Problems selecting a PCB Transformer

Cheers Nigel..

Many thanks for your help.

Declan

Nigel Goodwin said:

Yes it is, it's important though that the windings are in phase, but it's usually obvious how to do that.

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Nigel Goodwin said:

You've not studied the circuit! - the 15V one has four seven segment LED displays, so needs a much higher capacity supply!.

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Now this is confusing.

There are two different projects here.

Which one are you refering to?

If it's the first one "Darkroom Timer 1 6V.pdf", then I stick so what I've said.

If it's the second one "DarkroomTimer Schematic.pdf" then you're right, I have made a mistake, that tiny 1.5VA transformer just won't do.

However I do feel that 1A is still a bit overkill, those LEDs will only take 7*18.6mA = 131mA (they're multiplexed aren't they?) and I can't see the rest of the circuit using much; maybe the whole thing will use 200mA maxumum.

15V isn't needed a 12V will do.

What's the point of the opto-isolator? The relay should provide all the isolation you need.

Why bother with a separate 12V supply? You can get 5V relays you know.

NewcastleSAR,
Which project do you want to build, the first of the second?

Hero999 said:

However I do feel that 1A is still a bit overkill, those LEDs will only take 7*18.6mA = 131mA (they're multiplexed aren't they?) and I can't see the rest of the circuit using much; maybe the whole thing will use 200mA maxumum.

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Assuming the figures are correct, 200mA from the 22V after rectification, is still 4.4W.

Hero999 said:

Now this is confusing.

NewcastleSAR,
Which project do you want to build, the first or the second?

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Hi Hero999

At present, I'm haven't decided which of the circuits I am going to build although I am leaning towards the 2nd circuit titled the 'DarkroomTimer Schematic.pdf'.

As both used transformers, my questions were related to assistance in attempting to understanding and select the correct type of transformer from RS Components.

Looking at both circuits in a simplistic manner for my brain, I see what you are saying in relation to the 12V section. Is there any particular reason why I couldn't use the PSU Section of the 6V Circuit (adjusting Transformer to suit) and also integrating the Solid State Relay S201S01 (superseded by S201S05) into circuit 2 ('DarkroomTimer Schematic.pdf') thus doing away with the 12V section?

Declan

Seeing as you're having to ask for advise on a transformer, I don't suggest you attempt redesigning the circuits!.

Nigel Goodwin said:

Seeing as you're having to ask for advise on a transformer, I don't suggest you attempt redesigning the circuits!.

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Hi Nigel..

Yes, I understand what you are saying and wouldn't attempt to even consider redesigning the circuit but in terms of understanding concepts, would what I said work?

Declan

Yes, the principle is OK.

Nigel Goodwin said:

Yes, the principle is OK.

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Cheers for that Nigel..

This discussion has been very enlightening. I'm going to talk to the School's Technology Teacher to get further advice from him. I did electronics years ago but to be honest, the amount of maths in it put me off and being at a young age, that wasn't hard.

Now as I approach 40, I think I would be in a better 'educational state of mind' to learn.

Many thanks

Declan

NewcastleSAR said:

I'm going to talk to the School's Technology Teacher to get further advice from him.

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Good luck! - it's EXCEEDINGLY rare to find one who knows anything about Electronics - it's usually taught by a teacher who attended a short course (1 day or something!).

Nigel Goodwin said:

Assuming the figures are correct, 200mA from the 22V after rectification, is still 4.4W.

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But I suggested a 12V transformer.
[latex]0.2 \times \sqrt 2 \times 12 = 3.4W[/latex]

So I'd recommend a 12V 300mA transformer.

Good luck! - it's EXCEEDINGLY rare to find one who knows anything about Electronics - it's usually taught by a teacher who attended a short course (1 day or something!).

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That's true, I knew more than my technology at the age of 12.

I have a feeling that these circuits were designed by a PIC expert rather than an electronics expert.

Hero999 said:

I have a feeling that these circuits were designed by a PIC expert rather than an electronics expert.

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It's hardly a big deal fitting a 1A transformer! - it will run cooler for a start!, and it's likely the designer had a transformer available.

I was thinking more about the use of a separate 12V power supply and an optoisolator as well as a relay.