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Designing circuits at home

  1. Jun 22, 2009 #1
    Firstly, I am starting my senior year in Electrical Engineering once school starts, and I feel that I am in dire need of some more "hands on" experience with the equipment. At my uni, the EE degree is split up into a Physics path, and a technology path. I'm thinking that may be a bad idea though, because most of my class mates (in the Physics program) are surprisingly inexperienced with most of the equipment.


    At any rate, I would like to start playing around with circuits at home. I have breadboards readily available, resistors, diodes, mosfets, opamps, basic electronic devices like that. My biggest question is:

    Power supply. I could try a 9v battery, per se, but how "ideal" of a source would that be? Also, that seems pretty inconvenient.

    Also, perhaps something like an oscilloscope? I realize these cost thousands of dollars, but isn't there any software I can use to read an output signal, for example?


    At home, I also have a device to measure resistance, voltage, current, and probably some other stuff I'm not aware of. I'm afraid to call it what I think it is (multimeter), because I always hear a new name for it. That is a clear indicator of my technical inexperience, so I would really take any advice/comments I can get. Thanks in advance.
     
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  3. Jun 22, 2009 #2

    mgb_phys

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    A battery holder with a bunch of rechargable AA might be better for most digital stuff you only want 5V (4AA = 6v will do)

    There are a few companies that make USB scopes for $100-200 , if you are into digital/microprocessors it's an interesting project to make one with an atmel/arduino

    Multimeter / digital voltmeter / DVM it's all the same thing - from $20 to a lot!
     
    Last edited: Jun 22, 2009
  4. Jun 22, 2009 #3

    negitron

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    Oscilloscopes don't necessarily have to cost thousands. For a couple hundred you can get something in the 30-50 MHz bandwidth range which is more than sufficient to learn the basics of operation and troubleshoot most circuits you'd be liable to design at this stage. There are also good deals to be had on used equipment. I have a Tektronix 2465B 400 MHz 'scope I picked up for around $800.

    Multimeter is a perfectly acceptable term for the device; simply "meter" is used frequently in the industry.

    As for a power supply, a 9 V battery is perfectly fine for most basic circuits; combine with various regulators to provide different voltages and you're good to go.
     
  5. Jun 22, 2009 #4
    That does sound like an interesting project; I will look up more info on that.

    Is a 9v battery likely to quickly deplete if it is powering simple circuits with a couple of diodes/resistors?

    Also, I think the real learning comes from actually "seeing" your output. I wouldn't know how to interpret my design without seeing what's happening.
     
  6. Jun 22, 2009 #5

    mgb_phys

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    9V don't have a lot capacity - they have 6 tiny AAAA batteries (about 1/2 the size of AAA) in a box.

    Picoscope (in the UK) are good source of PC based instruments, but you can get cheaper versions in the US.
     
  7. Jun 22, 2009 #6

    negitron

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    True, but they can nevertheless power small circuits drawing a few tens of milliamps for many hours. If one wants greater longevity, most electronics stores carry various plug-in wall-wart power supplies; some even have voltage selectors to provide a range of outputs.
     
  8. Jun 22, 2009 #7
    Cell-phone battery chargers are a good source of regulated dc voltage. Read the label on it for details. If you aren't charging your cell phone all the time, plug your circuit into it.
     
  9. Jun 22, 2009 #8
    This is exactly what I had in mind, actually. That is what I am used to as a (DC) voltage source at my uni; I figured they would be too pricey (>$200) for what I'm willing to invest right now.

    But I imagine it can be designed. A diode rectifier coupled with a couple of voltage regulators. Of course, more complex than that, but I imagine it'd be a good project, especially educational.
     
  10. Jun 24, 2009 #9

    vk6kro

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    A good source of power for experiments is a holder for D cells. These are economical to run, especially if you put alkaline D cells into them. 6 D cells will give you a 9 Volt power source that should last for months.

