Question about a + - 30V power supply - oscilloscope [SOLVED] - Page 1
Forum section: electronics / beginners

author: Adrien_m8
author_rank: Newbie
timestamp: July 26, 2018, 08:55:36 am
content: 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 aFI 1333power 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 helpAdrien

author: ArthurDent
author_rank: Super Contributor
timestamp: July 26, 2018, 12:43:20 pm
content: 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.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: July 26, 2018, 01:09:29 pm
content: 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....

author: Adrien_m8
author_rank: Newbie
timestamp: July 26, 2018, 03:31:32 pm
content: 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

author: alsetalokin4017
author_rank: Super Contributor
timestamp: July 26, 2018, 04:08:29 pm
content: 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.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: July 26, 2018, 04:19:09 pm
content: It appears that this may be a GWInstek unit.https://produktinfo.conrad.com/datenblaetter/500000-524999/511529-an-01-en-ZWEIF_LABORNETZGERAET_GPS_2303.pdfGPS 2303Quote

author: ArthurDent
author_rank: Super Contributor
timestamp: July 26, 2018, 06:50:35 pm
content: If you check the diagram I posted in reply #2 you will see that is not how you wired your supply. The diagram I posted shows that the ground (mass, earth) connection on that supply is to the far right and is not used. The '+' of the left supply is connected to the '-' terminal of the right supply and that is your common (return) of the two supplies. As others have pointed out, neither the left or the right power supply channels is connected to ground and you don't need to connect any wire to the ground terminal.Having the switches set to the series connection allows the master supply knob to control both channels but you should have a jumper between the '+' of the left supply and the '-' terminal of the right supply and this is also your common connection of the two supplies.The following is basically an ad but it is short and gives you a quick visual on how to make the the connection and set up a similar supply.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: July 26, 2018, 11:34:33 pm
content: Arthur, I am 99 percent sure that this power supply automatically makes the series connection internally (see the excerpt from the manual I quoted above.) So it is not strictly necessary to jumper externally when the buttons are pressed for "Series". The "zero" level is available at both of the "unused" terminals of the two PSU channels. This can easily be checked with a DMM.You are of course completely right about the PSU's "Ground" jack; this would normally be NOT CONNECTED at all for this application.I'm a little worried since the OP has marked this thread "Solved" when it really isn't, according to his photo of the connections.

author: ArthurDent
author_rank: Super Contributor
timestamp: July 27, 2018, 12:08:23 am
content: Alsetalokin4017, I agree there is a high probability that the switches connect the two channels. The supply you show and the one the OP has are 'probably' the same internally and just relabeled but the output connectors are different so there is a possibility, even slight, that there may be other changes as well. Although the switched common is easily checked with a ohmmeter I would still use a jumper also as a quick visual reminder of how the supply is wired even if it is redundant.I did cringe a little at the SOLVED when I saw the common lead plugged into the ground jack.

author: Adrien_m8
author_rank: Newbie
timestamp: July 27, 2018, 09:08:26 am
content: Sorry for misunderstanding my picture was just for showing I had a -15 tp +15V voltage, indeed the ground is on the unused parts (left plus and right minus).For clarification, this is how I intend to wire my pins (picture attached).

author: ArthurDent
author_rank: Super Contributor
timestamp: July 27, 2018, 11:23:17 am
content: Ah, the wiring shown in your last photo is correct and I would now agree with SOLVED

author: alsetalokin4017
author_rank: Super Contributor
timestamp: July 27, 2018, 03:06:27 pm
content: I would double check the polarity of the servo connector.Using Green/Yellow wire for the Positive and Red wire for the Negative is a little confusing to me...And it is a little suspicious that the PSU's "overload" LED is on, but the LED on the servo driver board isn't... ?

author: ArthurDent
author_rank: Super Contributor
timestamp: July 27, 2018, 03:36:14 pm
content: I'm just assuming that + goes to + and - goes to - even though the colors are not what I would use either. The overload light I believe says '5v 3A' which may mean that there is a problem with the 3rd output from the supply and that shouldn't be a problem for the dual tracking outputs but should be looked into.

author: In Vacuo Veritas
author_rank: Frequent Contributor
timestamp: July 27, 2018, 07:42:32 pm
content: Don't your patients get upset that you're building this in their room instead of taking their temperature?