Do I need a galvanic decoupling between a controller and a power line?
Do I need a galvanic decoupling between a controller and a power line?
(OP)
Hi,
We would like to develop a simple temperature controller to control a resistive electrical heater. A sketch of projected schematic is attached to this message.
Does that insulating transformer (T1) make a lot of sense? Or it is a waste of money?
The maximum current through a heater (H1) is 20A. I did not estimate expected short circuit current through the heater but I assumed it would not exceed 2-3kA, while I could be wrong.
Thank you for your help!
We would like to develop a simple temperature controller to control a resistive electrical heater. A sketch of projected schematic is attached to this message.
Does that insulating transformer (T1) make a lot of sense? Or it is a waste of money?
The maximum current through a heater (H1) is 20A. I did not estimate expected short circuit current through the heater but I assumed it would not exceed 2-3kA, while I could be wrong.
Thank you for your help!





RE: Do I need a galvanic decoupling between a controller and a power line?
The transformer doesn't buy you anything in terms of touch access limitations, e.g. UL requirements. Shouldn't the secondary be grounded somewhere?
Where is the temperature sensing element? I.e., what's inside the 'PID'?
Mike Halloran
Pembroke Pines, FL, USA
RE: Do I need a galvanic decoupling between a controller and a power line?
3kA sounds low, even for a domestic installation at 230V. My domestic fault level is over 7kA which is pretty high by UK standards, but I'm not far from the substation. If it's an industrial installation you could easily exceed 3kA, unless you use an HRC fuse to limit the fault. Most semiconductor switches are protected by an ultrafast fuse designed for the purpose, although they aren't used as often as they should be because they are quite pricey. Sometimes it's cheaper to blow the silicon to protect the fuse.
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If we learn from our mistakes I'm getting a great education!
RE: Do I need a galvanic decoupling between a controller and a power line?
RE: Do I need a galvanic decoupling between a controller and a power line?
>> Shouldn't the secondary be grounded somewhere?
Mmmm... It is not planed to be grounded. Should it be? The heatsink of the SSR will be connected to PE but it is different story, I believe.
>> Where is the temperature sensing element?
PID is a standard temperature controller in DIN format, like OMRON or GEFRAN. A temperature sensor will be connected to it and will be physically placed close to the heater, while it can be anywhere. I omitted the temperature sensor in the sketch.
RE: Do I need a galvanic decoupling between a controller and a power line?
I believe, a sensor at the line potential is not a concern in this case. We are going to use something like GEFRAN 600. 230V will connect to a power supply input and a sensor will be connected to a temperature input.
The only reason for that transformer was protection of control block from surprises coming from faulty load. Problem is I do not clearly understand if a load fault can generate something dangerous for the controller from power line side.
RE: Do I need a galvanic decoupling between a controller and a power line?
RE: Do I need a galvanic decoupling between a controller and a power line?
RE: Do I need a galvanic decoupling between a controller and a power line?
RE: Do I need a galvanic decoupling between a controller and a power line?
I removed that transformer from the project as an obsolete part.