Question on transformer requirement for HVAC Controls Card 2

[ysh01]

hi,

found this requirement on power for a repeater card.

"24 VAC @ 5 VA. Utilizes a full-wave rectifier (do not ground secondary), and requires
power from a dedicated transformer"

want to understand basic of why utilising a full wave rectifier means no grounding of secondary and a dedicated transformer. can someone pls explain to me as i am totally lost.

saw other cards with same 24VAC @5VA which utilises half-wave rectifier asking for grounding of secondary and ability to share power from other transformer.

would appreciate a simple explanation as i am totally lost on this. thanks in advance.

 

[ScottyUK]

Sounds like the rectifier has one of the DC poles grounded. You can't ground one of the AC poles and also ground one of the DC poles - draw out the waveforms for a fullwave rectifier and work out which diode is conducting: you'll find that the negative pole is alternately connected to one end of winding then the other. The same applies to the positve pole.


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If we learn from our mistakes I'm getting a great education!

 

[ysh01]

still totally lost scott but thanks for your help. would appreciate simpler explanation for a dimwit like me. any way of doing this explanation simpler?

 

[ysh01]

perhaps break it into few portions. like -


1. since the control card A i assume from the specs is INTERNALLY rectifying the 24VAC to ? DC via a full wave rectifier and say we supply 24 VAC via a transformer
secondary, why is grounding of the transformer a problem?

2. why does a DEDICATED transformer be a requirement if control card A INTERNALLY rectifies 24VAC to ? DC via full wave rectifier?

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more questions might follow but if i understand these two requirements enough, it might answer my other questions. thanks again.

 

[rbulsara]

Review the circuitry and operation of a full wave rectifier, especially the "bridge" rectifier, which you most likely to have.

See for example:

http://www.play-hookey.com/ac_theory/ps_rectifiers.html

In the correct mode, two of the diodes and the load are in series.

As ScottyUK said, if a DC pole is internally grounded and you ground one of the AC legs, you will bypass one of the diodes in one half cycle and in the other cycle one the diodes will see full voltage, much in excess of its design resulting in some smoke perhaps. Dedicated ungrounded transformer avoids those scenarios.

Rafiq Bulsara

http://www.srengineersct.com

 

[DRWeig]

Try this pdf, it's from an old presentation of mine. Sorry for the quality, Windows ate the original and I had to scan a print.

The dark line on the bottom diagram indicates the short-circuit path that occurs.

Good on ya,

Goober Dave

 

[itsmoked]

It is as rbulsara said.

You can ground either the transformer,(one side of the rectifiers), OR the non transformer side,(other side of rectifiers). But, you cannot ground both sides.

Since the card maker is saying DO NOT GROUND TRANSFORMER SECONDARY they have already grounded the non-transformer side.

In circuits like this that is the most common method for various reasons.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ysh01]

many thanks dave for the diagram. and rbulsara for the link and itsmoked for the explanation.

i've gotto go thru the diagram and info and make sense out of it.

from the diagram dave provided, i would think during -ve half cycle the power supply is shorted with the internal circuitry (hidden gotcha?) but would it not happen even if don't ground externally (floating power supply).

trying to see why dedicated transformer called for now. thanks a million folks for the help so far and in advance.

 

[itsmoked]

I'm glad the drawing helped you.. I couldn't make any sense out of it

and I knew what I was looking for!


Sorry Dave..

A dedicated transformer is called for because normally one does ground the output of a control transformer. This is so the resulting circuitry can't float to some average of the local power wiring which could be something high and dangerous or at least noxious.

If you have one now that is ungrounded you could use its output and by grounding the DC side of your control card you will keep the rest of the circuitry tied to within a diode-drop of ground.

In some rare case this might cause issues with the other things sharing the same transformer. Likely not, but A possibility.

A short elsewhere could damage the card and or its rectifiers. Hence the card company saying "use an isolated transformer". They don't want to spend a bunch of bucks trying to technical-support some jungle mash-up they know nothing about or answer questions about why their card's diodes smoke or the card doesn't work.

Personally? I'd use a separate transformer.

