 pmSounds great. Now I just have to make sense of all this which will be tough in it's own right and put together a BOM to make 2 of them. This will for sure be a great learning experience. This should defiantly get me to crack open Practical Electronics for Inventors 3rd edition which I have been putting off.

author: blewisjr
author_rank: Frequent Contributor
timestamp: March 24, 2013, 03:39:00 pm
content: I have been doing even more research and looking into Dave's linear power supply. I think it could easily be modified to be able to do variable 0 - 12V with variable 0-2A controlled with rotary pots and a PIC 16F1829 as it has 2 pwm's and I think even 1 DAC. Do single output with 1 PWM controlling the voltage and a 2nd pwm controlling the current. Have the current limiting LED in there as well. Can get the 2 amps with the LT3080's in parallel. Power it off a plug pack 9V maybe? The limit the power to the PIC to 5V using a Linear regulator. The only issue I can see is getting the PWM to output a setting for the 12V output I am sure it can be done with a amp because a x2 will get 10V.Any thoughts?