    Maybe aim at a regulated variable power supply eventually. These are not hard to make and always useful. There are schematics for this on Google. Just hunt for regulated power supply.

    You can do without an oscilloscope, but sometimes you are really flying blind without one. They tell you the waveform and relative voltages of whatever you are looking at. Even quite a simple one is a huge improvement on nothing.
    There are lots of them on Ebay at present at prices ranging up from $100.
    Look for dual trace and 20 MHz or higher. Not that you always need these things, but the ones that have them are usually better quality than those that don't.

    Also try to get a digital multimeter and a small soldering iron. These don't have to be expensive either. They make good birthday presents if you have a birthday coming up.

    You are probably busy, but maybe you could contact your local Amateur Radio club and go along to a few meetings just to see what other people are doing. Also find out where the stores are locally that supply electronic components and kits.
     
  11. Jun 24, 2009 #10

    berkeman

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    You are getting great advice here. I can only add a bit and emphasize some of the points made already.

    Consider putting together a few kits for fun stuff that you are interested in. They help you to get started soldering and monitoring (and debugging!), and give you fun results in the process.

    For a simple general power supply, you could use two DC "wall wart" power supplies, and arrange the connectors to output +/- power supplies. You could use 3 of them to give you +5V and +/-12V as a standard output, which will serve you fine for 99% of your bench-top projects. Basing your power supply on batteries has some limitations, so using DC wall warts instead will give you more development flexibility.

    For your first 'scope, the USB PC/laptop based units are inexpensive and convenient. You will out-grow their capabilities at some point, and then you can look at the new $600 'scopes that are hitting the market (well, okay, from China). Pretty impressive 2-analog-channel + 16 digital signal logic analyzer performance.
     
  12. Jun 24, 2009 #11
    Pick up a computer PSU rated for about 200 watts on eBay for about $15. It'll give you regulated (about +/- 5%) 12V, 5V, 3.3V, -5V and -12V (negative voltages aren't always available on newer PSUs). I had one in an old computer, so you could check there.

    They usually have built in short circuit protection, over-voltage protection, over-temperature protection and are good for at least 5 amps on most of the outputs. You can combine the various voltages to get 24V, 17V, 10V, 8.7V, 7.0V, 1.7V (and any other ways you can combine the voltages). To switch them on you need to hard wire the green wire (PS_ON) to a ground lead. Then you can use the rear switch, if it has one, to control it.

    Note that you might need to load down the inputs. Use a 10W+ / 10R resistor to do that (on 5 volt line)... Not all PSUs require it though.
     
    Last edited: Jun 24, 2009
  13. Jun 24, 2009 #12
    I'm certain I have at least one extra PSU laying around somewhere, so I will definitely look into that.

    And about the suggestion to look for local amateur radio clubs, that is an excellent idea. Fun, small projects would be a great idea to keep busy and motivated. Thanks a bunch for the great content.
     
    Last edited by a moderator: Jun 24, 2009
  14. Jun 24, 2009 #13

    uart

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  15. Jun 24, 2009 #14

    mgb_phys

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    Be careful - the front panel switch is normally on the input side so at least one of the wires to it has line voltage when the thing is off.

    Speaks from experience - and it's 240V over here, and that hurts!
     
  16. Jun 24, 2009 #15
    This is why I only use ATX power supplies. Shorting two terminals is a easy price to pay for not making our hair stand strait up.

    Also, be sure to get a GOOD multi-meter. Not one of those cheap ones for $10 from harbor-freight but an actual good quality meter.
     
  17. Jun 25, 2009 #16
    The power supply I have requires no loading. Maybe the fan is enough for it, I don't know.

    PS_ON is not at line voltage. It is 5 volts standby power which is provided even when the unit isn't switched on (e.g. fan not spinning but the switch on the back is on).
     
  18. Jun 25, 2009 #17

    mgb_phys

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    It is on ATX, on the older AT supplies there was a line switch to the front panel.
     
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