Test and Measurement Forum

Problems when running off external DC supply

Post any questions you may have about our current range of oscilloscopes

Problems when running off external DC supply

Postby DarcyW » Thu Sep 17, 2015 4:01 am

I have a 3-4 week old 3406D MSO that seems to pack a fit when I use a 3m USB3.0 cable with it... On the 2m cable it works fine. In both cases I'm (supposedly) running the scope from the external adapter.

But... When I plug in the adapter there's no power indicator on the scope - until I plug in the USB cable. This just seems a bit weird. Should there be some kind of indication that external power is being used?

I've checked the supply and I'm seeing 5.2V out... but without hooking it up to an electronic load I'm unable to test if it's entirely functional. Equally, the problem could be internal to the scope.

I can't see in the user manual what to expect when running the scope from an external supply before I plug in the USB cable.

I'm surprised to be having this sort of problem when the device is so new! Any suggestions please guys?

Thanks
Darcy
DarcyW
Newbie
 
Posts: 0
Joined: Tue Aug 25, 2015 11:33 pm

Re: Problems when running off external DC supply

Postby Hitesh » Thu Sep 17, 2015 11:33 am

Hi Darcy,

The light on the front of the scope will not come on until it has got a USB connection to the PC as it is only then that the device can begin operation.

Does the scope work ok if you use the supplied USB 3.0 cable instead of your 3m USb 3.0 cable?

Regards,
Hitesh

Technical Specialist
Pico Technology
Hitesh
Site Admin
Site Admin
 
Posts: 2063
Joined: Tue May 31, 2011 3:43 pm
Location: St. Neots, Cambridgeshire

Re: Problems when running off external DC supply

Postby DarcyW » Thu Sep 17, 2015 10:52 pm

Hi Hitesh

Hitesh wrote:The light on the front of the scope will not come on until it has got a USB connection to the PC as it is only then that the device can begin operation.


That might be worth improving for future devices... otherwise how do I actually know it's running off external power?

Hitesh wrote:Does the scope work ok if you use the supplied USB 3.0 cable instead of your 3m USb 3.0 cable?

Yes, the scope works using the 2m cable provided. Other devices work using the 3m cable, just not the PicoScope, which is what had us wondering about the whole thing... So, then I found a high-power USB3 port on my laptop dock, plugged the 3m cable in to that and I'm good to go again.

So, our current options are...
a) The provided power supply is faulty, but still outputs ~5V2 (I can put this on an electronic load next week to see if it falls over under load), or
b) The scope itself has an internal fault

Using the PicoScope software, do I have a way of determining the external power supply status/voltage?

Thanks
Darcy
DarcyW
Newbie
 
Posts: 0
Joined: Tue Aug 25, 2015 11:33 pm

Re: Problems when running off external DC supply

Postby Hitesh » Fri Sep 18, 2015 9:53 am

Hi Darcy,

I have forwarded your comment to our Development Team.

Longer USB 3.0 cables may have an issue with voltage drop so the high-power USB 3.0 port looks like it has helped with overcoming this.

The driver can indicate if a power supply is connected or if a USB 3.0 device is connected to a USB 2.0 port so depending on your PicoScope 6 settings, there may be a prompt to connect a power supply or use a USB 3.0 port if it detects otherwise.

The power supply is there to ensure that sufficient current is supplied to the device if required. From the specifications:

Powered from a single USB 3.0 port or two USB 2.0 ports (dual cable supplied).
4-channel models: AC adaptor included for use with USB ports that supply less than 1200 mA


Hope this helps.
Hitesh

Technical Specialist
Pico Technology
Hitesh
Site Admin
Site Admin
 
Posts: 2063
Joined: Tue May 31, 2011 3:43 pm
Location: St. Neots, Cambridgeshire

Re: Problems when running off external DC supply

Postby DarcyW » Mon Sep 21, 2015 8:58 pm

Hi Hitesh,

It appears a contributing issue here is that we were not being presented with the "Power Options" dialog when we should have been! I unplugged with power adapter while plugged in to USB (we'd done this multiple times the day before) and up came a power options dialog warning about low power! I have no idea why this wasn't showing up earlier.

Right... I've just tried to reproduce this.

