Electronics > Beginners
Transformer voltage for dual power supply (+12V / -12v)
Ian.M:
N.B. bare halogen bulbs blow if you touch them before use. They have a quartz glass envelope, and if you leave a finger print on the glass, the oils in your sweat carbonise and cause the glass to overheat + the salts in your sweat fuse with the glass and lower its melting point. Always handle with a tissue and wipe with methylated spirits or IPA after handling.
mariush:
Just go to a car or motorbike parts store and you should easily find 10w/20w/40w incandescent light bulbs , and cheap...
Otherwise, any regular store should have cheap bulbs ... for example my desk lamp uses G4 bulbs , which are 12v 20w : https://www.amazon.com/CBconcept-10XG412V20W-Halogen-20-Watt-12-Volt/dp/7542904760/ref=sr_1_3?ie=UTF8&qid=1535300104&sr=8-3&keywords=g4+bulbs
Just use some paper napkin or something if they're the halogen kind... otherwise the grease can cause problems.
Oh... and it's overkill for this project, but when you really must reduce the voltage drop on diodes an alternative would be to get rid of the diodes completely and use a chip like Linear (now Analog?) LT4320: http://www.analog.com/media/en/technical-documentation/data-sheets/4320fb.pdf
Get this chip in a DIP package along with 4 through hole mosfets on a prototyping board and you got yourself a bridge rectifier with maybe 0.1-0.2v voltage drop, depending on your mosfet selection... the only downside is the cost... a few dollars for the chip and less than a dollar for the 4 mosfets.
Or get this in a surface mount package along with 4 surface mount mosfets and you can basically have a whole bridge rectifier in a TO220 style package, as the losses would be very small at 1.5-2A current output.
floobydust:
Measure your main's voltage, all this math can be wrong.
I did a linear (audio) power supply like this and it worked great in the lab.
At the venue, the line voltage was ~5% lower and the musicians had hum coming out of the power supply, worse when stage lighting was busy. Had to change the transformer from 12.6V to 14V. Schottky rectifiers can give you another volt - all assuming your line voltage is not high. It also changes throughout the day.
The PCB you are showing does not have ideal connections to the transformer center-tap, so expect more ripple on the -ve rail.
To get less than 5mVpp output ripple, you have pay attention to the PCB trace layout.
belzrebuth:
Okay I load-tested the transfomers, here are the results..
100VA transformer with 20W g4 halogen bulb on one of the secondaries-> 12.6Vac
50VA transformer with 14W g4 halogen bulb on one of the secondaries-> 13.2Vac
The parts for the PSU pcb should be here within 3 days so I will post results then..
I've ordered 4 of the low drop diode packages IanM suggested.
I have a KBU4J here I could also test.
I'll need to get find heatsinks though..
belzrebuth:
Should I have measured both the secondaries of the 100VA transformer?
I thought about loading each with a 14W bulb.
That's 1.2A at 12V.
I could also use the 20W bulbs,then it's 1.67A for each winding but then I'm too close to the max current rating..
I could do that for a short period though since I will still be within the transformer's tolerance.
If I still get more than 12V I think it's going to be ok if I use the schottky diodes..
I'm going to probably try to turn a heater or an airconditioner on while still monitoring the voltage..
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