| Electronics > Beginners |
| Is a triac on the primary side of a transformer a good idea? |
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| Ian.M:
30VA is far too optimistic until one of our transformer gurus has run the numbers for you given exact core dimensions, winding resistances, and voltages etc. I'm not that level of expert (although I can hum the tune if I've got a crib-sheet), but its a similar size to a Hammond Transformers 12VA single secondary model with the same primary configuration, but with an extra lower current auxiliary secondary and I therefore estimate you may get 10VA out of the center tapped (main) secondary. One way forward without the experts would be to measure the exact DC primary resistance, then hook it up with a 160 ohm >10W resistive load across the outer terminals of the CT secondary, and switch it on, briefly switching it off every 5 minutes at first then every 15 after the first 15 to measure the primary resistance again so you can estimate the average winding temperature using the temperature coefficient of resistivity for copper (see http://hyperphysics.phy-astr.gsu.edu/hbase/electric/restmp.html ). Keep it under 115 deg C as you are are measuring an average for the whole winding and there may be hot-spots. If after a couple of hours its still running cool enough, you can estimate how many more VA it can handle, from Newton's law of cooling, and the knowledge that resistive losses are proportional to the SQUARE of the current. If it starts to overheat, STOP, plot your readings, estimate the final temperature and scale back the VA rating accordingly. Finally retest with a load appropriate to the revised VA rating, finishing with an eight hour soak test, measuring the resistance hourly. Caution: if it has a thermal fuse in the primary with a low trip point this procedure may kill it. |
| oldway:
--- Quote from: daubmaso on April 29, 2018, 08:37:40 am ---By a big transformer, I mostly meant it's the biggest I've found so far. I have access to a much bigger one, probably 5 times the volume, but it's in an old receiver that I'm still using. I was hoping to get at least 2-3A out of the supply, and that's why I brought up the use of a buck-boost converter with some computer power supplies I have (I somehow managed to come across 4). I was hoping that transformer could dish out more current though, I didnt realizes it would only be about 30VA. I am just a second year physics student who likes to mess around with microcontrollers and robots, so I haven't learned too much about AC circuits, so I'm sorry that I'm not too savvy with this stuff. But anyone have any recommendations for a buck boost converter that would be able to handle 3-5A, or at least have the option of an external switching transistor? I think I might be better off with the computer power supply if I want this type of current capability. --- End quote --- As a student, you must learn that the method you follow to analyze and solve a problem is completely wrong. Before specifying what you want to do, you must first analyze what you have at your disposal to know the possible limits to your desires. You do the opposite, you say what you want, then, you try to match what you have to your specifications ... This is completely wrong. A transformer has very specific limits and especially with regard to its heating. If this is not respected, the transformer may be damaged and become unusable. EDIT: Another remark: someone who studies in the field of science or technology can not be satisfied with an imprecise and subjective language. We must adopt a scientific and technical language .... a huge amount of gas, a large transformer, a huge current, a high temperature, ... all this is not scientific and does not mean anything! .... It should be specified: for example, a quantity of gas of 10L per minute, a transformer of 500VA, a current of 2000A, a temperature of 250 ° C, .... |
| Circlotron:
Stick the tranny on a set of scales and weigh it. That should give us a good starting point as to guessing it's VA rating. |
| daubmaso:
Yes, I know I haven't been very precise, and I never was really intending to be very formal here with the way I present my questions. The reason I've been doing this is because I really was just curious if, as the name suggests, it was possible to drive the primary of a transformer to make a crude power supply, not so much discuss the details of the end product. But yes you're right I should be more precise with my wording, especially in a more technical setting such as this forum. I originally planned on using the transformer to make a 1.25-25v lm317 based power supply where current and voltage control would be done with an atmega328p microcontroller, quite similar to the design Dave made for his micro supply. I later changed my mind and wanted to use a switching regulator so that I could make it more efficient and increase the current capability at the lower voltage outputs. I never thought 5A at 25V was possible, but maybe down at 3.3-5V. Ideas that I thought of to design a switching based power supply were a tracking buck regulator with a linear output stage. This got me wondering if I needed the transformer at all, or if I could make use of some 300-400W PSUs I have. Using a tracking boost converter and a linear output stage came to mind, with either the 12V or 5V output of the PSU. Other ideas included a fixed boost regulator to around 35V and a buck regulator to change it from there, or a boost-buck switching regulator to try to reduce complexity. The main control IC I've decided on is a dsPIC33EP16GS506 because of its 12 bit ADC and 2 integrated 12 bit DAC outputs. Beyond this, I haven't decided on any switching or linear regulators. I'm aiming for 30v out max, although a greater output voltage would be more preferable, and a 3-5A output current. Do any of the afformentioned topologies posses the capability of creating the desired output without any major issues? Or should I abandon the higher output current and stick with an lm317? And thank you if you are still willing to answer my questions. --- Quote from: Circlotron on April 29, 2018, 11:30:57 am ---Stick the tranny on a set of scales and weigh it. That should give us a good starting point as to guessing it's VA rating. --- End quote --- I've heard this as a method before, but was never clear on the numbers. I'll certainly weigh it in the morning, thanks. |
| Jwillis:
Just a quick look you have approximately 4 square centimeter core .That will get you around 12 VA maybe . That,s around 300mA at 40VAC. If you want 3 amps at 20 volt you'll need around 150VA .The core area would be around 14 or 15 square centimeters . |
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