Problems selecting a PCB Transformer - Page 2 - Electronic Circuits Projects Diagrams Free

**Nigel Goodwin** · 2nd January 2007, 10:02 PM

Quote:

Originally Posted by NewcastleSAR

I can't seem to marry the 15VA and the output and 1A for this transformer to one at RS. The nearest I get to it is 347-2694 (16VA) but it only supplies 0.5A.

If it's like the other transformers (and I presume it is?), then it has TWO secondary windings (this make it far more versatile), paralleling them gives you the 1A you're looking for!.

Hero999 · 2nd January 2007, 10:23 PM

Quote:

Originally Posted by NewcastleSAR

Can you recommend a good book or website location that would help me understand transformers and how to select the correct one.

Google is your best bet.

Quote:

I'm still trying to tie in the values of the transformer in the circuit to understand the VAC (in/Out) versus the VA values and the output current it can supply.

VA is Volts times Amps, a 15V 15VA transformer can supply a maximum current of 1A.

Quote:

In simplistic terms and at the risk of getting a smacked, with reference to the 220VAC/15VAC 1A transformer, is this essentially a transfomer that steps down 220 Volts AC to 15 Volts AC and can supply an output current up to 1 amp?

Yes

Quote:

I can't seem to marry the 15VA and the output and 1A for this transformer to one at RS. The nearest I get to it is 347-2694 (16VA) but it only supplies 0.5A. The next one up is the 201-8738. Its supplies 0.73A but the VA has gone up to 22VA.

Don't worry about it 1A is far more than your PIC and optocoupler emmitter will ever use, just use the 1VA or 1.5VA unit I suggested.

Quote:

In the 6V transformer circuit, is the reason for specifying the 8V (or 9V) transformer due to the 7805 regulator needing a minimum voltage of 7.5V for it to operate?

Transformers voltage are always specified as Root Mean Square (RMS), an AC voltage that will do the same work as a DC voltage.

$V_{PEAK}=sqrt{2} \times V_{RMS}$

The voltage across your smoother capacitor is the peak voltage minus the losses in the rectifier (1.4V).

$V_{OUT}=sqrt{2} \times V_{RMS}- 1.4$

Now, if you look at the datasheet for you will find that the 7805 needs at least 8V to regulate properly, therefore the output needs to be kept above 8V, so rearanging the above formula:

$V_{RMS}= \frac{V_{OUT}+1.4}{sqrt{2}}$

So your input needs to be above:
$V_{RMS}=\frac{8+1.4}{sqrt{2}}=6.64V$

Quote:

Sorry if all the questions sound dumb.

I'm sorry I don't have either the time of will to explain all of this in great detail (that would take many pages).

Don't worry about the above formulae, try googling the following terms:

Voltage regulator.
Root mean square.
Dropout voltage.
Smoothing capacitor.
Transformer.

**Nigel Goodwin** · 2nd January 2007, 10:37 PM

Quote:

Originally Posted by Hero999

Don't worry about it 1A is far more than your PIC and optocoupler emmitter will ever use, just use the 1VA or 1.5VA unit I suggested.

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

NewcastleSAR · 2nd January 2007, 11:09 PM

Hi Nigel..

I think things are staring to click together now.

Having just studied a transformer circuit on the Internet and applying the 15VAC value if I wire the two secondaries together in series, it doubles the voltage but keeps current at same level but wiring in paralled keeps voltage at same level but doubles current.

Is that correct?

Declan

Quote:

Originally Posted by Nigel Goodwin

If it's like the other transformers (and I presume it is?), then it has TWO secondary windings (this make it far more versatile), paralleling them gives you the 1A you're looking for!.

**Nigel Goodwin** · 3rd January 2007, 09:18 AM

Quote:

Originally Posted by NewcastleSAR

Hi Nigel..

I think things are staring to click together now.

Having just studied a transformer circuit on the Internet and applying the 15VAC value if I wire the two secondaries together in series, it doubles the voltage but keeps current at same level but wiring in paralled keeps voltage at same level but doubles current.

Is that correct?

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

NewcastleSAR · 3rd January 2007, 12:05 PM

Cheers Nigel..

Many thanks for your help.

Declan

Quote:

Originally Posted by Nigel Goodwin

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

Hero999 · 3rd January 2007, 08:50 PM

Quote:

Originally Posted by Nigel Goodwin

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

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?

**Nigel Goodwin** · 4th January 2007, 09:15 AM

Quote:

Originally Posted by Hero999

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.

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

NewcastleSAR · 4th January 2007, 11:37 AM

Quote:

Originally Posted by Hero999

Now this is confusing.

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

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

**Nigel Goodwin** · 4th January 2007, 11:51 AM

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

NewcastleSAR · 4th January 2007, 11:59 AM

Quote:

Originally Posted by Nigel Goodwin

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

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

**Nigel Goodwin** · 4th January 2007, 12:30 PM

Yes, the principle is OK.

NewcastleSAR · 4th January 2007, 01:52 PM

Quote:

Originally Posted by Nigel Goodwin

Yes, the principle is OK.

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

**Nigel Goodwin** · 4th January 2007, 04:10 PM

Quote:

Originally Posted by NewcastleSAR

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

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!).

Hero999 · 4th January 2007, 06:50 PM

Quote:

Originally Posted by Nigel Goodwin

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

But I suggested a 12V transformer.
$0.2 \times \sqrt 2 \times 12 = 3.4W$

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

Quote:

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!).

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.

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4th January 2007, 11:51 AM	(permalink)
Nigel Goodwin	Seeing as you're having to ask for advise on a transformer, I don't suggest you attempt redesigning the circuits!. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

4th January 2007, 12:30 PM	(permalink)
Nigel Goodwin	Yes, the principle is OK. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk