| Electronics > Beginners |
| Transformer specs. (Answered) |
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| spec:
--- Quote from: MUDGEL on November 16, 2018, 04:38:06 pm ---I want to build a linear power supply. I’m in Australia so mains voltage is 240 VAC, 10 Amps. I want to have an output of 30 VDC, 5 Amps. What transformer specs are required to meet my desired output? I did a google search using a variety of the keywords used here but don’t understand about Volt Amps and their significance in spec’ing my desired transformer. By all means just tell me to get “xyz” transformer or teach me how to work it out for myself. I hope I asked the correct way. Thanks. Mike V. --- End quote --- Hi Mike, That is some lab PSU you are aiming at 8) Can I ask why you want to build it. Is it just for the experience or do you just need a power supply to use? If it is for the latter, as the other members have indicated, it is much better to buy one. You can get a 0V to 30V, 0V to 10A accurate linear lab PSU with digital read out for £50UK or the equivalent in Oz (a guy on another forum from NZ bought one and I have three). To answer your question about the transformer specification here it is: 40V RMS at 5A RMS if you go for the common E/H transformer. That means a 200W rating. But I would advise to get two 20V secondaries at 5A (see pre switching below). It does not matter if the two secondary windings are connected or not. But, best go for a toroidal transformer. They have a better electrical performance, are more compact, and have less stray magnetic fields. With a toroidal transformer you may have to limit the inrush current on the primary at switch on, but that is a simple matter. I suppose you know that you will need a pretty hefty reservoir capacitor to keep the ripple voltage down and to handle the ripple current. You will also need a very low thermal resistance heatsink with additional fan cooling. Without any kind of primary supply switching you will need six output power transistors. With 50% pre switching you will need three output power transistors. A good bridge rectifier spec is : 20A upwards at 100V upwards. If you are wondering why 20A for a 5A supply, it is to limit the forward voltage drop when the huge gulps of current are charging the reservoir capacitor. If you can make a bridge rectifier from Schottky diodes, even better. If you want to know how to calculate/guesstimate the transformer parameters, the reservoir capacitor parameters, and even the transistor/heatsink parameters just shout and I will run through them. It is pretty boring stuff though. :palm: |
| IanB:
--- Quote from: MUDGEL on November 16, 2018, 10:26:14 pm ---Sorry! I neglected to mention that the transformer will be used in a lab bench power supply. The range is 0-30VDC in 0.01 Volt inc/decrement & 0-5A in 0.01A inc/decrement. If I can do it I would like to be able to provide miilivolt, milliamp values at least below 1V, 1A if not the full range. I’m not asking for instructions on a whole build just the transformer specs or how to derive them. Thanks for all the useful input thus far. Mike V --- End quote --- As someone already mentioned, a lab power supply with these specs purchased off the shelf would cost you an arm and a leg, and maybe a kidney too. It would have taken expert electronics engineers many hours to design and test and debug. This project is way off the scale for a hobby build. You do not seem to understand what you are contemplating. If you want to succeed I recommend you to set your sights a bit lower. Go for something like 0-15 V and 0-1 A and you will have a much better chance of success. |
| MUDGEL:
I already have a cheap and cheerful Chinese made 0-30V, 0-5A linear power supply. Korad KA3005P It works fine, especially for the price. I want to challenge myself not by assembling a kit but actually building an LPS from the ground up. Thanks to those who helped and encouraged me. I think I have what I need now. Clearly I have a lot to learn so I’ll go continue doing that now. |
| IanB:
--- Quote from: MUDGEL on November 17, 2018, 08:51:11 am ---I already have a cheap and cheerful Chinese made 0-30V, 0-5A linear power supply. Korad KA3005P It works fine, especially for the price. I want to challenge myself not by assembling a kit but actually building an LPS from the ground up. Thanks to those who helped and encouraged me. I think I have what I need now. Clearly I have a lot to learn so I’ll go continue doing that now. --- End quote --- OK, cool. You could learn a lot about how to build such a thing by disassembling your Korad and reverse engineering it. Look at the transformer and see what windings and voltages it has, trace out the circuit board and draw the schematic, look at what kind of pass elements are used, what size heat sinks, what cooling fans and air flow arrangements, etc. Electrically, look at the precision at which you can set and regulate the voltage and current. If you have an oscilloscope look at the regulation under different kinds of loads and load changes, also the regulation and response time at switch on and switch off. See if you can find what limitations it has that let it be designed down to the price. |
| mariush:
if you want to make something fancy, you can replace the bridge rectifier with a LT4320 and four mosfets : https://www.analog.com/media/en/technical-documentation/data-sheets/4320fb.pdf Saves you around 8-10 watts of heat when you're close to 5A of output, and around 1-2v of voltage drop (that would be across the diodes in the rectifier). As for the transformer, yeah, see the post above with the hammond link ... use 0.62 for a reasonable approximation. If you want 30v out and 5A, then you're looking at around 35v dc before going through transistors so let's say a peak of 40v dc ... that's around 28v AC ... so you'll need a 2 x 15v AC transformer or a 2x18v AC transformer. So you want 5A ... with the 0.62 factor, that means you need something around 8A .... so you're looking at 2x15v 8A = ~ 240VA (round it up to 250VA) or 2x18v 8A = 288VA (round it up to 300VA) So either of those two you mentioned in a previous post would work, if they're 300VA. You'll probably also want a separate 2 x 15-18v ac transformer for +/-15v for opamps, 12v, 5v etc |
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