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Question about a + - 30V power supply - oscilloscope [SOLVED]

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Adrien_m8:
Greetings,

I'm a good physician, but lacks many knowledge in electronics... I need to supply a servo driver with a +-18V (to +-30V) power supply.
I got a FI 1333 power supply for this task than has a mass, a - and a + output and delivers voltage from 0 to 30V.

But when I look at its output on my oscilloscope, connecting one wire on mass/ground and the other on + I don't see a continuous +voltage that i can shift but a sinusoidal oscillation. Same goes for mass and -. I only see a continuous voltage when i measure between - and +, excluding the mass.

I'd assume my servo needed -18V on one pin and +18V on the other, so a differential of 36V at least, and my power supply doesn't provide it. So do I need a ~40V power supply ? Or is my power supply enough and I just don't understand something ?
Why is the voltage indicated on the power supply not the voltage between the mass and + but the one between - and + ? If I set my power supply to 30V and connect the -, the mass, and the + to my driver will it be powered with +-15V or +-30V ?

Many thanks for your help
Adrien

ArthurDent:
It looks like your power supply can be used as a dual tracking supply (outputting equal + and - voltages) if wired correctly. Make sure it is wired similar to this one and that the two voltmeters have identical readings. It should work.

alsetalokin4017:
This ^

On this type of power supply the "mass" or GND is not connected to any of the actual power outputs, unless the user deliberately "straps" one of the outputs to the GND (mass) terminal.

So when you have your scope connected to GND (Mass) and one + or - output terminal-- you aren't seeing the power supply at all, you are actually seeing your home's mains wiring leakage.

To have the bipolar supply your servo circuit needs, use the connection above. Left channel minus is the minus output. Left channel plus and right channel minus connected together, this is your circuit's "zero volt" reference point (COMMON or GND for the servo driver). Right channel plus is the plus output. Tracking buttons select "SERIES" mode. Voltages set to 15 volts on each side. Current limit set as appropriate on each side. The power supply's "mass" or ground terminal may be left disconnected.

RTFM....    ;)

Adrien_m8:
All right ! Thanks you all for your replies.
Putting the power supply in serie mode and wiring between the left - and the right + indeed provides up to a 60V differential output, allowing me to satisfy the servo +-30V needs.

RTFM => I would gladly read it if only i had one... they tend to disappear quickly in labs... and I couldn't find it online. But who needs a manual when you've got such an active forum to help you out ;)

alsetalokin4017:
OK.... in your photo it looks like your PSU makes the series connection internally when the buttons are pressed for serial connection. So you have the two outputs connected to the scope in series, giving you the 30 volts total. But you have both channels of the scope reading the same connection (the green jumper). And you have the PSU's minus output connected to scope and PSU ground (through the scope's BNC shields.)

This isn't really the way you will connect to your servo driver. With the series connection as you have it, the PSU's Ground terminal is NOT at the center, zero voltage level of the differential. Rather, you have it connected to the minus of the differential output. The servo plug will be looking for the zero level in the two center pins, not the PSU's ground (which will be the mains ground if you have the right mains plugs.)

Since your PSU evidently makes the series connection internally, you should be able to see the Zero level at both of the "unused" terminals of the channel outputs.  You can test this with a multimeter.

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