1 - Open PicoScope software, do not use demo scope. Plug in scope to power adapter, use 3m USB3 cable to plug in to standard USB3 port on laptop. Device fails to properly appear in device manager. PicoScope PC software briefly shows "opening devices" before that small box closes and we go back to the blank PicoScope interface with no frames etc (i.e. same view as when selecting not to use a demo device). No device in device manager

2 - Do the same thing again, but this time without the external power adapter. Same symptoms. No device in device manager.

3 - This time, no power adapter, and close PicoScope PC software. Plug in 3m USB cable to standard USB3 port. PicoScope 3000 correctly appears in device manager. Now, start up PicoScope software - device is removed from device manager. Software complains it cannot find a device.

4 - PC software closed. Plug in power adapter. Plug in 3m USB cable to standard USB3 port. Device correctly appears in device manager. Open PicoScope Software. Device disappears from device manager. Software complains no suitable device can be found. This needs to be fixed. This should work.

5 - PC software closed. No power adapter. Plug in cable to battery charging USB2 port. Device appears in device manager. Open software. I am now presented with a "Power Options" dialog that tells me about the different ways to power the device. Selecting to operate from USB power fails with the message "Insufficient USB power"

6 - No power adapter. Plug in 2m USB3 cable to USB3 port. Device appears in device manager (same as #3, but with 2m cable). Open software. Now presented with "power options" dialog. Plug in external power - device operates as expected.

7 - Same as #6, but 3m USB cable in to same standard USB3 port. This is now a repeat of #3. No suitable device found. No display of power options dialog. Device removed from device manager.

8 - Same as #6 but with power adapter connected before starting PC software and 3m USB3 cable. Device removed from device manager upon starting PicoScope PC software. No suitable devices found. I just worked out this is a repeat of #4... This should work!

So, from all this we can only assume there is some kind of internal fault with this device. It fails to run off USB3 ports when using a 3m cable with the external power adapter plugged in. Unless you've got something else for us to try we'll be returning this device as faulty.

Cheers
Darcy
DarcyW
Newbie
 
Posts: 0
Joined: Tue Aug 25, 2015 11:33 pm

Re: Problems when running off external DC supply

Postby DarcyW » Wed Sep 23, 2015 1:20 am

Hi guys, before I arrange to send this back to our distributor, can you please confirm whether this is unexpected operation, or functioning as expected?

Thanks
D.
DarcyW
Newbie
 
Posts: 0
Joined: Tue Aug 25, 2015 11:33 pm

Re: Problems when running off external DC supply

Postby Hitesh » Wed Sep 23, 2015 9:26 am

Hi Darcy,

Thank you for the detailed steps describing the problem.

If you are able to operate the PicoScope with the supplied 1.8m USB 3.0 cable with/without the external power supply then this is expected behaviour.

When you connect the device to the PC, the USB interface should enable the Vendor and Product IDs of the device to be found and the device is displayed in the Windows Device Manager.

When the PicoScope 6 software is started, the software will go looking for devices and display a list of devices that have been found if there is more than one. When a device is opened, the firmware will be downloaded to the device, and the device will briefly be disconnected and reconnected in order to restart before it begins to operate. At this point it may become a bit more power hungry depending on the settings applied at start up (e.g. number of enabled channels, sampling rate etc.).

What could be happening is that the 3m USB 3.0 cable is providing insufficient power at start-up. We provide cables with a higher core to prevent voltage drop. The PC end of the USB cable also has the ground and shielding connected together.

Is there a particular requirement for a 3m long connection from the PicoScope to the PC?

The option to display the power options dialog is stored in the preferences.xml file (see section 3 of the PicoScope 6 Self-Help Guide on how to locate the trace.xml file as this file will be in the same directory).

Open the file in a text editor such as Notepad and look for the following line:
Code: Select all
<preference name="switchtousbpowerdialogsuppressed" value="True" />

Change "True" to "False" and save the file then start PicoScope 6 and the dialog will be shown in future if the driver reports that

If you wish to discuss the above further, please e-mail support@picotech.com and I will be happy to assist.

Best wishes,
Hitesh

Technical Specialist
Pico Technology
Hitesh
Site Admin
Site Admin
 
Posts: 2063
Joined: Tue May 31, 2011 3:43 pm
Location: St. Neots, Cambridgeshire

Re: Problems when running off external DC supply

Postby DarcyW » Wed Sep 23, 2015 8:41 pm

Hi Hitesh, I'm replying on the forums so that this is captured for anyone else who may be experiencing similar problems.

