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VELO HV Diagnostics


This guide is written to assist in VELO High Voltage diagnostics. This information was collected from 19/1/15 to 22/1/15. If the apparatus or software described in this guide has changed, please update the information.

This diagnostic can be performed by a minimum of two people but for safety reasons it is recommended to have at least two people when working in the pit. Therefore I recommend that these tests are performed by a team of three people; two in the pit to change connections and one in the control room to perform the IV scans.


To perform the diagnostics you will need the following equipment:

  • One small flat-head screwdriver (width 1.5mm)
  • One Digital Multimeter (DMM) with reading limits of 20V and 20MΩ
  • One "Resistor Cable" with 16x3MΩ resistors attached to a male SLA.H51.LLZG plug
  • One "Resistor Socket" with 1x2.3MΩ resistor attached to a female EGG.1B.303.CLL socket
The above mentioned cable and socket should be located within the VELO storage room above the LHCb control room at Point 8.

The resistor cable

The resistor cable used in the HV tests


Before Starting the Tests

Before starting and HV diagnostics, the following documentation about the HV setup should be read:

  1. The HV Connectivity guide detailing the wiring for each channel
  2. The HV System Verification Procedure manual ( from before Run I)

Performing the Tests

Step 1: Checking the Voltage Limits

Before turning on the VELO, the voltage hardware limit on each of the ISEG modules should be checked to avoid damaging the sensors. This is performed by going to the second floor of the counting house in the pit (D3). This can be reached by taking the lift to DELPHI, turning right before the staff enterance to LHCb and going up the stairs beside the visitor enterance to LHCb. The VELO modules are located in the first aisle of the module room.

You will need the DMM and screwdriver at this stage.

To check the hardware limit, place the ends of the DMM to the voltage limit socket and set the DMM to read up to 20V. The conversion between what you read on the DMM (V_{lim}) and the hardware limit (V_{max}) is 275.4*V_{lim} = V_{max}.

V_{max} increases in steps of 7V and has a maximum value of 700V so, for example, to set V_{max} to 350V then V_{lim} must be set to 1.27V. This achieved by turning the screw to the right of the V_{lim} socket.


There are six modules, labelled Module 1 - 6 which contol the 12 VELO boards, what module controls what board is as follows:

Module 1 BOARD00 & BOARD01
Module 2 BOARD02 & BOARD03
Module 3 BOARD04 & BOARD05
Module 4 BOARD06 & BOARD07
Module 5 BOARD08 & BOARD09
Module 6 BOARD10 & BOARD11

For this initial stage of testing, we will only be required to go up to 150V but it is NOT recommended to take the sensors beyond 350V so a suitable voltage limit should be applied.

Step 2: Performing the first IV scan

Now that we have ensured that the voltage limit is set to a sensible level we can perform an IV scan on the sensors. After logging into your online account for LHCb, open VELO_HV panel (can be reached from velo_user_panel or velo_top):


You should see the following panel:


Open up the VELO HV monitors for both the A & C side by clicking VELO[A/B]_HV under "Sub-System." This will allow you to quickly notice any tripping behaviour.

  • In order to start the scan at 0V, first turn off the HV
  • In the "Manual Operations" frame, select the required scan region from "Selection" (you can scan the whole VELO, a specific side or a specific board).
  • Click on button START (by "Start an IV scan for [ALL]"). The following panel will appear:

Set the voltage range to go from 0V to 300V, make sure that the voltage steps are sensible (10V or 20V) with an appropriate waiting time at each step (between 5s and 20s) then click "START."


Step 3: Analysing the IV scan

After the IV scan has completed then the results can be analysed by logging into plus and performing the following commands:

ssh -Y plus

The following GUI should appear:

To open the IV Scan results, go to "Special analyses". The latest 2 IV scans will be loaded as data and reference files. Different scans can be selected from the drop down list. You can use "Temperature corrected" to scale both plots to the same temperature (currently -10C). A successful scan on a sensor will look like the following:


and temperature corrected


The above test was performed on the first top and bottom downstream sensors on the A-side of the VELO with the low voltage off. The sensor naming convention is VLxx_[A/C][T/B] where xx is the sensor number, [A/C] refers to what side of the VELO is being tested and [T/B] refers to whether the sensor is a top or bottom sensor. To convert between sensor number and sensor name you can use the module data base or open the sensor conversion text file located in:



Step 4: Checking the ISEG units

If the first three steps have been successful then all the sensors are properly connected. The sensor cables should now be disconnectd from their corrosponding ISEG module and putting in the SLA.H51.LLZG resistor cable in their place. This will allow the ISEGs to be tested to their full voltage of 700V. This can only be done for one module at a time which will allow for testing of two boards per trip to the pit. The cable should be connected to a module as seen below for Module 1 ( REMEMBER TO PRESS THE EMERGENCY STOP BEFORE DISCONNECTING THE SLA.H51.LLZG CABLE TO THE SENSORS!) and V_{lim} should be increased to 2.50V as outlined in Step 1:

As the emergency stop button has been pressed, it will not be possible to read V_{lim} until the button has been pressed again. The wiring of the resistor cable and the modules is detailed in the HV Connectivity guide, the link can be found in the section, "Before Starting the Tests."

Be careful when removing the sensor cables as they are VERY delicate.You can now return to the control room and perform IV scans up to 700V safely. This is achieved by going to the Manual Operations box on the VELO_HV: Top panel as seen in Step 2 and selecting the correct board from the drop down list, "Selection." Remember to change the voltage test range. A successful test should look as follows:


Step 6: Checking the Sensors Upper Voltage Limit

If the ISEG units have successfully went to 700V with no faults then a VERY slow (1V/s) ramp to 350V on all sensors can be performed as previously outlined. If any current trips are noticed (they will possibly appear at 320V and above) then the EGG.1B.303.CLL socket can be attached to the cable leading to the sensor at the site of the VELO (a map of the VELO with corrosponding sensor designations should be located above the VELO station in the control room) and the IV scan can be manually performed on that specific sensor by clicking on the tripping sensor and changing the voltage by your own doing then recording the current after it has settled (note, there is no software written to perform an IV scan on an individual sensor and sensors should not be repeatedly tripped as to avoid damaging them so you cannot perform an I/V scan as detailed previously).


This guide has been written to compliment the other manuals for VELO operation; specifically the manual mentioned in the section "Before Starting the Tests" and has not been created as a stand alone. If there are any uncertainties with the instructions please contact Cameron Dean at

-- CameronThomasDean - 2015-01-30

Topic attachments
I Attachment Action Size Date Who Comment
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PNGpng lovell_iv_scan_temp.png manage 335.1 K 2016-10-25 - 14:08 CameronThomasDean lovell_iv_scan_temp
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Topic revision: r8 - 2016-10-25 - CameronThomasDean

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