Bench Supply Schematic

Discussion in 'The Projects Forum' started by spinnaker, Dec 21, 2009.

  1. spinnaker

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    My first project in a very long time is going to be a bench power supply. Yes I am aware that I could have probably bought one for around the same price.
    But I wanted this to be a project.

    Yes I am aware that I can use a walwart but I wanted to use a transformer instead.

    Yes I am aware that I can add a negative supply. That will come later.

    The project is based on the one found here. I chose to break out the variable supply on it's own as I will need this for a future project. The supply will have a second output with a selectable 3.3, 5V, 9V and 12V. I might add 3V in the future.

    The supply is not designed to be high amps. I figure 1 - 1.5 will do me for a while.


    I plan to add a digital volt meter powered by as PIC1845K20. There might be better choices but I have a demo board with this chip and thought it was a wise choice to select a ucontroller the same as my demo board.

    The ucontroller will also act as the selector for the various fixed voltages.

    Attached is the schematic and parts list.


    The PIC schematic will come later.


    If someone could please have a look and make sure if I designed everything correctly. If there are things that will work but I could have done better please let me know. I plan to build the supply on a perf board first, so I could always improve it as I go.
     
  2. SgtWookie

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    S1 and F1 should swap places; the fuse should always be the first item. J-1 should connect to the "hot" wire. In the States, that is the black wire. The plug will have three prongs, one round (ground) that connects to a green wire, and two flat blades.
    One blade should be wider than the other. That is the neutral wire, connected to a white wire.
    The narrower blade is hot.

    24VAC is excessive for a low voltage supply. 18VAC would be good. Otherwise, your regulator will make a good room heater.

    C2 should be located as close as possible to the input of the regulator IC.

    Just a technicality, but you seem to be using input-type connectors for both inputs and outputs.

    R3 should be 120 Ohms. This guarantees that the minimum required current for guaranteed regulation will flow through R3, and hence R4/R5/R6/R7, whichever is selected.

    This will also change the values for R4 thru R7.

    3.3v -R4: 195 Ohms
    5v -- R5: 358 Ohms
    9v -- R6: 740 Ohms
    12v - R7: 1026 Ohms

    [eta]
    R2 should also be 120 Ohms.
    Thus, if R1 is 1k, you would have a range of about 1.25v to about 11.73v.
    If you kept R1 at 5k, you would only have use of about the first 1/5 of the pot.

    The values for R4 thru R7 and R1 are based on a Vref of 1.25v (measured from the LM317 OUT to ADJ terminals) and a typical Iadj of 50uA. Your mileage may vary, up to +/-5%.

    B1 - you've specified 1.5A. That's OK, but if you use one rated for higher current, it will have less of a voltage drop across it; and there will not be much of a difference in price (if any).
     
    Last edited: Dec 21, 2009
  3. spinnaker

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    Thanks for the input Sarge.

    Glad to see I am not too far off the mark.

    I guess the reason I chose 24VAC because that is what I have. :) And I was thinking I could vary it from 0-24V (or whatever max I could get put of it).

    I hope Rat Shack carries 120 ohms. Did not order any of those from Mouser. Those could be very expensive resistors considering shipping. :) I'll have to check, I might have a couple on hand.


    As far as "connectors". I just kind of used what was available in the drawing package that I have on hand.
     
  4. SgtWookie

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    You can use resistors in series and parallel combinations to arrive at 120 Ohms.
    Here is a calculator:
    http://www.qsl.net/in3otd/parallr.html
    120 Ohms is a standard value of resistance.

    You can use the 24vac transformer. Just be aware that as your output voltage decreases, the power dissipation in your regulator will become very high.

    For example, if you are supplying 5v @ 1A to your load, and your filter caps have 26VDC on them, you will be dissipating (26v-5v)*1A = 21 Watts in the regulator, and 5 Watts in your load.
     
  5. spinnaker

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    Thanks. I know I could put resistors in series (or parallel) and that would probably be what I will do if I have to do it. Just thought it might be kind of messy. :)


    I guess I could always swap out the transformer, when I run across another in a good deal.

    Speaking of loads. I have heard that it is a good idea to have a permanent load across my outputs. But that would mean a 26W ( worst case) resistor, correct?

    Good idea or bad?
     
  6. SgtWookie

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    It's not messy at all, if you plan ahead for it when laying out your PCB.

    If the transformer has 240/120 primary windings, you could run it wired for 120 for the higher voltages, and wired for 240 to get lower voltages.

