Transformers - Page 1
Forum section: electronics / beginners

author: Moondeck
author_rank: Regular Contributor
timestamp: April 25, 2014, 08:17:54 pm
content: Hi. I am designing a variable PSU and i need to know a few things about choosing a transformer:1. Does a toroidal, iron core, air core transformers work all the same?2.If i have 3 regulators, 1.5 A each do i need a 4.5A transformer? or can it be larger output (like transformed 230V to 30V 5A or does it have to be 4.5A on outputs)?3.How should i isolate a toroidal transformer? any Faradays cage needed or something?/Olgierd

author: mariush
author_rank: Super Contributor
timestamp: April 25, 2014, 08:44:41 pm
content: 1. Yes, they all work the same. There are some differences that make a particular type preferred for some applications (for example torroidals for audio, because the magnetic field is staying within the toroid), but they work the same.3. You don't have to isolate a toroidal , the magnetic field is pretty much inside the transformer. The wire is also insulated and the whole toroid is usually covered in tape or some other method for extra protection2. It depends on how you design the power supply and how you connect the regulators3.A transformer's output is defined in RMS voltage and VA ... for example 100VA. Depending on how you rectify the RMS AC voltage to DC, the amount of current can change. Typically, a bridge rectifier is used to convert the RMS AC voltage to DC, in which case the peak dc voltage becomes [1.414 x Transformer AC Voltage, minus the voltage drop on the bridge rectifier ] and the current becomes ~ 0.62 x AC current.As a real world example, if you have a 12 Vac 100VA transformer, you have a transformer that can output 100/12 = 8.33 A of AC current. However, if you use a bridge rectifier you will have a peak DC voltage of 1.414 x 12 - 1.6v voltage drop on rectifier = 15.3v and the current will be about 0.62 x 8.33A = 5.16A ... it makes sense, if you multiply 15.3 x 5.16 and add the losses in the bridge rectifier, you get about 85-90VA out of the 100VA the transformer is capable of.As for variable power supply ... it really depends how you want to design it.If you want it to be pure linear power supply, then you need quite big transformers. You also have to be careful, because linear regulators work by dissipating the voltage difference as heat, and you can't just dissipate lots of power as heat.For example, a regulator like LM337 can output 1.5A of current, but it can't do it in any conditions. For example, if you have an input voltage of 30v and you want output of 5v at 1.5A, then the regulator will dissipate (30v - 5v ) x 1.5A = 37.5w as heat, which is simply not possible (it can do about 15w maximum). Variable power supplies work around this limitation by using transformers with several taps (secondary windings) and connecting in series only as many as needed.For example, let's say you have a 24Vac transformer, split into two 12v secondary windings. Converted to DC, that's about 0-15v , 15-30v ... so if the output voltage is less than 12v dc, only the first winding is used and now you're only wasting (15v - 12v ) x 1.5A = 4.5 watts. If you want 24v, then both windings are connected in series (using a relay or mosfet or whatever) and you have 30v input, so the power dissipated is (30v-24v) x 1.5A = 9w

author: Moondeck
author_rank: Regular Contributor
timestamp: April 25, 2014, 08:57:17 pm
content: Well, thanks!About question 2, it goes like "Mains->Transforming->Rectifying->Filtering->Linear regulators like LM317 and 78xx->Out"What about a fuse?

author: sean0118
author_rank: Frequent Contributor
timestamp: April 26, 2014, 04:15:52 am
content: Quote from: Moondeck on April 25, 2014, 08:17:54 pmDo you have any isolation requirements? Some transformers are better for isolation than others.edit: I just saw #3, aren't toroidal transformers poor for isolation because they can not provide the required creepage distance?

author: Moondeck
author_rank: Regular Contributor
timestamp: April 26, 2014, 10:32:24 am
content: Umm, no, i just want to mount it in a plastic enclosure and make it isolated so it wont kill me

author: sean0118
author_rank: Frequent Contributor
timestamp: April 27, 2014, 01:01:36 am
content: yeah, but your transformer is going to isolate your output from mains. If you are just buying a transformer it should be easy to find one that is suitable, but if you are designing your own transformer then it gets harder.

author: Moondeck
author_rank: Regular Contributor
timestamp: April 28, 2014, 02:33:33 pm
content: No, i dont make my ownI just want to know what trans and fuse should i buy, like if i have 3x 1.5 A regulators should i have a 4.5A or more transfo and a 4.5A fuse to the mains?

author: kolbep
author_rank: Frequent Contributor
timestamp: April 28, 2014, 03:05:57 pm
content: If you jse a Tordial, jus make sure that the mounting bolt is not touching a conductive surface on both ends. If it does, then it acts as a shorted turn, drawing a lot of current, heating things up, andthat is not good. Unless you are making a bench heater as well....

