Basic Parallel/Series Question

Discussion in 'The Projects Forum' started by Phrazemaster, Dec 21, 2010.

  1. Phrazemaster

    Thread Starter New Member

    Dec 3, 2010
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    Hi, I'm creating a small board to support some relays and LEDs in a microphone preamp. I am new at electronics, and at board design, so please bear with me.

    I am attaching a file of the board, simplified (some circuitry not essential to this question deleted).

    My question is, if I have a 15v input, can I use the power across 4 boards like this, using connectors between the boards (shown)...I have a single power rail, and a single ground, going across. I'm wondering when relays/LEDs engage, if the voltage or current will drop.

    My understanding is, voltage will remain constant since the individual components will be connected in parallel with the main + and - rails. This would mean the current would be divided amongst all the boards.

    I added a capacitor across the power rails at input to smooth out any power drops when the relays/LEDs are engaged. But will this compensate for lost current due to the topology of the board?

    Is there a problem with this design; have I missed something fundamental here? I don't want the LEDs to get dimmer as I turn on more and more switches, and I don't want the relays to fail to engage when more and more switches are turned on, due to a lower current. This design would work fine for 1 board, but what about for 4 boards connected together using the Unimate connectors shown, each to each other? Or do I need to get a new power +15v power supply for each board directly from the power supply itself?

    Thank-you for answering such a simple question.

    Mike
    led-board-question.jpg
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Are you certain that you want to use relays? Relays are electromechanical devices. They're generally pretty good at isolation and in power switching applications, but consume a fair amount of power due to the coil having to be energized, and can be noisy and slow electrically.

    If you're switching line-level audio, you could use an analog switch, such as the CMOS 4000 series 4066 quad analog switch/multiplexer. Have a look at OnSemi's datasheet for the MC14066B, available from this page:
    http://www.onsemi.com/PowerSolutions/product.do?id=MC14066BFG

    You could use an RC network to turn the switches on and off slowly, which will help prevent loud "popping" noises like you'll get with relays.

    Why don't you post your schematic so we can see what you have in mind.

    As far as whether you're going to run out of power or not - we can't tell from the information provided, as we don't have any specifications on the parts you're using, nor the power supply specifications.
     
  3. Phrazemaster

    Thread Starter New Member

    Dec 3, 2010
    15
    0
    Hi, thank-you for your thoughtful response.

    I decided to use relays after a great deal of consideration. I just wanted a lighted switch for the high pass, phase, and phantom functions of the preamp I'm building. Ha. Try finding a cheap DP3T switch, for less than an arm. I needed 3P since the LED portion takes up one position.

    Other folks at the prodigy pro forum mentioned they use relays without incident, and they are very reliable. I chose a Panasonic TQ2 series relay for its low current switching - it's a telecom relay.

    I looked at your suggestion of the quad analog switch...it's an IC, I see. I really don't know how to integrate something like that into the design. I'm sure it's easy if you know. How hard is this to implement?

    All I really want to do is have a nice, lighted switch that is functionally equivalent to a DPDT switch with an integrated LED, for which I resorted to using a SPDT switch with integrated LED and a DPDT relay in its place.

    I already have the SPDT lighted switches at this point, non-returnable. They have integrated LEDs. How could I go about using the quad IC switch you mentioned, in lieu of the relays? I was always leery of them anyway.

    I couldn't believe how complicated it ended up being, to this newbie.

    I am posting the schematic for the switch/relay portion so you can get an idea of what this is. The power supply is a 15V DC supply onboard the preamp. What is not shown is the actual switching contacts of the relay (the DPDT portion). Shown instead is just the relay coil.

    Thank-you so much in advance for your kind opinions.

    Mike
    relay-circuit---layout-concept2-corrected-again.jpg
     
    Last edited: Dec 21, 2010
  4. Phrazemaster

    Thread Starter New Member

    Dec 3, 2010
    15
    0
    I looked up some specs and the 4 channels of preamp use about 20ma each at about 15v.

    The transformer is a torroidal Avel-Lindburg 30VA Y236103. I am also posting the closest to the schematic for the PSU as I have. Does this shed any light whatsoever?

    _PSU-All.jpg
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Gee whiz, you sure know how to make it hard on a guy. ;)

    Can you post the schematics, or a link to where you found them?

    What you've posted so far (except for the relay/LED/resistors) are the board layouts and part placement diagrams.
     
  6. Phrazemaster

    Thread Starter New Member

    Dec 3, 2010
    15
    0
    Yes of course, sorry. Here's a basic schemo, not including the DPDT part of the relay (only relay coil circuit is included).

    Thank-you so much.

    Mike

    relay-circuit.jpg
     
  7. Phrazemaster

    Thread Starter New Member

    Dec 3, 2010
    15
    0
    Oh, of course you want the psu schemo too; sorry! Here it is...

    green_psu_schem_v2.gif
     
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