    Using 120 Ohms from OUT to ADJ eliminates that necessity entirely; as it provides a built-in permanent minimum load of just the right amount to guarantee regulation.

    That is why I suggested it in the first place. It's right in the documentation for the LM117/LM317. It may take several reads of the datasheet before that becomes clear.
     
  7. spinnaker

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    Thanks. I need to learn to read beyond the sample schematics they have in the datasheets, Most of the samples show 240 ohm but the details are in elsewhere in the sheet as you pointed out.
     
  8. SgtWookie

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    Exactly.

    The 240 Ohms for R1 works for the LM317L, as it only requires 5mA for guaranteed regulation. The LM317L is limited to 100mA total output current.
     
  9. spinnaker

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    One thing I need to learn is how to know what I need to know from a datasheet. I am sure there is a lot of information in a datasheet that while might be of interest to the average hobbyist, it is not all that important? In other words do I really need to be able to understand everything?
     
  10. SgtWookie

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    The better you understand the contents of the datasheet, and apply the information correctly to your circuit design, the more satisfactory the result will be.

    If you won't be stressing a component anywhere near it's maximum limits, then it may not be that important. However, unless you understand the contents of the datasheet, how would you be able to determine where the component's strengths and limitations lie, and how to best utilize it?
     
  11. spinnaker

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    I starting wiring the supply up. It os going slowly but it is going. Hey it is my first project in about 30+ years, I want to be sure to make as few mistakes as possible. :)

    The selectable portion is started.

    The variable portion of the supply is complete except for the problem below. Note: I did go against Sarge's advice a bit and chose a 150ohm for R2 instead of his advised 120. The schematic posted above is pretty much correct except R2 is now 150 ohm.

    The supply varies from 1.2 V to 34.4 V with no load.

    It is difficult dialing in an exact voltage. R1 is a 5K linear taper 20% tolerance from rat shack. Would I see better performance with another pot? Precision pots are pretty expensive. Would I have to go that route if I want things to improve?


    Since the supply will be controlled by a PIC, I was thinking of adding one of these . But I am thinking that maybe a bit too involved right now. I already have a learning curve with the PIC and LCD. Not sure if I want to add a digital pot to that right now.
     
  12. SgtWookie

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    If you want more resolution, you'll either have to use a multi-turn precision pot (yes, they are expensive) or use two pots in series; one a higher value, one a lower value.

    Since you're using a 150 Ohm resistor for R1, the current through R1 will be 8mA. You'll need a load of at least 2mA to get guaranteed regulation - which isn't much.

    Anyway, since the current through R1 will be 8mA, every 125 Ohms of resistance between ADJ and GND will add 1V to the output - over 1.2v.

    So, you're basically using 0 Ohms to 4125 Ohms on your 5k pot.

    If you put a 500 Ohm pot in series with your 5k pot, and center the 500 Ohm pot before you go for the "fine adjustment", you'll have a much easier time of it without the expense.
     
  13. spinnaker

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    Thanks for the tip on the two pots.

    It is 8ma through R1 but does that limit t the current my supply is capable of? If I read the datasheet correctly, it limits my adjustment current.

    Why is a higher current important here? Every other design I have found uses 240 ohm (yeah I know they all could be wrong).

    Does the higher adjust current make my adjustment a bit finer?
     
  14. spinnaker

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    I see you answered above.

    "This guarantees that the minimum required current for guaranteed regulation will flow through R3"

    So it is for good regulation then.

    OK I will swap this out when I get some 120ohms . Yeah I know I can put some together to ge wjat I want but now it is pretty much a space problem.

    Guess I will have to go with two pots or get a more steady hand. :)

    Maybe when I add a dial, that will make adjustment easier.
     
  15. SgtWookie

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    It reduces the output capability by 8mA out of 1.5A.

    You've read it incorrectly.

    It has to do with the minimum current required to guarantee regulation.

    It means less of a load current necessary to maintain guaranteed regulation.

    If the regulator has less than a 10mA load, the output regulation is not guaranteed.
     
  16. SgtWookie

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    Yeah, I did. ;)

    Yep.

    If you cramp things too closely in a linear regulator supply, you will have problems.

    The dual pots make a good substitute for a steady hand. ;)

    Maybe when I add a dial, that will make adjustment easier.[/QUOTE]

    You can get mechanical reduction dials for pots; or you used to be able to get them. They're available on the surplus market, but you'll need better pots than the RS variety.
     
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