author: mariush
author_rank: Super Contributor
timestamp: April 28, 2014, 04:04:03 pm
content: Quote from: Moondeck on April 28, 2014, 02:33:33 pmMoondeck, the answer is IT DEPENDS.A linear regulator takes an input voltage and outputs a voltage at a particular current, and will dissipate the difference between input voltage and output voltage as heat. The regulator will use the same amount of current from input, as it delivers on the output.This means that, for example, if you power a 7805 with 9 volts and you have something connected to it that "eats" 1.5A ( so 5v x 1.5A = 7.5w), then the 7805 itself will consume 1.5A from the 9v input (so it will use 9v x 1.5a = 13.5w)Now it's your job to think how to connect those 3 linear regulators, how to arrange them in a way to reduce the wasted energy.Let's take a simple example ... let's say you want an adjustable regulator to output 1.25v-9v at 1.5A (LM317), a 7809 to output 9v up to 1.5A , a 7805 to output 5v at up to 1.5A.To output a stable 9V, the LM317 regulator needs at least 11v at input. Same for the 7809 regulator. The 7805 needs at least 7v to work.So you will need a transformer that, after rectification, will provide the regulators with at least 11v DC ... that transformer would be a 10Vac or 12Vac one or higher.. Rectification will give you a peak voltage of 10Vac x 1.414 = 14v , minus about 1.5v on the bridge rectifier gives you about 12.5v ... with enough input capacitance you have 11-12.5v input.If you put the tree regulators in parallel, each will need 1.5A, so in total you have 4.5A of current... at 11-12.5v. The transformer is however labeled for 10-12Vac (depending on what you choose) so the transformer actually has to be rated for more current than 4.5A at 10Vac or 12Vac ... a formula that approximates the current is Iac / 0.62 .. so 4.5 / 0.62 = 7.25ASo the transformer would be Some AC input -> 10Vac 7.25A ( or 10x7.25 = 75VA). The primary current will be proportional to the secondary current, so if you know the current needed on secondary side, you know the current on primary side ...230v x ? current = 10v x 7.25a? = 10 x 7.25 / 230 = 0.31A ... approximately. If you have 110-120v input, then the current is obviously twice that.Now you have to keep in mind that because you have so much capacitance after the bridge rectifier, when you plug the transformer in the mains, the transformer may pull a bit more current for a bit of time, so you want to use a fuse that won't snap the instant you reach that current... those are called time delay fuses. You also want to add a bit of margin, about 10-20% of the current.So for 230v input, if I know the current is going to be 0.3A, I'd probably use a 0.4A-0.5A time delay fuse.Now, this all above matters if you put all the regulators in parallel, so the current adds up.You can also be smarter and use a transformer with two secondary windings or with a center tap, for example a 20Vac or 24 Vac with two 10Vac or 12Vac secondaries, both with equal amount of current. In this case, you could put the LM317 on one winding, and the two other regulators on the other winding.One winding will have to be capable of 1.5A and the other will have to be capable of 3A.. so overall your 24Vac will have to be capable of 3A after rectification, or about 4.8A on the label ( 3/0.62=~4.... the same amount of power (72VA), but an easier to find transformer. The fuse will be about the same because the primary side uses the same current to produce those 72VA in the secondary side.---No matter how you go, you still have three regulators that will waste a lot of energy as heat. With the 10Vac transformer above, you'll have :LM317: 11-12v in , 1.25-9V out @ 1.5A ... so 14.6w --- 3w waste7809 : 11-12v in, 9v out @ 1.5A , so 3w waste7805 : 11-12v in, 5v out @ 1.5A , so 9w wasteYou can reduce this waste by replacing the 7809 with an adjustable LM1085 linear regulator, configured for 9V. This LM1085 can output up to 3A, so instead of connecting the 7805 directly to 11v input, you can connect the 7805 to the output of LM1085, so now you have7805: 9v in, 5v out @ 1.5A , so 6w waste (you reduced the heat by 3 watts)

author: tombi
author_rank: Regular Contributor
timestamp: May 16, 2014, 10:04:38 am
content: Dumb question on a slightly tangential topic; Take a supply like one of the RIGOL DP832s with triple outputs which Dave took apart. Two outputs go from 0-30v so would they have a couple of pairs of windings per output channel (say 15x2) or would they share the same output windings across the channels?Seems hard to find Toroidal transformers with more than two secondary windings. So then how would you make your own dual channel 0-30v supply without making the regulator glow like a light-bulb (from heat)?

author: mariush
author_rank: Super Contributor
timestamp: May 16, 2014, 11:00:27 am
content: The channels are isolated, each channel has its own multiple windings. Keep in mind that the transformer also has several windings for internal operation (+/- 12v or +/-15v at low current for operation amplifiers, 5v for logic / microcontrollers, 12v or something like that for the lcd display and for its led backlight etc etc)Yes, the commercially available toroidals usually have only 2 secondary windings. Some have a third low voltage, low current winding as well.The toroidals or classic transformers in the linear power supplies are custom made, and there are companies that make such transformers on order.You can get around this by connecting multiple secondary windings in series or in parallel, to get the voltage you want. The problem with this is that (at least with the classic transformers) you have a much heavier power supplies (2 transformers = 2 x more metal and wire) and a more expensive one.

author: madires
author_rank: Super Contributor
timestamp: May 16, 2014, 11:55:08 am
content: Quote from: mariush on April 28, 2014, 04:04:03 pmI'd like to add that the LM1085 has to pass those extra 1.5A creating 3W more heat (11V in, 9V out @ 1.5A)The benefit of stacking regulators is the spreaded power loss.