| Electronics > Beginners |
| DIY 0-30V 0-3A ...Again |
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| Jwillis:
Something like this to control voltage for pin 7 .Since that pin gets its voltage directly for the rectifier ,changing the voltage to something like 28 or 30 volts should be plenty but or interfere with the rest of the circuit. |
| Cliff Matthews:
Using 3 separate zener's could produce temperature drift. U1 doesn't need any help, but pin 4 of U2 and U3 might work fine with a 2.7 or 3.1v zener at D7.. |
| Jwillis:
--- Quote from: Cliff Matthews on January 07, 2019, 12:44:13 am ---Using 3 separate zener's could produce temperature drift. U1 doesn't need any help, but pin 4 of U2 and U3 might work fine with a 2.7 or 3.1v zener at D7.. --- End quote --- I was wondering because zeners of higher values >5.6V tend to have positive temperature coefficients as compared to lower value zeners <5.6 volt which tend to have a negative coefficient. So wouldn't that reduce the possible drift because It's in the upper voltage range? |
| Kleinstein:
The only important zener is D8. The other zeners are only for the supply of the OPs. So there TC does not really matter. U1 and U3 don't need a high positive voltage, so they could run from the 12 V regulator. The zener diodes should go the other side of the shunt, not to include the zener currents in the current limit. How much negative supply is needed depends on the OPs used. U3 may need a little more, to safely get it's output to about -0.8 V and work with an input near or slightly below ground. With the TL081 from the kit the 3 V lower limit (though often better) for the input common mode range can be a problem - to be on the safe side an extra diode from the output of U3 to ground could help here, by preventing the output to go too negative on transients. |
| Audioguru:
Why are you adding Zener diodes instead of using the correct 44V opamps, a -1.3V supply for them and a 28V transformer like the improved version of this power supply? The output transistors are simply emitter followers that have some voltage loss and no voltage gain. If you use a 30V Zener diode for U2 then the maximum output from the project will be only about +20V at 2A, instead of 30V at 3A. If you want to use the low voltage TL081 opamps that need a negative supply of -5V then use an 18VAC transformer instead of adding Zener diodes to reduce the voltage. Then the single output transistor will not overheat as much. |
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