Is there an equivalent to a 24VDC diode in 24VAC

Discussion in 'General Electronics Chat' started by gerryr1gerry, Nov 21, 2011.

  1. gerryr1gerry

    Thread Starter New Member

    Nov 21, 2011
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    Electronic nerd needs help at a very basic level please.
    I have a sprinkler system which uses a programmable timer OR illuminated manual switch to turn things on and off at the right time.This system operated perfectly for years using a 24V DC power source with diodes throughout the line to stop current feed back. The 24VDC programmable timers became unavailable here (Australia) and I was forced to change the system over to 24VAC.I have 12 different station solenoids which are controlled by either timer or manual switch.Each station must also turn on the master pump switch to pressurise the system.My problem stems from the master pump switch feeding back current to all the solenoids when just one is activated. In DC this was overcome by having a diode in each line to prevent this.I have looked at relays but the quantity needed becomes too high. As I see it each station would require 5 relays to overcome the problem.

    1 relay to operate Station Solenoid by timer
    1 relay to operate a warning light to indicate station is on timer.
    1 relay to operate the main pump start via Timer.
    1 relay to operate the Station Solnoid via manual switch.
    1 relay the main pump start via Manual switch.

    If there was a component which did the same thing with 24VAC as a diode does with 24VDC I am sure it would solve my problem.
    I would appreciate any help or advice I could get.
    If you are kind enough to reply I would ask you keep it understandable for a real amateur

    Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Perhaps you can use a bridge rectifier in order to change the 24VAC to rippled DC that has an average voltage near 24. That way, you can still use diodes like you did before, and your DC solenoids should still work OK.

    A bridge rectifier has four terminals; two marked AC or "~", one "+" and one "-". If you can't find a bridge rectifier or they are too expensive, you could make one using four diodes that are of sufficient voltage and current rating. Since it's for outdoors, you should use a higher voltage rated diode; the higher voltage rating shouldn't detract from the performance, but may cost more.

    [eta]
    Here's what I'm talking about:

    [​IMG]

    The upper portion represents your old DC timer. The lower represents your new AC timer.

    Everything to the right of the bridge will work like it did on DC.
     
    Last edited: Nov 21, 2011
  3. gerryr1gerry

    Thread Starter New Member

    Nov 21, 2011
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    Thanks SgtWookie for your suggestion.
    I can see what you mean.
    I have some bridge rectifiers to give it a try but I thought I was once told it wouldnt work because the diodes would still sense the signal as (Not sure of the term used but think it was this)"half sine wave AC current".
    Would I have to use a special diode?I am only using 24VDC at 1miliamp(I think.Its not much amperage anyhow).

    If the bridge rectifiers work it would sure be a cheap and simple way to overcome my problem.

    Would appreciate your comments.

    Thank you
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    AC is when the signal traverses the 0v reference point, because current is flowing in both directions.
    By using the bridge rectifiers, you wind up with rippled DC. The current only flows in one direction. The voltage varies quite a bit; anywhere from zero to roughly 1.414 * V RMS, less the voltage drop across a pair of diodes (let's just call that 1.4v for simplicity's sake), so you wind up with roughly 0v to 32.6v out, peak-to-peak. If you took the RMS value of that rippled DC, you would wind up with roughly 23V DC.

    Your diodes will still function as they did with the DC system, and your solenoid valves should still work as well.

    I think that it is very likely that you are using a good deal more than 1mA when a solenoid valve is engaged.

    You might wish to try making some bridge rectifiers using 1N400x series diodes; anything from 1N4003 to 1N4007; the higher the number on the end, the higher the PIV (peak inverse voltage) rating is:
    1=50v, 2=100v, 3=200v, 4=400v, 5=600v, 6=800v, 7=1kv.
    I suggest the 1N400x series because they have made so many of them, and they are normally very inexpensive; a couple of pennies apiece here in the States even in small quantities.

    RS Australia has 1N4005 diodes for $0.04/ea.
    Website: http://australia.rs-online.com
    Search for 1N400
    Then on the resulting page, sort by price (descending).

    I don't know how many bridge rectifiers you'll need. You might consider these instead:
    http://australia.rs-online.com/web/p/bridge-rectifier/6875939/
    1.5A, 200V, $0.23/ea when you buy 20 at a time; so $4.60 + shipping.
    That would save you some time.
    You will still need some way to protect the connections / bare wires from the environment.

    [eta]
    However, I REALLY do not know how much current your system requires. If you use diodes/bridges that do not have a sufficient current rating, you will have failures quite often.

    You really should find out what is the maximum current required from any of these bridge rectifiers.
     
    Last edited: Nov 24, 2011
  5. gerryr1gerry

    Thread Starter New Member

    Nov 21, 2011
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    It would seem my amps requirements are fairly low.The two items switched by the 24VDC are

    Pump start relay appx 400 mA

    Solenoid valves appx 100mA

    The Max load on the circuit at any one time would be

    2 Pump Start relays and 4 Solenoids....say 1.2 amps however 90% of the time it would only be 1 pump start and 2 Solenoids.....say 600mA

    I think I would prefer "store bought" bridge relays to making my own.They aren't that expensive and at the moment I only need 6.

    Would appreciate any extra advice you may share and I thank you
     
  6. SgtWookie

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    Jul 17, 2007
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  7. gerryr1gerry

    Thread Starter New Member

    Nov 21, 2011
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    Thanks you for your help will follow your suggestions.

    Appreciated
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Let us know how it works out. :)
     
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