FAQ Toroid Low voltage power
Contents
AC Power for Low Voltage Light Strings.
Low voltage in this article will refer to all AC voltages under 50v. Whilst these may be available in other parts of the world this initial article will be for Australian Christmas Lights people.
Australia
Lets get some important stuff out of the way.
IMPORTANT NOTE: Mains power IS lethal, if not used correctly. If you do not have experience and knowledge about mains connected equipment you must let others do this work.
WORKING WITH ALL VOLTAGES CAN BE DANGEROUS, CARE SHOULD BE TAKEN AT ALL TIMES AND IF YOU ARE NOT SURE THEN DON'T.
FIXED 240V AC WIRING MOST BE DONE BY A QUALIFIED AND LICENSED ELECTRICIAN. Fixed AC wiring is that stuff behind the power points in your house or garage.
Kit's and hobby projects containing 240v may be constructed by hobbiest people and may have a normal power cable wired in that has the appropriate 3 pin connector, or a IEC socket may be used in the project case. This is a hotly debated topic and may not be true for all Australian states, if anyone can quote actual chapter and verse please PM Aussiephil at DLA to update this. Correct earthing of metal chassis cases should be implemented.
Please add any extra legal or safety issues needed.
DETAILS CONTAINED BELOW IS FOR INFORMATIONAL PURPOSES.
General
The most common voltages seen would be 24v and 36v nominal AC volts. Other voltages are used in Austraila such as 12v and 18v.
Light strings in Oz normally come with the appropriate plug pack (wall wart – transformer) for the number of bulbs and voltage requirments of the strings.
As we use SSR's to actually switch the AC to the light strings the SSR will need to go between the plug pack and the light string so there will be some wiring required.
As there are so many options this article will deal with the most common voltage of 24v.
24vAC light strings will typically have 210 to 260 bulbs per string with a 30VA transformer supplied.
Longer strings of 400 and 800 bulbs will have larger transformers supplied
So for every 2100 to 2600 bulbs you would have 10 plug packs equal to 300VA of power.
Now you could buy lots of power boards and use the plug packs to power the SSR's but this is neither practical or sensible.
It is normal to replace those little plug packs with a single larger transformer. The most widely used due to both price and availability are the 300VA (watts) units from Jaycar, DSE or Altronics
How do i work it out
Now the easy bit, as suggested by Synnie in a thread at DIYC, grab all your existing plug packs, add up all the VA ratings listed on them and write down the total. All caculations will assume the worst case of all lights being turned on.
Example 1. You have the following:
- 24 x 30VA = 720VA
- 2 x 60VA = 120VA
This gives a total of 840VA required.
Now take the 300VA from the Toroid and divide into 840 to give 2.8. As you can not buy 0.8 of a transformer we better round that up to 3 required. People will say to then add some over head but the original transformers already had a safety margin so it may actually be possible to run more strings per toroid than the simple calculations above indicate.
Example 2.
- 40 x 30VA = 1200VA
- Equals 4 * 300VA Toroids.
Example 3.
- 10 x 24VA = 240VA
- 35 x 30VA = 1050VA
- 4 x 80VA = 320VA
Total 1610VA. 1610/300 = 6 So six toroids needed.
Shops like Jaycar will normally have price breaks for multiple units with up to 20% of for buying 4 or more, so it is important to work out how many you need and make a good guess as to any future requirments. It is almost assured that you will acquire more lights!
Wiring them up:
OK you have identified you need four transformers and have gone and bought them, what now.
Two factors need to be considered. How many SSR boards do you need to power. How far will the cabling need to run.
The common practice in OZ is to mount all the transfomers in one place and run the low voltage power to the SSR's. This has the benefit that they can be mounted inside out of the weather and simplifies the 240v wiring a little. It has a downside of voltage drop if the power feeds are too long and have high currents.
Lets assume that the you plan to put them all inside. The practice that is seen often, is to wire all the 240v inputs in parallel and to wire all the outputs in parallel so that you have one big transformer. Whilst “wrong” is not the right term it's pretty close.
Issues
Issue 1. Toroids transformers have significant inrush currents when they are first turned on and can be measured into the hundreds of amps for 1KVA units, what 4*300VA? So turning on all four from the one power switch may actually be enough to start tripping breakers on you main board. Solution wire each to it's own power switch and flick each one on in turn, better yet a set of time delayed high power triacs could be used to electronically turn on the toroids at the start of the night. Issue 2. Wiring the ouputs in parallel. This is done all the time by many people and yet all the technical information you will find recommends that this is not good practice as there is no simple way to ensure that the load is shared equally between the transformers.
Due to both of these issues this author can not recommend wiring these up in parallel.
So what's best. Best is to document the estimated current draw of all your light strings and the SSR's they will be connected to. This could be in VA or straight Amps whichever is easier.
The easy way is to say we have about 1A /ch for a SSR, so 4A total, this equals around 4 * 250 bulb strings. A 300VA will deliver around 12Amps at 25v so we can have maybe 3 SSR's per transformer. Wow you say this is getting expensive. That is why documenting expect current is better as you can maybe run more SSR's on each transformer
Let's deal with some direct questions asked over at DIYC
1.How do I work out how many transformers I need - will one do or is there a formula to work out how many I need?
- That has been dealt with above, and is as much a science and calculation as it is a guess, always buy more then you need as your lights will grow.
2.I've come across enough discussion about the best places to get them, but are there different types? What type do I need and would question 1 answer this for me?
- Toroid Transformers are the ones to use, mainly because they are the easiest to get hold of. Any transformer that converts 240v to 24v at the required current will work though.
3.What parts do I need other than the electric donut?
- The bare minimum would be some 240v power cables to connect to the input side of the transformer and to plug into the power point and some solder and electrical tape on the outputs.
- Better setup would be to mount one or more transformers into a case and add a IEC power socket as used on computer power supplies for each transformer. On the output side add a fuse connector inline with each transformer them add some form of power distribution conectors. A simple PCB could be made for this.
4.Is there anything special about putting it together?
- The diagrams included in this article should answer this question, the main thing is to ensure that you don't overload any one transformer.
5.Does it just plug into the wall or does it wire into the mains somehow?
- Answer 3 covers that, it is best to build it in a kit/hobbiest manner and have it plug it.
6.How does it connect to the lights?
- Covered in the article.
7.FINALLY and most importantly, do I need to get an electrician to do anything?
- You only need an electrician if you are doing fixed cabling as discussed at the top of the page, however if you are unsure then do NOT continue and ask an electrician to help connect the transformers up.
- This is a hotly debated topic and may not be true for all Australian states, if anyone can quote actual chapter and verse please PM Aussiephil at DLA to update this.
Wiring Calculations and Sizing
- Wire Sizes common in Australia
- Voltage drop calculations and just why they are important
- Some ideas to choosing the correct cables
