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| Is a triac on the primary side of a transformer a good idea? |
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| daubmaso:
Hi there, I've been wanting to make a variable power supply for some time now (above an lm317), and was considering either using the 12v output of some spare computer power supplies I have laying around and using a buck-boost converter or a buck converter on the 55v output of a nice big audio transformer I salvaged a while back. So I got to thinking if it were at all practical to just use a triac on the primary side of the transformer to step down the voltage. So would this be at all practical? Could it produce a stable output voltage? Would it take too much filtering if I tried to go down too low (say 3.3v)? Does this idea have any merit at all or should I just stick with a buck boost on the 12v supply? I'm sorry if the answers to these questions are obvious, or if it is a completely terrible idea. Any input would be greatly appreciated, thank you. |
| Circlotron:
--- Quote from: daubmaso on April 28, 2018, 10:51:03 am ---the 55v output of a nice big audio transformer I salvaged a while back. --- End quote --- Do you mean the 55VAC output of a mains transformer that came out of an audio amplifier? You can use a triac but make sure both the positive and the negative half cycle are exactly equal or you will get a net flow of DC in the transformer primary and possible burn it out. Use a phase control IC, not a simple light dimmer. Secondly, use a choke input filter after the rectifier so that it utilises the whole of the AC waveform coming out of the secondary. If you use only a capacitive filter the resulting short sharp current peaks of the chopped output waveform means the transformer will be greatly loaded for not much DC power output, e.g. a 100 VA transformer might only be able to supply 30 watts DC. Even with no waveform chopping a 100VA transformer can only supply typically 60 watts DC when feeding a capacitive input filter. |
| daubmaso:
Yeah it was a mains transformer and I usually get around 55V after it's rectified, it has a few other taps but most of them only get a couple of volts which I imagine was for the filament of the VFD. And for the control I was planning on just making a 0 crossing detection circuit and control the switching with one of the PICs I have laying around here. The whole plan was to make a digitally controlled power supply with rotary encoder controlled voltage and current limiting, so no the use of a light dimmer was never the intended method. I have plenty of beefy inductors and stuff from old TVs I took apart, so at least I could make use of those finally. And would this really be any different than a half bridge rectifier? And if so, would I be better off using switching transformers? I guess what I'm really asking here is not so much if it's possible to do this, but more if it's practical over traditional switching supplies. I have plenty of high power mosfets, igbts and thyristors just collecting dust in my junk bin, but I thought the triac route might be interesting to see if it was worth looking into. |
| oldway:
Hello, as this is your first post on this forum, I welcome you. To answer your question directly, no, it's not a good idea. Controlling a transformer through the primary causes a lot of problems: 1) Safety: working on circuits connected to the main's supply is dangerous, even lethal. As a beginner, you must avoid unnecessary risks. 2) highly inductive load: the phase control of highly inductive loads poses great problems of dV / dt when switching the triac. This can lead to a loss of control of the triac which goes in full conduction. 3) saturation of the transformer: any assimetry in the control causes a continuous voltage which saturates the transformer and will blow the fuse and perhaps the triac. 4) the very bad waveform of the current: with a normal transformer, the shape of the current waveform is very unfavorable: they are pulses of great amplitude and short duration ... The rms value is very high for a low average value ... It is an operation to avoid because it will heat up very hard the transformer, the triac and the rectifier. For older battery chargers that used this principle, the transformer was "special" with increased short circuit impedance. (20 to 25% instead of 7%) A far better solution would be to supply the transformer primary with a much lower voltage than the nominal voltage. |
| Ian.M:
Alternatively, working with what you've got, a switched mode buck converter (after the bridge rectifier and main reservoir cap) could be used as a preregulator to drop the secondary side DC bus to a few volts above the desired output voltage, followed by a good filter and large reservoir cap to clean up the switching hash and provide a sufficient reserve to supply load transients then finally a linear regulator to get the exact voltage and current limit required. However the devil is in the details, and unless you've got a *LOT* more PSU design experience than most of us, it wont be an easy project to get right, and the mistakes along the way are likely to involve a lot of smoke and expensive parts. Can you post a photo of the transformer with a ruler in it for scale, and measure *ALL* secondary voltages (AC V RMS please) + the primary voltage the measurements were taken at. There may be other options like commercially available DC input digitally controlled bench PSU modules. |
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