Electronics > Beginners
Is a triac on the primary side of a transformer a good idea?
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Ian.M:
LM317HV goes up to 60V, and with external pass transistors, as many amps as you can afford to heatsink away the dissipation for!

However LM317 based 'lab' PSUs are madness over about 500mA - they are just too inefficient, with excessive voltage drop. 

I suspect the winding on the middle two pins of the secondary side is an independent secondary giving 15V AC at a lower current (from the thinner wire gauge).  That could be very useful to power the regulator control circuit floating with respect to the +output terminal. 

If you measure the primary and both secondary winding resistances*, and also the core thickness, one of our transformer experts can probably give you an estimate of its current capability on the main and auxiliary secondaries. 

When you said "55v output of a nice big audio transformer I salvaged a while back", I had visions of a brute of a transformer up around 1KVA or even higher that takes two hands and a bit of 'grunt' to pick up, not a little thing you could sit on the palm of one hand with your arm out straight.   I'm fairly disappointed!  :--

The mean DC current available after a bridge rectifier and reservoir capacitor is only 62% of the RMS current into it, and that's a fairly small transformer.  I'd guess somewhere around 10-15VA, so it may only be good for 150mA continuous DC output and maybe 300mA for a 5 minute overload rating from cold.  When the output's low enough you could tap-change down to the center tap, reconfiguring the bridge as a two diode full wave rectifier, and get 50% more output current.   If you want to get the most out of it a LM723 floating regulator based design should allow you to build a 0-50V PSU with current limiting.  Adjustable limiting is more complex - it may be worth compromising on the maximum output voltage and using a pair of LM317HV regulators in the LM317 datasheet 'Lab' psu circuit with the negative bias rail + a cooling fan run off the auxiliary secondary.  That may actually be the best simple option for turning it into something usable on the bench.

* Reasonably accurate (+/-5%) measurements of what may be very low resistances are *ESSENTIAL* if we are to estimate the relative wire gauge and thus the VA distribution between the main and auxiliary secondaries.   If you dont have a bench meter that does four wire (Kelvin connection) low ohms measurements, set up a LM317 as a 100mA current source, put 100mA DC through the secondaries (you can wire them in series) and measure the DC voltage drop across them in mV.   With that setup the resistance is ten milliohms per mV measured.
oldway:
So it's a transformer with a center tap 20V-0V-20V .... This is perfect for making a dual power supply +1.5 to + 20V and -1.5 to -20V adjustable.
Use a LM317 and a LM117.
No need for current limitation, the LM317 and 117 have their own internal protection.
The current limitation value will depend of the heatsinks you have used and of the input and output voltages with a maximum of 1.5A.
Ian.M:
I think you mean a LM317 and a LM337 for the negative rail. 

1.5A on either rail would fry that transformer - that's 30VA and its just not big enough.  If its got a thermal fuse in the primary, a sustained overload will kill it dead.  If you dont want to implement current limiting, a thermal trip would be a good idea - a sub-miniature glass bead thermistor tucked into the secondary with a dab of heatsink grease  and a comparator circuit that senses the temperature and if its over the setpoint, latches, lights a LED and drags both regulators ADJ pins to 2V the other side of the 0V rail.  If you do want to implement current limiting that's appropriate for the transformer, a LM723 based design would be simpler. 

If the O.P's interested in logic circuits, it may be worth adding an EBAY CC/CV buck module to derive an independently floating fixed +5V output from the Auxillary secondary.  My best guess at the moment is it would be good for about 200mA.
oldway:
Yes, indeed...
Sorry  :palm:
I used a LM7805 and a LM7905......
daubmaso:
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.
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