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Basic Electronics » Relays and Controlling them

May 08, 2012
by exussum
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Hi guys,

Just putting together a relay and while i can control it with the nerdkit no problem, if i pulse it quickly i lose power to the nerd kits display.

I assume its a similar problem to the Motor problem where i need a flyback diode ? Or is it something else.

The relay coil is 100 ohms of resistance and if left on the display works fine. Just wondering the best way to wire this ?

Thanks

May 08, 2012
by exussum
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as a better help

https://www.circuitlab.com/circuit/666zy9/relay/

Is what i have where SW1 is a psychical switch, SW2 is nerd kit, and 240V is AC.

Physical switch is a manual over ride. so regardless of what the nerd kit is doing the relay will be on. any help is appreciated

May 08, 2012
by rajabalu21
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Yes. You will need to add a diode across the relay coil. Please see this.

-Raja

May 08, 2012
by exussum
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Thanks. It seems to be working how ever fast i switch it now. For the physical switch ive got just 2 wires, 1 from the +5 rail and 1 to ground, Should i put a resistor in or is the 100 ohms of the coil enough ?

Thanks for your help

May 08, 2012
by rajabalu21
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As per your circuit diagram, you have one physical switch SW1 in series the +ve 5 volt and the relay coil. The relay is connected to the ground directly. If your relay is rated for 5V then you do not have to add any resistor. I presume your SW2 from nerdkit is driven by a MOSFET/BJT transistor and not directly from the microcontroller. If so all is well.

-Raja

May 10, 2012
by exussum
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Im running it though a 2N7000L but when i switch on, The voltage only hits around 2.2v - i need at least 4, Any reason for this ? Connecting the + rail to Source, one of the pins to gate, and the last to the relay.

Is there any reason for the voltage drop ? to be so big (same wire directly to the + rail works)

May 10, 2012
by exussum
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Connecting after the relay works fine, Why does it not work before the Relay ?

I can post pics if that helps

May 10, 2012
by sask55
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Without seeing the setup I am quesing. You may have the 2N7000 between the load(your relay coil) and the + 5 V rail. That is what is indicated in the circuit lab symatic and will work fine with simple manual switches. The mosfet should be between the load and the ground. ie control the current flow to the relay coil with the mosfet on the low voltage side of the load.

[servo] (http://www.nerdkits.com/videos/servosquirter/) and the Nerdkits Guild for more detail.

May 10, 2012
by sask55
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The link was incorrect

servo and the Nerdkits Guild for more detail

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