Long (300m) run impedance transformation
Long (300m) run impedance transformation
(OP)
thread240-218990: Long cable induction???
This question was asked and answered on the referenced thread. However, this engineering newbie didn't quite understand. I have a relay whose signal switch is about 300m from the relay. A tester (DC) "sees" the circuit as open, but the relay (240 AC) "sees" the circuit as closed. I am assuming this is the same issues as discussed in the reference thread (i.e. impedance transformation).
What I don't understand is how to solve the problem. From what I have read going DC for the signal circuit is the best solution. I would assume that I would put an AC/DC converter in my box and then change my relay to one that accepts a DC signal to switch an AC circuit?
Someone also recommended putting a resistor or capacitor to ground. If I understand this correctly, I would hook one leg of the signal circuit to a resistor and then just shunt that to neutral or ground? This is obviously a simpler solution, but does it make sense?
TIA
This question was asked and answered on the referenced thread. However, this engineering newbie didn't quite understand. I have a relay whose signal switch is about 300m from the relay. A tester (DC) "sees" the circuit as open, but the relay (240 AC) "sees" the circuit as closed. I am assuming this is the same issues as discussed in the reference thread (i.e. impedance transformation).
What I don't understand is how to solve the problem. From what I have read going DC for the signal circuit is the best solution. I would assume that I would put an AC/DC converter in my box and then change my relay to one that accepts a DC signal to switch an AC circuit?
Someone also recommended putting a resistor or capacitor to ground. If I understand this correctly, I would hook one leg of the signal circuit to a resistor and then just shunt that to neutral or ground? This is obviously a simpler solution, but does it make sense?
TIA





RE: Long (300m) run impedance transformation
DC solves the issue completely.
In the AC situation you cannot just "put a resistor to ground". You have to actually put it to ground or elsewhere in a specific spot of the system.
The problem is typically a relay in the control panel and then one side of the signal to the relay coil being run way over to a switch. Then right back to the control panel.
The long run with an AC signal appears to be a low impedance - even when the switch is open. It is the distributed capacitive coupling between the wires of the long run. The current that can flow is limited to the overall situation. If it is enough, what happens is once the switch is closed the relay coil is energized and the relay pulls in. The problem comes when the switch is opened. The low impedance is often enough to keep the coil pulled in.
The solution is a resistor across the coil in this case. So this low current is shunted so the coil can fall back out. So the value would vary, and you may have to pay attention to the power dissipation of the resistor.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Long (300m) run impedance transformation
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If we learn from our mistakes I'm getting a great education!
RE: Long (300m) run impedance transformation
That being said, is the solid state relay a "good enough" solution and if so, which is cheaper solid state or AC/DC conversion? The relay I used in the first installation cost me here in Costa Rica about $120US. How much is a suitable AC/DC converter?
RE: Long (300m) run impedance transformation
Can anyone suggest why my AC system over a much larger distance worked when using a ZELIO SR38101FU 100/24OV 6E/45? It seems like it should have the same problem, but perhaps they've put some resistor or capacitor in the box to keep this sort of problem at bay.
RE: Long (300m) run impedance transformation
I used the DC SSR solution on a cable run roughly a mile in length, i.e. a loop approaching two miles in length, sucessfully. My predecessors had tried using 24V and 36V to drive 12V relays, paralelled cores, and all manner of tricks. The SSR made the rpoblems go away. I certanly don't advocate a 2 mile loop, but when some idiot has already installed it then it's a way out.
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If we learn from our mistakes I'm getting a great education!
RE: Long (300m) run impedance transformation
Of course a power supply has its own failure points but they will all be fail OFF. The mechanical relay tends to be fail-off too. Presumably the relay's point of use is energized = system running, de-energized = system off-safe-shutdown. So the aforementioned failures all result in a safe condition. With the SSR: It can fail shorted, so I wouldn't use it in a safety position.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Long (300m) run impedance transformation
RE: Long (300m) run impedance transformation
One point. You have never actually said what this is for. If it is indeed for a safety shutdown don't use a bulb as filaments fail and so your system will revert. If it isn't safety related - have at it.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Long (300m) run impedance transformation
RE: Long (300m) run impedance transformation
Keith Cress
kcress - http://www.flaminsystems.com