Very simple circuit review

[SMUGangsta]

Hi

I am now getting fed up soldering/de-soldering bits, and my breadboard works great - but some of my components dont fit in my breadboard - an example being relays.

I want to make a little box with 4 relays in it, using banana connectors for an input, output, and a common earth.

The circut that will be used is attached, i just wanted to know if I need to add anything due to the fact that the relays will be mounted remotely (up to 300mm) away from the signal.

The input will be direct from a pic (or similar) or other 5v source (will this circuit work with other voltages, would be ideal if it would operate from a 3v to mabey 12v input)

The output will be 12v

Will noise on the connecting lead cause problems and if so how can i combat it?

thanks

 

[crutschow]

Physically connect D1 directly at the transistor drain lead so you will suppress any inductive spike from the interconnect lines as well as the relay inductance. Add a 0.001µF cap from Q1's drain lead to ground to provide a path for the inductive spike of the wiring.

Place a 10kΩ resistor in series with the input (Q1's gate) and add a 0.1µF cap from Q1's gate to ground the to suppress any control input noise and spikes.

The operating voltage will depend upon the gain and gate threshold voltage of Q1. Since the maximum threshold voltage of the 2N7000 is 3V it won't work reliably for that input voltage. With 5V (or above) control input the maximum Q1 drain current is 200mA so that's the maximum relay coil current you can control.

 

[SMUGangsta]

Thats Brilliant, thanks crutschow

I will redraw the circuit with your recommendations, and post it up, that way we'll know if I understood properly.

 

[SMUGangsta]

This is roughly what I am trying to achieve, if you have any suggestion to make it better, please let me know.

The 230v outputs will be fused on the back of the box, as will the internal 12v outputs

 

[SMUGangsta]

just redone the circuit, with the modifications suggested.

Do i need anything else?

And will D4, D7 be enough to prevent 12v flowing back into the external circuit? eg. im using a pic and I connect an output pin to the input of this circuit - when i press the 'test' button - will the pic be safe?

Thanks

 

[crutschow]

You didn't post a schematic with D4 and D7 so I can't answer your question.

 

[SMUGangsta]

Sorry, I have added it to the post now, was sure I attached it

 

[crutschow]

A few things:

1. The 100nF capacitors should be connected to the gates, not the inputs. That gives an RC filter at each gate.

2. The diode will provide isolation on the input so the PIC is safe but it will reduce the gate voltage by about 0.7V so the maximum transistor current will be less. That may be okay depending on how much current the relay coils draw. What is the relay coil resistance?

3. I forgot to have you add 1 megohm resistors from each transistor gate to ground to sink any leakage current and maintain the transistor off with no input.

 

[SMUGangsta]

Have uploaded a revised diagram.

The coil resistance is 87.03Ω for the CHANNEL A and B, will have to go home to measure the mains relays.

Thats 0.138Amps that the coil will draw, is that right?

Thanks for your help so far

 

[crutschow]

Diagram looks good.

That's the coil current for 12VDC. It could be more than that depending upon your transformer voltage. A 12.6VAC transformer will usually generate over 16VDC unloaded from a rectifier-capacitor converter giving a coil current of about 0.184A.

You could add a 7812 regulator to give 12VDC regulated from the rectifier output.

 

[SMUGangsta]

Thanks, thats awsome!

I will look at adding the 7812, but only for the control section, but leave the output unregulated (if switched to internal 12v) as it will most likely just be used for light bulbs, and mabey some *small* motors etc.