| Electronics > Beginners |
| Help me design a PSU |
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| rstofer:
There is mention of a 48 VAC transformer above and the output, when rectified, is about 62V (plus or minus the filter capacitor and current draw/voltage drop. Nevertheless, a really big number. When the PS is outputting, say, 1A at 1V, the pass transistor(s) drops 62V - 1V and dissipates 61 Watts (more or less). That's also a really big number. That's why real power supplies use transformers with tapped secondaries and some number of relays that kick in at various voltage settings. If you want to output 5V, there is no point in having more than 8v DC going into the pass transistor (more or less). Clearly, not 62V... |
| Efe_114:
--- Quote from: rstofer on November 14, 2018, 02:28:40 pm ---There is mention of a 48 VAC transformer above and the output, when rectified, is about 62V (plus or minus the filter capacitor and current draw/voltage drop. Nevertheless, a really big number. When the PS is outputting, say, 1A at 1V, the pass transistor(s) drops 62V - 1V and dissipates 61 Watts (more or less). That's also a really big number. That's why real power supplies use transformers with tapped secondaries and some number of relays that kick in at various voltage settings. If you want to output 5V, there is no point in having more than 8v DC going into the pass transistor (more or less). Clearly, not 62V... --- End quote --- thats more than some heaters i cant imagine running it at full power 3A 1v which would create 183 watts heat my aquarium heater is 60 watt :palm: |
| Efe_114:
I dint know how but i made a lm317 circuit go under 1.25 volt without anything I just connected like this: Pot pins at the outwards go to ground and output Slider goes to adj Vcc goes to Vin I wonder how did it work i will probe everywhere and test it under load when (if) i find out whats going on i will post here |
| wraper:
--- Quote from: Efe_114 on November 14, 2018, 03:56:05 pm ---I dint know how but i made a lm317 circuit go under 1.25 volt without anything I just connected like this: Pot pins at the outwards go to ground and output Slider goes to adj Vcc goes to Vin I wonder how did it work i will probe everywhere and test it under load when (if) i find out whats going on i will post here --- End quote --- Dunno what you made but LM317 won't go below 1.25V without negative voltage on ADJ pin. |
| VEGETA:
--- Quote from: Efe_114 on November 14, 2018, 03:56:05 pm ---I dint know how but i made a lm317 circuit go under 1.25 volt without anything I just connected like this: Pot pins at the outwards go to ground and output Slider goes to adj Vcc goes to Vin I wonder how did it work i will probe everywhere and test it under load when (if) i find out whats going on i will post here --- End quote --- That cannot be, please post schematic and pictures if possible. I explained to you why it happens and you could see its internal schematic to see it. --- Quote ---Dunno what you made but LM317 won't go below 1.25V without negative voltage on ADJ pin. --- End quote --- Yes because you need to compensate for the +1.25v of its internal reference. |
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