# How many amps.

Discussion in 'Power Supplies' started by cleevesinoz, Dec 14, 2014.

1. ### cleevesinoznewbie here

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Hi Guys,
This may be a stupid question but I am a complete novice when it comes to playing with power and my wife doesn't want me to blow myself or the house up.
I had a chat with a local electronics supplier how can supply power packs to replace blown Big W transformers and says as long as the lights don't draw more than 8 amps all will be good .
My problem is trying to work this out ,I know amps equals watts /220v but I am not sure what figures off the transformer i need to use.
it says:- input 230-240v~50-60hz
output 22v~ 10va
If I worked it out right that would mean 10/220 which equals 0.045 amps per string which seems very low.
Any help would be greatly appreciated

2. ### AAHI love blinky lights :)Community Project Designer

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Amps definitely doesn't equal watts/220.
Watts are Volts times Amps. The input current is sort of 10VA/220V but the secondary current, which is the low voltage side, 22V @ 10VA equates to 0.45A of secondary current. The primary (240V) current isn't directly proportional to the specs of 22V 10VA as small transformers like that are typically only about 70% efficient.
The term VA is used instead of Watts (which are Volt Amps) to differentiate between the output power and the input power when there is the 30% (ish) difference like there is with little transformers.

3. OP

### cleevesinoznewbie here

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I am totally confused now as a lot of websites i have read today are saying Amps equals total wattage divided by 220v

4. ### scamperDedicated Elf

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Forget about the primary side (220v) It is the secondary side you need to worry about like Alan said.
Just remember that the power companies transmit high voltage like 22,000 volts, because there is very little current and the wires can be small. (relatively) same for 240v down to 22v.
So if the secondary side is 22v at 10va. then like Alan said you are looking at around .45a

5. ### GrumpyNew Elf

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Let me chime in here…I learning lights and controllers, but I know transformers.

To properly apply a transformer or power supplies, you need to understand the current input (primary, plug side) and load (secondary, lights side)

A 10VA is 10VA primary or 10VA secondary - - minus a little for the efficiency losses.
So your 10VA / 220 = .045 amps input and 10VA / 22 = .45 amps load
Not sure how this less than ½ amp transformer is going to give you 8 amps of load.

Eight amps load would require a 10A Power Supply - or -
8 x 1.25 (1.25 multiplier is a safety factor, or service factor) X 22V = 264VA - - standard size would be 250VA, so you need to look for a 250VA or a 0.25kVA transformer. A 250VA would be about 11A lo load. Not the full 20% service factor, but good enough.

For power supplies, typically rate the secondary side only when having a variable input. To keep the wife’s kitchen running and your TV recording, you still need to know the current load of the primary side. If its’ a 5A input, its’ a 5a input, but you need a input service factor too, so consider your load as 6 to 7A. If looking at a watt load rating, ,then divide the watts by the input voltage and add 20-25%

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