AC/DC transformer VAC question? - Page 1
Forum section: electronics / beginners

author: stev.dk
author_rank: Regular Contributor
timestamp: August 26, 2012, 01:03:08 pm
content: I need to make a powersupply for my guitar pedal board, with multiple 9v outputs. Im planning on having a 230VAC EMI filter and some 230VAC-2x12VDC transformers, and 9VDC voltage regulators. I need about 1000mA @ 9VDC. All transformers i have seen is rated in VAC, and im not familiar with that, so im not sure how much i need.Can any of you guys give a quick explanation, or link to a detailed explanation of the VAC term? Thx in advanceNOTE: this post is misleading, i refer to my other post belov at:https://www.eevblog.com/forum/beginners/acdc-transformer-vac-question/msg141262/#msg141262

author: PSR B1257
author_rank: Frequent Contributor
timestamp: August 26, 2012, 01:52:08 pm
content: QuoteCorrect. More precisely it is the RMS-voltage or so called quadratic mean.Therealvoltage is by the factor sqrt(2) higher (only true for sin-wave), than the RMS-voltage.If you got for instance a transformer with 12VAC output voltage, a fullwave rectifier and aidealfiltercap, you get about 15V output voltage.If you want 9VDC output voltage, you need a 7.5VAC transformer - then you get pretty exactly 9VDC out, even without a voltage controller.QuoteDo you have any information about the tolerance of this voltage?

author: stev.dk
author_rank: Regular Contributor
timestamp: August 26, 2012, 02:22:44 pm
content: Oh i just realiced my original post might have been misleading... I was bit confused, when typed.I need a AC to AC transformer. Transforming 230VAC to 12VAC, rectifier bridge and voltage regulator 9VDC, and having an output capable of 1000mA. The transformers aren't rated at mA or Amps but by VA (volts/amps?). The VA is the part im woundering how to calculate...About the tolerance, most guitar pedals will work on a high range of voltages (only limited by the components in the pedal, usually about 12-16VDC) , but a stable voltage fairly close to 9VDC is preferable.Hope this have cleared the confusion

author: T4P
author_rank: Super Contributor
timestamp: August 26, 2012, 02:35:57 pm
content: Usually a transformer is calculated as VA as they will have voltage fluctuate across the shot as well as apparent VA and actual VABut that's another topic and oh, you do not need a EMI Filter!A EMI filter is only for a SMPS as they might impose switching frequencies back onto the mains but in a mains frequency transformer there's no need to be all paranoidIf you want a 9VDC output you need precisely a 9VAC transformer and a LDO like the LM2940But with a 12VAC Input you waste more heat with a normal linear regulator but you do not need to use LDO's or Buck Switching Regulators

author: IanB
author_rank: Super Contributor
timestamp: August 26, 2012, 02:45:47 pm
content: Quote from: PSR B1257 on August 26, 2012, 01:52:08 pmThis is not accurate. Therealvoltage is the quoted RMS voltage. For example if you connect a 12 V bulb to a 12 V RMS AC supply the bulb will light up at the correct brightness. If you pass that 12 V AC supply through a full wave rectifier and then connect it to the bulb the bulb will be slightly dimmer than before due to power losses in the rectifier. This shows that if you want to draw the rated current from an AC supply like a transformer you cannot expect more than the rated AC voltage after you convert it to DC. After various losses the DC voltage will usually be somewhat less than the AC voltage.What you get when you multiply the AC voltage by sqrt(2) is the peak voltage, but you can only obtain this voltage from the smoothing capacitors if you don't draw any current. As soon as you start to draw current you will see ripple on the supply and the ripple will reduce the available smoothed and regulated voltage.

author: IanB
author_rank: Super Contributor
timestamp: August 26, 2012, 02:48:41 pm
content: Quote from: stev.dk on August 26, 2012, 02:22:44 pmYou could obtain this using a 12 V transformer, a rectifier, filter capacitors and an LM317 regulator on a suitable heat sink. See the LM317 data sheet for examples of how to set it up.

