serial resistors project 500 in 1

Discussion in 'Homework Help' started by sparkler, Oct 11, 2011.

  1. sparkler

    Thread Starter New Member

    Oct 11, 2011
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    0
    Hi I'm new :) but drowning:eek:
    I'm using the maxitronics 500 in 1 project 8 which is just showing what happens to LED brightness when serial resistors are summed (in this case 2 x 470s). The circuit is such that either 1 resistor is play via the "switch" or both are.

    Having wired up as described the book says the LED should be darker with the switch in the UP position (towards terminal 72) as 2 resistors come into play. I can see why that should work from the schematic.

    However, I find that the opposite happens - the LED becomes (very slighly darker) with the switch in the DOWN position, with only one resistor in play.

    Result: I'm more ignorant than if I had not done the project!

    Anyone know this kit? Would be very grateful for any help!!
    Thanks
    Sparkler
     
  2. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    Welcome to the forum!

    Could you post a schematic? Even a picture of the circuit will do.

    Do you have access to a multimeter - one that can measure resistance and voltage?
     
  3. RiJoRI

    Well-Known Member

    Aug 15, 2007
    536
    26
    General troubleshooting tips:

    (1) Verify that your parts have the correct values.
    (2) Make sure the parts are connected correctly.
    (3) Mark your schematic with the expected voltages
    (3a) Verify the expected voltages are there. ("Close" -- +- 10% is OK)

    Now comes the fun part: Replace the switch with a wire, and see if that helps. What happens when you swap the resistors?

    Actually, 3/3a should give you the answer to where the problem lies.

    --Rich
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Are you sure that the switch state corresponds with the toggle position in the way that you think it does? Some switches do the reverse of what you might think due to their internal mechanism. You may even be really unlucky and have a switch that does not comply with the kit instructions, either because the original instructions were wrong, or because you have a different switch than originally planned.

    A wiring error would still be my first guess though.

    As for the result, more resistance should give less current and hence less light. There is however one caveat: given far too much current, LEDs can get dimmer. If therefore you have too low a resistance or even no resistance in one switch setting, the LED could be dimming due to overload. Unfortunately if this has occurred the LED may be damaged.
     
  5. sparkler

    Thread Starter New Member

    Oct 11, 2011
    3
    0
    :) Hi thank you so much all three. I shall try to attach photos. As you will see the circuit is dead simple, I've done (slightly) more complex ones ok. Actually I looked at the next project which wires resistors in parallel and that worked exactly as expected!!

    But it worried me that the serial resistor project was a 'footnote' about resistors and if the thing is in reverse it defeats the point and will mess up other projects down the line. Yes, the idea that the toggle switch may be inverted did occur to me. Though it's difficult to see in this kit, unlike the cardboard ones. Good idea! I will try replacing the switch with wires. No, I don't have a multimeter yet, but I can now see the need for one! That'll be overtime on the saturday job!

    I will go through all the trouble-shooting steps and report back - thanks again all of you it really has helped!
    cheers
    sparkler
     
  6. RiJoRI

    Well-Known Member

    Aug 15, 2007
    536
    26
    Yes, a multimeter is invaluable. I took an electronics course where the multimeter was used for measurements I was positive needed an oscilloscope!

    You can probably get away with a very cheap multimeter. We can get them here in the States for less than the price of a cheap meal. ( < $10 )

    --Rich
     
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  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    300
    I note the term "serial" resistors. In British English, "series" is more commonly used for components connected this way. I have heard French people use "serial" like this in English, and I think the same may be true in India. What is the normal USA usage?

    To return to the topic, with an LED and resistors in series, Current I = (Vs - Vf)/(R1+R2+...RN), where Vs is the supply voltage, Vf is the LED forward voltage. Current and brightness reduce with increasing resistance, always provided the current does not get too big.

    For an LED, and most other diodes for that matter, Vf does not vary much as the forward current changes over quite a wide range. Typically we can assume it to have a constant value, except in quite accurate work, or where the current will be quite a long way from the normal operating level. The Vf is different for different designs of LED, in particular it is higher for blue or white devices, less for longer wavelengths. It also does vary a bit between devices, and generally it falls with increasing temperature.

    The near-constant but not entirely predictable value of Vf is why LEDS almost always have to be used with series resistors or more elaborate constant-current drives (too little current = too dim, too much current = short life). We may see cheap lamps that manage to run LEDs directly from small batteries, but such arrangements tend not to get the best out of the battery life. The brightness may also be very variable, and worst of all the LEDs may even burn out if there is an unfavourable combination of low Vf and high battery voltage. A series resistance always wastes some power, but this is usually preferable to having an unpredictable current.

    I seem to have rambled on a bit here, but the point of the necessity for resistors or other current-determiners with LEDs deserves attention: it is the source of a lot of trouble for beginners who try to run LEDs directly from voltage supplies, with zero or minimal resistance.
     
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  8. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    Like our cousins from across the pond ... series is the term used.
     
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  9. sparkler

    Thread Starter New Member

    Oct 11, 2011
    3
    0
    Well, I'm learning a lot today! Cheers good people. Hi Rich, yes it turns out I shall be a multimeter man in a few days as I ordered one on Amazon for a few quid - so I won't have to do overtime on Saturday!

    Yes, Adjuster, I think there are some language issues here! OK so 'series' it shall be. I suppose I was using 'serial' as an adjective, I guess it doesn't work like that? The Maxitronics book I am using was written in Taiwan, or possibly China, it doesn't look like it has been proof read by a native English speaker at all (or more worryingly it has!).

    Report: I examined the 470 resistors I was using and found they were different, one of them was like all the other 470s in the packet - but, although from the same packet, the second resistor has different markings. Anyway, I put another 470 in and it worked! 2 resistors 'in series' (?) did dim the LED. I believe it now!

    Thanks very much everyone for all your help! :)
    Sparkler
     
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