Anonymous asked in Science & MathematicsEngineering · 1 decade ago

Rating of dry reed relay for a line carrying 25-50 mA current?

I'm working on a project and need the current through a wire to be switched off and on at a fast rate.

The wire will be carrying a constant current of 25 - 50 mA.

I'll be using a reed relay for this switching purpose (Is there a better option?)

What i need to know is :

1> What should be the rating of the reed relay ?

2> Is contact protection essential in this situation ?

3> If it is essential, how should it be done ? using a diode or a series RC snubber ?

4> In case i' m using a diode, is a single diode 1N4007 (general purpose rectifier diode) across the relay enough ? Or should I use some other diode ?

5> In case i'm using a RC snubber, can somebody please explain how it should it be done, or post a suitable link regarding the same.(i have searched, btw, and am LOST).

6> Also if possible, can someone explain or post a link regarding the working and connection of a dry reed relay.


Ok, ill give you some more details.

My project includes a landline telephone.Now, it is illegal (and may be a cause of harm to telephone company property) to connect any unauthorised device.

The wire (in my question) is acting as the phone line,(because i dont want to take chances connecting anything to the phone) which operates with a voltage of about 48V(i'll be using a much smaller voltage, may be 9V) and a current of 25-50mA.

By using pulse dialling (read here for more details: i.e. opening and closing a switch in series with the phone line, a number can be dialled.

For example, to "dial" number 9, the switch will have to be closed 9 times in a second.

The pulses of current so generated through the wire will be seen on an LED(simulating the outgoing telephone signal).

Do let me know the solution, or whether any more information is required.

2 Answers

  • 1 decade ago
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    You mention only the steady state current, not how the current reaches this value. The contact protection depends almost as much on whether the load is resistive, inductive or capacitive and linear or non-linear than it does on the load current. You also don't mention how much voltage the contacts must block when they open. And "a fast rate" is not a spec. how many cycles per second? A hundred, a thousand, a million per second?

    Add these details and, perhaps, I can offer a useful suggestion.


    Okay, almost any reed relay is fast enough for pulse dialing that has a maximum pulse rate on the order of 10 pulses per second.

    And the impedance of the load is on the order of 600 ohms, since that is about the impedance of the lines. I would expect a reasonable life on the order of 100,000 to 1 million pulses without any contact protection.3000 to 30,000 numbers dialed).

    The only thing you might add is a diode (a 1N4148, or other small signal diode) across the coil, though it will very slightly slow the coil dis-energize time. Adding a resistor about equal too the coil resistance in series with the diode will give almost as much speed as allowing the voltage to reverse spike without the diode, but will limit the coil voltage during the dis-energize to about the normal on voltage, and this will add to the supply voltage that the coil driver must withstand. For instance, if you are using a 5 volt relay coil with a resistance of 1000 ohms, and you parallel the coil with a diode in series with a 1k resistor, the coil will add about an additional 5 volts to the 5 volt supply (10 volts peak running quickly down to 5 volts) that whatever is driving the coil must withstand as the coil is turned off (a transistor I assume).



    John Popelish

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  • 1 decade ago

    A reed (or any other kind of) relay is not the ideal device for switching on and off in rapid repetition. It will have a limited lifespan, though it will do for a short-term project. Why not use a transistor as the switch instead?

    If you have to use a relay you do not need to worry about protecting the contacts. The sensitive bit is the transistor you use to drive the coil, which needs protecting by putting a diode across the coil - the 1N4007 will do.

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