Circuit to control 25 devices: 24V, 1A each

Discussion in 'The Projects Forum' started by dullard, Oct 10, 2006.

  1. dullard

    Thread Starter New Member

    Oct 10, 2006
    I have a 24VDC supply, and the ability to provide several 5VDC (20 mA) signals.

    I need to operate 25 devices (one at a time, randomly selected). Each device is 24V, 1A.

    The easiest way to do this would be to have 25 relays (each controlled by the 5VDC signal). However, is there a better way of doing this? Is there a way that is (A) cheaper and (B) uses fewer of those 5VDC signals?

    I know that if you have 5 bits of information, you can have 2^5 = 32 conditions. But is it possible to operate devics with just 5 on/off relays?

    I can easilly picture a grid, with five switches forming rows and five switches forming columns. Then have an AND gate at each intersection. If the row AND column are turned on, then that lightbulb turns on. That gives just 10 relays needed. Is there a better way though? Also, are there AND gates available that operate at 24V and 1A?

    Or is there any rotary switch that can handle 24V at 1A and is controlled by a 5VDC signal? I could use this to move from device to device. Then use one relay to turn on/off the power so that only the desired device is operating as the rotary switch moves to the next device.
  2. kubeek


    Sep 20, 2005
    I suggest using the grid, with 10 relays you can operate 25 devices.

    You have to connect the 5 "horizontal" relays to switch the + voltage, the other 5 to switch - voltage, and conect the devices you need to control to the crosses.
    This is the easiest way to implement AND function in this case, and I don´t think there is any other way with less relays. Instead of relays you can also use transistors, low power mosfets are quite cheap.
  3. dullard

    Thread Starter New Member

    Oct 10, 2006
    Thank you for the advice, kubeek.

    So what you are saying is that I make a circuit that is connected in this order:

    1) 24+ VDC source
    2) MOSFET1 Drain Pin
    3) MOSFET1 Source Pin
    4) Device + pin
    5) Device - pin
    6) MOSFET2 Drain pin
    7) MOSFET2 Source Pin
    8) 0 VDC

    Then the device acts as the AND between MOSFET1 and MOSFET2. Only if both are activated, then the current will flow.

    Is that the correct interpretation?

    I was talking to a friend and he suggested that I use a couple of decoders. Then I send the signals to the decoders which activate 25 MOSFETs which activate the 25 devices.

    Which method would be preferable?
  4. Søren

    Senior Member

    Sep 2, 2006

    Almost, but you need to use both N-channel and P-channel devices and don't get 24V from gate to source.

    Like this (only two rows and two columns drawn):

    Depends on whether you want to use 5 N-ch (logic level), 5 P-ch and 5 NPN BJT's devices or 25 N-ch devices.