Twelve bucks is a problem??

http://www.grainger.com/Grainger/FUNCTIONAL-DEVICES-INC-RIB-Transformer-2ETC4?Pid=search

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ysh01]

thanks itsmoked for your reply.

your clarification doesn't answer my question why can't one transformer power example 2 of these "internally grounded" control cards, or does it? i am lost on this. sorry for that and if you can explain this to me it'll b good. thanks in advance.

 

[ScottyUK]

Normally a grounding scheme will tie to earth at one, and only one, point. Multiple connections to earth result in currents flowing in unexpected paths such as through the chassis or on big systems through earth itself. It's a great way to pick up noise and create havoc with the circuit, which is why it's not done. Of course you might get away with it but for the cost of a control transformer it's not worth the risk.


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

It would put both bridge rectifiers in parallel which would probably not be good.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ysh01]

thanks all....itsmoked, how would putting both bridge rectifiers in parallel not be good? sorry, still don't understand this part.

 

[Skogsgurra]

Dear ysh01

I have followed this thread from the beginning and i do not think that much progress is being made. Have you REALLY tried to follow the diagrams and the text in the DRWeig posting 12 Aug 10 13:47?

If you REALLY did that, then no more explanation can be given to you. It is all there in plain English and very good diagrams.

Could the problem be that you do not understand the symbols in the diagrams? The spiral (curly) lines are primary and secondary windings in the transformer. A transformer is a device that takes AC voltage from one system to another system that is isolated from the first one. It also usually does something to the voltage level, too. In this case taking voltage from grid voltage to a lower voltage - like 24 V or so.

Then, there are triangles with little bars at the pointy end. Those are diodes. A diode is a rectifier element. A rectifier is an electronic component that lets current through in one direction and doesn't let current through the other way - much like a check-valve. You do understand check-valve, do you?

One symbol that is very important to understand is the ground symbol. It looks like a triangle built from three lines of differing length. That symbol doesn't mean actual ground (not possible to build houses on or plant flowers). Instead, it means an electrical potential that is connected to a grounded electrical potential which, eventually, is buried in that ground that houses are built on and where trees and flowers grow.

Now, if you have got all this. And also understood that the lines going between transformer windings (the curly lines) and the diodes (triangles with lines) and ground (three lines of differing length) are conductors (that is where current flows (the 'plumbing' of electricity if that feels better). Then some effort from your side really should make the thing understandable. If it does not, then I am afraid that your description of yourself as 'totally lost' and 'a dimwit' is correct. But that can't be? Can it?

Anyhow, just trust the manufacturer and the expertise that you have met in this thread and go buy that transformer. It hurts. Yes. But it will hurt more if you persist in not doing so.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ysh01]

Thank you all for the explanations.

 

[DRWeig]

Itsmoked,

Sorry if the diagram was not easy to follow for all -- I usually am standing there describing it with a pointer.

Quick clarification might be:

Top diagram shows all four diodes in the bridge.

2nd diagram I erased the two diodes that are not conducting during the positive half cycle of supply voltage -- in that diagram, there is no problem.

3rd diagram I erased the two diodes that are not conducting during the negative half cycle of supply voltage, and put the other two back in. Then I put a heavy line over the circuit that shows H and G meeting up at the COM terminal of the actuator. Scotty is right, it's not an absolute short circuit, there's still the two-diode forward voltage drop in there -- but it's usually enough to at least blow the primary fuse, power supply fuse, or (often) the transformer primary winding.

Happens all the time in our business -- so we sell a second power supplie and/or transformer to the folks who don't pay attention to the instructions. In fact, on our power supplies we even tell you which diode to clip out if you want to avoid all such issues (turning power supply into a half-wave device, with corresponding de-rating of output current and ripple).

As for multiple power supplies from one transformer, it's not really an issue -- we do that all the time too. Just be careful about grounding. Only one point please (or none at all if the code allows).

Good on all y'all!! I often feel like a novice when I'm in this forum (in spite of my 30+ years in commercial and industrial power and controls), so I was extra happy to see a question that I'm actually an expert with. You guys amaze me daily.

Goober Dave

 

[LionelHutz]

Why, since it's obivious you are not an electronics person and don't really understand electronics at all, can't you just accept the recommendations from the engineers that know and recommended the transformer?