Circuit design

Syntax99 · Jun 4, 2018

Hi everyone.
I’m new to plc and am in the process of designing a plc test/learner so that I can self study at home.
I have purchased an S7 1200 with a separate 24v- psu and a tp400 hmi.

My first question.
Is it normal practice to fit MCB’s for each mains powered consumer and in the case of the PSU do I also need to fit DC MCB’s on the outputs to protect its consumers.

Second question.
Do I need to fit AC surpression to all AC control / latching relays, and DC surpression to each 24v- relay?

Thanks in advance

Syntax99 · Jun 5, 2018

Any help would b really appreciated

janner_10 · Jun 5, 2018

Just fit an MCB for 240v to the power supply, rated to the cable size which is in turn rated to the draw of the unit.

You can fit a fuse on the 24v supply to the plc, i personally wouldn't bother at home.

Again, I wouldn't bother with suppression on 24v relays.

Syntax99 · Jun 5, 2018

janner_10 said:
Just fit an MCB for 240v to the power supply, rated to the cable size which is in turn rated to the draw of the unit.

You can fit a fuse on the 24v supply to the plc, i personally wouldn't bother at home.

Again, I wouldn't bother with suppression on 24v relays.

Thanks for the reply.

I have got the recommended MCBs for both AC consumers. But Siemens recommend one at 10A or 16A b or c curve because they have 20A inrush current. The fact that they both use less than a few amps in normal use concerns me.

I had intended using fuses on 24v supply lines but couldn’t find any low amp DC fuses to fit din rail fused terminals

so I am considering 2A mcbs

I was considering putting clamping diodes across each dc relay coil, and thyristors on the ac ones.

I would like to build the plc to Industrie standards if possible.

Thanks once again for your reply

tragically1969 · Jun 5, 2018

Syntax99 said:
I would like to build the plc to Industrie standards if possible.

That entirely depends on the industry.

Syntax99 · Jun 5, 2018

tragically1969 said:
That entirely depends on the industry.

Hi

Just to clarify. By industry standards I mean that I want to incorporate common industry design and safety elements.

tragically1969 · Jun 5, 2018

Syntax99 said:
Hi

Just to clarify. By industry standards I mean that I want to incorporate common industry design and safety elements.

Well the industry standard for a test bed would be not to put any suppression on relays, or worry about fusing every element ;-)

But if you want to use suppressors/snubbers then go for it, or buy relays with it built in.

Follow the manufacturers guidelines on fusing/MCB's and you wont go far wrong.

Mad_Poet · Jun 5, 2018

As I recall, adding a reverse biased diode across the relay coil is a
pretty much 'standard practice'. This is done so when the coil is
'turned off' and the magnetic field collapses it doesn't induce
a current to blow out the output transistor.

Why? Try this site :
https://electronics.stackexchange.c...a-diode-connected-in-parallel-to-a-relay-coil

In most electrical circuits with a relay, a diode is connected in parallel to the coil of the relay. Why?

Since an inductor (the relay coil) cannot change it's current instantly, the flyback diode provides a path for the current when the coil is switched off. Otherwise, a voltage spike will occur causing arcing on switch contacts or possibly destroying switching transistors.

Is it always a good practice?

Usually, but not always. If the relay coil is driven by AC, a bi-directional TVS-diode (or some other voltage clamp) and/or a snubber (series RC) need to be used. A diode would not work in this case as it would act as a short-circuit during the negative half-cycle of the AC. (See also Red Lion SNUB0000 for application info)

For DC driven relays, a diode is usually used, but not always.

>>>
(Article does continue. Poet.)

Syntax99 · Jun 5, 2018

tragically1969 said:
well the industry standard for a test bed would be not to put any suppression on relays, or worry about fusing every element ;-)

but if you want to use suppressors/snubbers then go for it, or buy relays with it built in.

Follow the manufacturers guidelines on fusing/mcb's and you wont go far wrong.

..

Syntax99 · Jun 5, 2018

Mad_Poet said:
As I recall, adding a reverse biased diode across the relay coil is a
pretty much 'standard practice'. This is done so when the coil is
'turned off' and the magnetic field collapses it doesn't induce
a current to blow out the output transistor.

View attachment 47881

Why? Try this site :
https://electronics.stackexchange.c...a-diode-connected-in-parallel-to-a-relay-coil

In most electrical circuits with a relay, a diode is connected in parallel to the coil of the relay. Why?

Since an inductor (the relay coil) cannot change it's current instantly, the flyback diode provides a path for the current when the coil is switched off. Otherwise, a voltage spike will occur causing arcing on switch contacts or possibly destroying switching transistors.

Is it always a good practice?

Usually, but not always. If the relay coil is driven by AC, a bi-directional TVS-diode (or some other voltage clamp) and/or a snubber (series RC) need to be used. A diode would not work in this case as it would act as a short-circuit during the negative half-cycle of the AC. (See also Red Lion SNUB0000 for application info)

For DC driven relays, a diode is usually used, but not always.

>>>
(Article does continue. Poet.)

Thank very much for the info MadPoet

janner_10 · Jun 5, 2018

I imagine a home test bed to be a few lamps, switches and perhaps an analogue input if feeling flush.

If you're considering something with an inrush current of 20amps, then that's a mighty substantial test bed.

Expect journeys to the consumer unit!

Syntax99 · Jun 5, 2018

janner_10 said:
I imagine a home test bed to be a few lamps, switches and perhaps an analogue input if feeling flush.

If you're considering something with an inrush current of 20amps, then that's a mighty substantial test bed.

Expect journeys to the consumer unit!

My workbench has an inbuilt rcd.
My test bed has an rcbo and mcb’s

got to keep my other half happy. God forbid the electric cuts out in the middle of the soaps

Circuit design

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