Driving a 5v relay from a 5-18vac source.

Discussion in 'The Projects Forum' started by Miles1962, Oct 30, 2009.

  1. Miles1962

    Thread Starter New Member

    Oct 30, 2009
    Hello, this is my first posting to this forum, looks like there is a lot of knowledge here, maybe you can help me with a basic circuit problem.

    I am designing a model railroad train detection circuit that will turn on a 5v relay when 18vac is sensed on a pair of wires coming from the track. The 18vac will signal that a train is on track. What i need is something simple that will watch for the 18vac and switch the relay (5v?) on without drawing too much power, and as a plus, work at lower ac voltages as well.

    Here are some relay specs i am considering:
    5v Dip Relay 57ohm coil, or 12v Dip Relay 740ohm coil.

    Currently, forgive the pun, i have a circuit that rectifies the 18vac, regulates to 5v and powers a small relay. But the regulator generates too much heat at 18vdc, and i was hoping for a simpler, more efficient solution.

    I suspect that you might recommend a transistor type circuit that gives an "on" output when the circuit input ranges between 5 and 18 volts AC. Problem is, i am not savvy enough about such circuits to design one on my own.

    Thanks in advance.

  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    First, the 5V 57Ω relay is not very sensitive; it takes 0.44W to operate it. The 12V, 740Ω one takes only 0.19W.

    Second, do you want to operate the relay off the track voltage directly, or can you power the relay circuit through a separate power source, and then only "sense" the track voltage?
    Last edited: Oct 31, 2009
  3. Miles1962

    Thread Starter New Member

    Oct 30, 2009
    Howdy, yes the circuit is currently connected to the track 3rd rail, so that when a train enters the area, ac flows down the sensing wires to my circuit. But I do have 12vdc onboard, so keep that in mind.

    I only used that 5vdc relay because my rectified and regulated voltage would trip at around 4vac which was nice. But the heat made by the regulator was quite noticeable at 18vac track power.

    The 12vdc detector output would need to be strong enough to drive 3 relays of 12v and 320 ohms, and 6 LEDs. In my circuit, i used the 5v relay to switch that 12v output on and off. If this is a little too much, i'll just use the output of one 12v relay to do all that, so all i really need is a solution that puts out a 12v signal when voltage at the track goes above 5vac.

    Just a dumb question:
    I just found some 5 vdc relays with coils of 360 ohms, would the 7805 regulator run cooler if i used this relay instead?

    Thanks in advance,
    Last edited: Nov 2, 2009
  4. soda

    Senior Member

    Dec 7, 2008
    Connect a TIP3055 to the output of the regulator and put it on a heat sink about 3inch by 3inch(75mm) . The base go to the regulator output, collector to the input of the regulator and the emitter is then the output. Just add a fuse to the output because if you short the leads,the transistor will go.

  5. Miles1962

    Thread Starter New Member

    Oct 30, 2009
    Looks like that would work, but would i still have my excess heat issue? Per the recommendation ,I am considering using the more sensitive relay with the 720ohm coil, that should help.
  6. tkng211

    AAC Fanatic!

    Jan 4, 2008
    Take the rough calculaltion about the heat dissipation on the regulator as follows,
    Voltage across the regulator: 18-5=13V
    Current flows through the circuit/relays: 5/57=88mA, 5/360=14mA, 5/720=7mA
    Approx. heat dissipation on the regulator: 13x.o88=1.15W, 13x.014=0.18W, 13x.007=0.09W.
    Based on the above , I believethe 5V relay witth 360 ohm can meet your requirement in heat problem.