| Electronics > Beginners |
| Voltmeter for negative voltage |
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| AngraMelo:
Hey guys, I built the basic LM317/LM137 dual power supply that is shown on the LM317 schematic. Works fine and Im happy with it. Now, Im trying to use the same volt/ammeter meter (that basic mini volt/ammeter pic below). I tried using a 3PDT switch to make the commutation and switch the current probes and the voltage probe. The thing does not work to measure the negative voltage, it measures current on both sides fine but only measures positive voltage. Ive spent such a long time thinking about this that Im embarassed. Switch connections pic below The thin black lead is not connected to anything The thin red lead is connected to a 7812 fed from the positive rail of the supply |
| Wimberleytech:
I found this and believe it is your meter. I think you need a 4PDT switch to solve this. |
| perieanuo:
you can do it just switch correctly the negative voltage measurement wire and voltage power line for ammeter Envoyé de mon HUAWEI NXT-L29 en utilisant Tapatalk |
| AngraMelo:
I think I got, this new way o wiring it keeps the black current lead on the ground or negative depending on the switch position and also toggles the thin yellow voltage line based on which current the meter is measuring, either from + to ground or ground to the negative rail of the PS. The volt/ammeter keeps being powered by a 7812 that is fed from the positive rail of the PS. So the meter receives 12V when measuring the positive side (given that the black current lead is not tied to ground) or receives 24V when the black current lead is tied to the negative rail. Schematic attached What do you guys think? will it work? |
| AngraMelo:
Wait, if I wire it like the post above wouldnt it be receiving 42V on the thin red wire when measuring the negative current? Because Im powering it with a 7812 that is fed by the positive rail so when measuring positive current the meter sees 12V on the thin red line. But given that the power supply is a +-30V wouldnt there be 42V between the 7812 and the negative rail? The meter can take 30V on the powering leads. Sh#t, can anyone help? |
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