0v or +5v

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Is there an advantage to 0v switching ? IE: if i had a long run of cable there may be a volt drop with 5v. Would this not be a problem with switching 0v.
 

PhilP

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Is there an advantage to 0v switching ? IE: if i had a long run of cable there may be a volt drop with 5v. Would this not be a problem with switching 0v.
The voltage drop is due to the resistance of the cable, and the current you are drawing..

V=IxR Ohm's law.

Hence we recommend larger cross-sectional cables for higher current. - Less resistance, so less voltage-drop, for a specific current.

You 'see' the same effect looking for a bad joint in your track..
A good meter will show 'full' track voltage, because a meter needs a very small current to register a voltage.
You only 'see' the voltage drop across the bad joint, when you are drawing a significant current.

Example :

5V supply, long run of cable 4 Ohms.

If your load at the end of the cable draws 1A,then Ohm's law gives a voltage drop at the load-end of;

1(amp) x 4(ohms) = 4Volts

If you are only drawing 100mA (0 1A) then the voltage drop is;

0.1(amp) x 4(ohms) = 0.4Volts

In the real-world, the resistance of your cable will be very small, and (for controls and lighting) possibly only a few Amps.

It is probably 'better' to run heavier cables, with a higher voltage, as an accessory power-bus.
Then use local regulators, to give you a local supply for the accessories.

PhilP
 

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The voltage drop is due to the resistance of the cable, and the current you are drawing..

V=IxR Ohm's law.

Hence we recommend larger cross-sectional cables for higher current. - Less resistance, so less voltage-drop, for a specific current.

You 'see' the same effect looking for a bad joint in your track..
A good meter will show 'full' track voltage, because a meter needs a very small current to register a voltage.
You only 'see' the voltage drop across the bad joint, when you are drawing a significant current.

Example :

5V supply, long run of cable 4 Ohms.

If your load at the end of the cable draws 1A,then Ohm's law gives a voltage drop at the load-end of;

1(amp) x 4(ohms) = 4Volts

If you are only drawing 100mA (0 1A) then the voltage drop is;

0.1(amp) x 4(ohms) = 0.4Volts

In the real-world, the resistance of your cable will be very small, and (for controls and lighting) possibly only a few Amps.

It is probably 'better' to run heavier cables, with a higher voltage, as an accessory power-bus.
Then use local regulators, to give you a local supply for the accessories.

PhilP
Sorry, What i should of said, if i have a 5v relay with the choice of 5v or 0v to energise the coil & lets say it was 500 meters cable length, the 5v might struggle, with 0v switching would this not be effected.
 
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So, maybe you can explain what you mean by 0v?

Are you meaning that the "smart end" is an open collector transistor that "pulls to ground" when energized? If so, there HAS to be +voltage some where else... and both ends would need a common ground.

From an electronic engineers point of view, it's more dependent on the transistors that are doing the switching, and the load that could cause voltage drop, and really it is the same.

Perhaps you could be a little more descriptive of what you are switching, and your two different scenarios.

Greg
 

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I have a relay that with the moving of a jumper it can be energised with 5v or 0v, my choice. Are you saying it wont make any difference which way i choose as there will still be a volt drop. This is just to energise the relay coil
 

Paul M

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Energised with 0 volts? Sorry I'm obviously missing something
 
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No, I'm still not getting through.

One side of the relay coil is got plus 5 volts, and the other side, ground (maybe you are calling 0 volts).

0 volts makes no sense unless it is referenced to something, for example a hamster is 0 volts.

Kidding aside, voltage has to be present, and current needs to flow.

how about showing a picture of the relay, or a part number or a circuit schematic?

perhaps this is a "solid state" relay?

Greg
 
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No, I'm still not getting through.

One side of the relay coil is got plus 5 volts, and the other side, ground (maybe you are calling 0 volts).

0 volts makes no sense unless it is referenced to something, for example a hamster is 0 volts.

Kidding aside, voltage has to be present, and current needs to flow.

how about showing a picture of the relay, or a part number or a circuit schematic?

perhaps this is a "solid state" relay?

Greg
My apologises, its a Optocoupler Relay in that you can use 5v or 0v to trigger the coil

relay10.JPG
 

My45G

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It doesn't make any difference which leg you switch. The voltage drop is caused by the current flowing in the circuit
 
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OK, so you are switching most likely a logic level, very low current.

Makes no difference. Remember that your "0v" needs to be referenced to the 0v (ground) that is powering the relay module.

Greg
 

Paul M

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It doesn't make any difference which leg you switch. The voltage drop is caused by the current flowing in the circuit
Gotcha
 

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The main difference between +ve and -ve switching is that, depending on where the switch is located is;
-with -ve switching you will have 5V at the relay input all the time and the cable will be "live" all the time.
-with +ve switching the 5V line will only be "live" when the switch is made.

Personally I prefer +ve switching as I don't have a large number of live wires running all over the place that may be damaged which may affect the power supply.

As said previously voltage drop is still present when the switch is made and current is flowing.
 

PhilP

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Power left on in the garden, combined with the inevitable slight dampness, will cause copper-tracks on pcb's to disappear over time..

This can be controlled, but is a consideration.

PhilP
 

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The Shed

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For anyone remotley interested in the description and operation of the 'jumper', those who own and operate Massoth equipment for track contacts and Feedback Modules will be familiar with the concept and principle of triggering a device.
 
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The Shed

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And, just when you thought it was safe to peek over the parapet..........2 Channel 5V Relay Module

Dual Relay Point Motor.jpg

Then there is this one, you'll have to work how to use this one, on your own!
Helpful hint, there is NO positive trigger!!

Four Relay Module.jpg
 
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