why have a switch in the 0v side ? 1

[DTSFA]

hi

what are the advantages, if any, of inserting control switches in the negative side of control circuit?

( say you have 24v dc relay, and a control switch is inserted in the negative side of the relay )

for testing, a field control switch, I can see there may be sligth advantage,

all comments welcome ( sensible ones that is ! )

 

[VEBill]

If the 'switch' that controls the relay is a transistor, then it's often much simpler to use an NPN bipolar switching transistor in the ground seeking side of the circuit. Easy to drive the transistor's base, referenced straight to ground.

 

[BrianG]

Another reason is that if the switch, either mechanical or transistor, is in the positive supply side any wiring fault between the switch and the load (e.g. relay) to the grounded negative rail can result in a short circuit on the positive supply. Unless the individual circuit is protected by a fuse this can cause lots of damage to either the power supply, the switch, the wiring, or all three. Note: don't rely on fuses to protect solid state switches under these conditions as they usually don't blow quickly enough.

 

[VEBill]

Also, Google-up the phrase 'High Side Driver' to see what's sometimes involved (the complications) in the option of switching the 'hot' side of your +24VDCrelay.

The advantage of high side switching can be human safety - e.g. in AC circuits one never switches just the neutral side as it would leave the load wiring 'hot'. (Except I've seen just that within some products like water heaters and air exchanger.)

 

[DTSFA]

BrianG

that a good point about shorting,

I will google 'High Side Driver' ,

I am sure I read something about less 'stress' on mechanical operated ( I mean non-silicon type ) switches, if they are used in the 0v line ? ( less arching , perhaps ? I am only guessing here )

 

[dArsonval]

It's what your "logic", or "circuit" requires to function.
Curious to know if this is the "Conventional Flow" verses "Electron Flow" question?
If so... I'm staying tuned.

John

 

[VEBill]

It can be informative to perform some thought experiments.

Imagine a series circuit of your 24v battery, your load and your mechanical switch... ...all floating in the interstellar void. Please feel free to reverse the battery polarity at any time, or even just replace it with a self-contained AC voltage source.

Next, you can swap the positions of the load and switch, or (to save yourself the time and effort of rewiring) just move yourself to look at the circuit from the opposite side. Read that again if you missed it.

Next, someone joins you in the interstellar void with one end of a VERY long single wire. The distant end of this wire is, he assures you, connected to the dirt of a planet. You can connect this single ended wire to various nodes of the circuit.

Finally, the ghost of Ben Franklin shows up to apologize for getting conventional current bassackwards to the electron flow. For bonus points calculate the average electron speed in a 14-ga wire powering a 60-watt light bulb. And then explain why the light doesn't require three minutes to turn on. Maybe Ben was actually right?

 

[DTSFA]

It's what your "logic", or "circuit" requires to function

good point,

we use a lot of safety interlocks , for elevator controls, and some are in 0v leg/line.


is there an advantage to have switch in 0v line, from a safety switch perspective ?

 

[bacon4life]

In substations DC supply is ungrounded so that it continues to function after the first unintentional ground connection, so we use isolation switches in both the positive and negative leg. We also use breakers/fuses in both legs.

Corrosion can also be a problem for equipment connected to a negative ground battery, thus the use of positive grounded communication gear. This type of gear would of course use switches in the negative side.

 

[cranky108]

How about testing? Is it possible to test the circuit without this switch?