author: stev.dk
author_rank: Regular Contributor
timestamp: August 26, 2012, 02:56:43 pm
content: Ehm, im not saying any of you are wrong, but i just wanted to know about the amp ratings of the transformer...230VAC to 12VAC, i need to draw 1000mA, how do i calculate the needed VA not to blow the thing up when drawing 1000mA?

author: PSR B1257
author_rank: Frequent Contributor
timestamp: August 26, 2012, 03:04:51 pm
content: QuoteAs I wrote...But you are right, the expressionreal voltagewas a bit dodgy. I meant withrealthe voltage you see on an oscilloscope.QuoteNo,volts/ampswould beV/AVA means V*A which is for a resistive load equal to W (watts)Quote12V*1000mA=12000mVA=12VABut you should select a transformer with a higher VA-reading, as mentioned by IanB.You still don't mention the tolerance for the voltage. Maybe you don't even need a voltage regulator.

author: stev.dk
author_rank: Regular Contributor
timestamp: August 26, 2012, 03:09:30 pm
content: Quote from: PSR B1257 on August 26, 2012, 03:04:51 pmThx a lot

author: mariush
author_rank: Super Contributor
timestamp: August 26, 2012, 03:25:25 pm
content: So like the people already said.When you rectify the AC voltage to DC you multiply by sqrt(2) ... so if you have a 230v to 12 VAC transformer rated at 12 W , it means the transformer can do 12 VAC at 1A. But the DC voltage after rectification is ideally 12v x 1.41 = 16.92v . In reality, as the current goes through 2 diodes at any moment, you have another drop of about 0.5-0.8v per diode.Therefore, you have now about 16v output when you have no load on the transformer. When you have some load, you will have about 15.5v output.But the transformer is still rated for 12 W, so now you have 15.5v at some amps = 12W ... the current is 12 w / 15.5 = 0.774 A ... so now you only have about 700 mA at 15.5vThe linear regulators have also a certain efficiency, going to 9v from 15.5 would be done at around 85% efficiency if you use a plain lm317 or something similar to the classic 7805, so you're actually looking at generating about 9v @ 600 mA from a 12 VAC 12 W transformer.You should pick a 8-10 VAC 1.5-2A ( so about 15-20 VA minimum) transformer, for example something i found very fast :http://www.digikey.com/product-detail/en/VPS16-1600/237-1260-ND/666146It's 16 VAC 1.6A or 8 VAC 3.2 A8 VAC @ 3.2 A -> rectifies to 11,3v -> drop ~ 1-1.5v to the couple of diodes -> you're left with about 9.5-10v at about 2-2.5A with a decent smoothing capacitor (I'm thinking 1000-3300uF)...Now you can use one or a couple of low drop linear regulator like MIC2941 (http://www.digikey.com/product-detail/en/MIC2941AWT/576-1138-ND/771607) which needs at most 0.35v drop to generate 9v @ 1A so as long as the input voltage won't drop below about 9.4v you will be good.Since you probably won't max out both outputs at 1A, you could use such transformer that can give you max 9.510V DC @2-2.5 A and two linear regulators to get 2 x 9V DC @ 1-1.25A eachps. You don't need to pick one that has such low margin for the regulators, you can for example pick one with 10vac 2.5a-3a for about the same price:http://www.digikey.com/product-detail/en/VPS20-1250/237-1264-ND/666150http://www.digikey.com/product-detail/en/VPP20-1500/237-1077-ND/242521This will give you a bit more leeway for the voltage drop.

author: T4P
author_rank: Super Contributor
timestamp: August 26, 2012, 04:37:30 pm
content: mariush, how many amps that goes into a regulator goes out exactly the same. Not 600mA But still about 750mA

author: IanB
author_rank: Super Contributor
timestamp: August 26, 2012, 06:52:41 pm
content: Quote from: stev.dk on August 26, 2012, 02:56:43 pmWell VA is nearly the same as watts, so 12 V and 1 A would be 12 VA. However, when you use a DC regulator the load on the transformer is not ideal and you have to increase the rating a bit to compensate. So 18 VA or 24 VA would be more suitable.However, by the time you have finished building this power supply it will be large and heavy and expensive. It would be much more practical to simply look for and buy a plug pack or wall adapter of the right rating. For instance, like this one:9 VDC 1000mA regulated switching power adapter