Hitesh wrote:If you are able to operate the PicoScope with the supplied 1.8m USB 3.0 cable with/without the external power supply then this is expected behaviour.

Given the cable and scope are both USB3 compliant, and I'm using the external power supply, whether I use a 1.8m cable or a 3m cable is somewhat irrelevant.

Hitesh wrote:When the PicoScope 6 software is started, the software will go looking for devices and display a list of devices that have been found if there is more than one. When a device is opened, the firmware will be downloaded to the device, and the device will briefly be disconnected and reconnected in order to restart before it begins to operate. At this point it may become a bit more power hungry depending on the settings applied at start up (e.g. number of enabled channels, sampling rate etc.).

This makes sense... That's why I have the external power supply plugged in.

Hitesh wrote:What could be happening is that the 3m USB 3.0 cable is providing insufficient power at start-up.

This is a well known problem with USB powered devices, but isn't this the entire point of the external power supply?! If not, what's the point in the external power supply! I can only assume there is a fault or a design flaw in this device - because if I have the external power supply plugged in, I should be able to use any length USB3 cable that complies with the standard.

The specification for this device states:
4-channel models: AC adaptor included for use with USB ports that supply less than 1200 mA
So, lets say I either lose the supplied 1.8m cable, or someone else grabs it and I need to buy another one. So long as I have that 5V@3A supply connected, what difference does it make?

Hitesh wrote:We provide cables with a higher core to prevent voltage drop. The PC end of the USB cable also has the ground and shielding connected together.

This is a great thing to do, especially if someone needs to run the scope exclusively from USB power

Hitesh wrote:Is there a particular requirement for a 3m long connection from the PicoScope to the PC?

Yes, because the 1.8m cable doesn't reach the other side of my bench

Hitesh wrote:The option to display the power options dialog is stored in the preferences.xml file (see section 3 of the PicoScope 6 Self-Help Guide on how to locate the trace.xml file as this file will be in the same directory).

Open the file in a text editor such as Notepad and look for the following line:
Code: Select all
<preference name="switchtousbpowerdialogsuppressed" value="True" />

Change "True" to "False" and save the file then start PicoScope 6 and the dialog will be shown in future if the driver reports that

This doesn't actually fix the problem in any way, and as per the scenarios I listed this doesn't work some times, regardless of the saved preference.

Unless you can provide a reason not-to, or a firmware update, or something... I'm left with no choice but to return this device to the distributor. If the next device has the same problem, I'll repeat the process... This should not be happening.

Regards
Darcy
DarcyW
Newbie
 
Posts: 0
Joined: Tue Aug 25, 2015 11:33 pm

Re: Problems when running off external DC supply

Postby Hitesh » Thu Sep 24, 2015 12:42 pm

Hi Darcy,

No problem with continuing in this thread.

The 3406DMSO will always draw a certain amount of current from the USB port even with the PSU connected.

This allows the device to communicate with the PC, and identify the power options available to it. As the device is failing to operate correctly when using the 3m USB3 cable, it is likely that the voltage drop across the cable is taking it below the threshold to operate correctly. Our own cables have thicker cores to minimise the risk of this.

There are a couple of comments in relation to some of the test points that you mentioned in a previous post which I hope provides clarification:

DarcyW wrote:5 - PC software closed. No power adapter. Plug in cable to battery charging USB2 port. Device appears in device manager. Open software. I am now presented with a "Power Options" dialog that tells me about the different ways to power the device. Selecting to operate from USB power fails with the message "Insufficient USB power"

This is exactly what should happen when the USB port/cable can provide the current to operate the digital interfaces but not the full current to power up the whole scope.

DarcyW wrote:6 - No power adapter. Plug in 2m USB3 cable to USB3 port. Device appears in device manager (same as #3, but with 2m cable). Open software. Now presented with "power options" dialog. Plug in external power - device operates as expected.

Again this is exactly what should happen. The fact that plugging in the PSU causes the device to operate is a pretty good indication the PSU is not faulty.

If possible, is there another 3m USB 3.0 cable that you can try?

It would also be useful to know the model of the PC that you are using.

Regards,
Hitesh

Technical Specialist
Pico Technology
Hitesh
Site Admin
Site Admin
 
Posts: 2063
Joined: Tue May 31, 2011 3:43 pm
Location: St. Neots, Cambridgeshire


Return to USB PC Oscilloscopes

Who is online

Users browsing this forum: No registered users and 0 guests