18v AC/5v DC Power Supply

[Hayato]

Hello.
You don't need full wave rectifiers just for turning leds on. A half wave will do the job, fine.

20VA is too low, you gonna have only 13W at a power factor of 0.65. 13W means about 0.65A at 20V.
If you want 20V at 2.5A, you'll need, at least, 77VA (as Stevez said).
If you want 20V at 4A (2.5 + 1.5), you'll need, at least, 123VA.

You can substitute the 2 1000uF caps for a 2200uF only. And the 4 2200 ohms resistors for a 560 ohms only.

Tell me something, why do you need to turn so many leds on?

 

[angie1199]

The 2.5 amp load at 0.65 power factor and 20 volts (that's what you'll be delivering) is 77 VA. The 1.5 amp load is 46 VA. The challenge - is the load continuous? If it is the answer is easy - if not and you want to get by with a smaller transformer you'll need to determine the load based on the expected operation of the circuit. Note that you can reduce the 46 VA part by going to a switching supply as it is more efficient.

Hi Stevez

Thanks for the reply. I've contacted the tech help from an electronics supplier and they say for what I need 100VA will do, giving 18v AC @2.5A max and 5v DC at 1.5A max.

So I think that's where I'#ll leave it.

Thanks for your help

Angie

 

[angie1199]

Hello.
You don't need full wave rectifiers just for turning leds on. A half wave will do the job, fine.

They flicker when they're on half wave, and for an extra few pennies I think I'll go with the full wave.

Tell me something, why do you need to turn so many leds on?

The LED panel is a control panel indicating the status of signalling points. Each LED is tri-color to indicate 'main route' or 'turnout' using only two colors.

Thanks for your help.
Angie

 

[Hayato]

They flicker when they're on half wave, and for an extra few pennies I think I'll go with the full wave.

The LED panel is a control panel indicating the status of signalling points. Each LED is tri-color to indicate 'main route' or 'turnout' using only two colors.

Thanks for your help.
Angie

Well, they will flick, but you won't notice.
The human eye can distinguish up to 36 frames per second.

The led will blink 60 times per second, so you won't notice it is blinking.

 

[angie1199]

Well, they will flick, but you won't notice.
The human eye can distinguish up to 36 frames per second.

The led will blink 60 times per second, so you won't notice it is blinking.

LOL, but I did notice it! That's why I'm going the full wave route

 

[Hayato]

LOL, but I did notice it! That's why I'm going the full wave route

Wow.
Eagle eyes.

Don't forget to add ballast resistors, to limit the current passing through the leds.

 

[Nigel Goodwin]

Wow.
Eagle eyes.

Bear in mind he's in the UK, 50Hz mains, so 25 flashes per second with half wave, as opposed to 30 (NOT 60) with your 60Hz mains.

 

[Hayato]

Bear in mind he's in the UK, 50Hz mains, so 25 flashes per second with half wave, as opposed to 30 (NOT 60) with your 60Hz mains.

Hello,
Nigel, I have to disagree.
In 1 cycle, you have 1 crest and 1 valley (trough).
So in 1 cycle, you are going to have 2 peaks, one positive and one negative.

In 1 second with a 60 Hz wave you have 120 peaks (60 positive and 60 negatives) or 60 crests and 60 valleys. If you put a diode there, you are going to "ignore" one of those peaks, then you are going to remain with 60 crests or 60 valleys.
Take a look at a 10 Hz graph:

 

[Nigel Goodwin]

Hello,
Nigel, I have to disagree.
In 1 cycle, you have 1 crest and 1 valley (trough).
So in 1 cycle, you are going to have 2 peaks, one positive and one negative.

Very sorry, you are quite right

With half wave rectified AC you get one peak, and one valley (of NO voltage) per cycle (giving 50/60), if you fullwave rectify then the negative peak fills the empty valley (giving 100/120)

Still it's easier to see 50Hz ripple than 60Hz.

 

[Hayato]

Still it's easier to see 50Hz ripple than 60Hz.

Yes, but they are british, and the british do not fear ripple.

 

[Nigel Goodwin]

Yes, but they are british, and the british do not fear ripple.

Perhaps this lost something in the translation?, or is it just me?.

 

[Hayato]

Perhaps this lost something in the translation?, or is it just me?.

No, I was just joking about. Grand part of the world uses 60Hz, and the british use 50Hz.

 

[angie1199]

I think I'll stick with Hayato's first comment, 'Eagle eyes'

 

[Nigel Goodwin]

No, I was just joking about. Grand part of the world uses 60Hz, and the british use 50Hz.

We've had the 50/60 discussion before, there are good reasons (or at least acceptable reasons!) for both choices - but it's not just Britain that uses 50Hz, much of the rest of the world does as well - it's hard to say which is most common?.

 

[stevez]

25 Hz was a standard in the US and believe it or not some industrial customers still use it.

 

[Hayato]

We've had the 50/60 discussion before, there are good reasons (or at least acceptable reasons!) for both choices - but it's not just Britain that uses 50Hz, much of the rest of the world does as well - it's hard to say which is most common?.

Yes, you are right.
My joke was unfortunate.

 

[Hero999]

25 Hz was a standard in the US and believe it or not some industrial customers still use it.

I've read that somewhere on Wikipedia, all transformers would have to be twice the size and the same for motors which would run at half the speed. There would be less losses in distribution but the distribution system would cost twice as much.

 

[Optikon]

We've had the 50/60 discussion before, there are good reasons (or at least acceptable reasons!) for both choices - but it's not just Britain that uses 50Hz, much of the rest of the world does as well - it's hard to say which is most common?.

It's not hard to say, I looked it up. Here's the deal:

Out of all countries world-wide with industrialized power distribution:

Exclusive 50Hz = 110 total countries.
Exclusive 60Hz = 29 total countries.
Both 50Hz & 60Hz = 7 total countries.

Hehehe... 2 countries support direct current.

So by almost 4 to 1 50Hz is most popular.

 

[OutToLunch]

was there any breakdown by population?

i mean, if 110 countries the size of rhode island are using 50Hz, then it may not be the most popular.

 

[Nigel Goodwin]

was there any breakdown by population?

i mean, if 110 countries the size of rhode island are using 50Hz, then it may not be the most popular.

I think a LOT depends what China uses?

Interesting breakdown by countries, but as suggested population would be helpful as well?

A quick Wikipedia https://en.wikipedia.org/wiki/List_of_countries_with_mains_power_plugs%2C_voltages_and_frequencies makes it quite clear with a nice coloured map!.