One of the most troublesome and most occurring CIJ printer faults that contributes to downtime and repeated operator intervention is EHT fault or sometimes called HV trip.  This fault usually and often occurs as a result of a short circuit of one or both (if applicable) of the live EHT plates to a lower potential (ground) .  This short maybe as a result of a gradual or sudden formation of a conductive path between the live plates and a lower potential or ground.  CIJ electronics/firmware/software implement a HV short circuit detection or more precisely a method for detecting a drop, a decrease in EHT voltage below a predetermined level for a set short period of time (milliseconds).

1. SELLENIS LIMITEDINNOVATION IN INDUSTRIAL INKJET TECHNOLOGY

2. EHT OR HV FAULTS/TRIPS – WHEN?
 One of the most troublesome and most occurring CIJ printer faults that contributes to downtime and
repeated operator intervention is EHT fault or sometimes called HV trip.
 This fault usually and often occurs as a result of a short circuit of one or both (if applicable) of the live
EHT plates to a lower potential (ground) .
 This short maybe as a result of a gradual or sudden formation of a conductive path between the live
plates and a lower potential or ground.
 CIJ electronics/firmware/software implement a HV short circuit detection or more precisely a method
for detecting a drop, a decrease in EHT voltage below a predetermined level for a set short period of
time (milliseconds).

3. EHT OR HV FAULTS/TRIPS – HOW?
 If the EHT voltage drops below a given level for longer than a certain set time the CIJ printer system
generates a fault and switch off the HV supply to the deflector plates.
 This is to prevent charged drops being deflected randomly and uncontrollably onto other critical
printhead parts and degrade the print further.
 It is also important for safety purposes to cut off the HV supply to the deflector under these
circumstances.
 The fault & switch off of EHT occur only after a drop in voltage for a minimum period of time to ensure
the printer system distinguishes between a real problem and an instantaneous drop in EHT due to
electrical noise (false alarm)

4. EHT OR HV FAULTS/TRIPS – WHY?
 There are several root causes for the occurrence of EHT fault or trip. These can be
classified into two areas:
a. Design robustness
b. Operating conditions
 EHT trip as a result of a printhead design that is not robust enough
 Non-robust design specification of deflector plates shape, form and surface finish
 Non-robust design specification of deflector plates spacing or inter-gap
 Non-robust design specification of deflector plates mounting geometry, shape, surface finish and
location
 Non-robust printhead system design (ie designing the printhead taking into account all the
interractions of the deflector plates and EHT voltage with other components of the printhead during
operation.)

5. EHT OR HV FAULTS/TRIPS – WHY?
 EHT TRIP as a result of operating conditions
 EHT trip sensitivity setting
 EHT voltage setting [too high or too low for the operating parameters such as raster in use, drop
velocity (or pressure setting), charge voltage setting, modulation voltage setting)
 Operating temperature (room temperature; the higher this is the more likely EHT trip in the case of a
non-robust design)
 Moisture/humid /wet operating environments such as in drink bottling , steam laden air applications
plants, food canning plants and the like)
 Inks formulation quality that leads to the creation of microsatellite droplets
 Ink splash back from gutter

6. EHT OR HV FAULTS/TRIPS- HOW TO SOLVE?
 EHT Robust Design
 The designer needs to design the whole printhead not just the deflector plates taking into account the interactions
between the various printhead components. During operation./
 The design needs to take into account and design for the various operating conditions listed previously including possible
manufacturing and assembly variations.
 EHT TRIP consideration of operating conditions
 EHT trip sensitivity setting – understand this setting when applicable and ensure it is adequately set
 EHT & Drop velocity voltage settings :
• Ensure EHT voltage is not set too high for the raster in use so as not to deflect drops into the defector plate. This is
particularly relevant when printing 2, 3 or 4 lines or large fonts
• Ensure pressure or drop velocity is not too low for the raster in use so as not to deflect drops into the deflector plate.
This is particularly relevant when printing 2, 3 or 4 lines or large fonts
• Ensure EHT voltage is not set too low so as the least deflected drop does not clear the gutter edge. This is
particularly relevant when printing 2, 3 or 4 lines or large fonts
 Ensure EHT voltage is not set too low so as the least deflected drop does not clear the gutter edge. This is
particularly relevant when printing 2, 3 or 4 lines or large fonts

7. EHT OR HV FAULTS/TRIPS- HOW TO SOLVE?
 EHT TRIP: Consideration of operating conditions
 Moisture/humid/wet operating environments
• Use of a positive DRY air system.
• A positive air system only such as those supplied by a standard air pump will probably not be
effective and might even increase the occurrence of EHT trips.
 Ink splash back from gutter
• Investigate ink quality and stability
• Adjust jet into gutter
• Printhead design robustness
• Microsatellite droplets
• Consider changing ink or nozzle

8. Sellenis engineers have the expertise and the know how in this field. This ensures that Sellenis
designs of printhead and CIJ components and assemblies are robust incorporating optimum geometry,
shape, surface finish and optimum system configuration to operate reliably under varying conditions of
operations and environment.
Sellenis Limited
INNOVATION IN INDUSTRIAL INKJET TECHNOLOGY

9. CONTACT SELLENIS
Sellenis Limited
Unit 1, Saxon Way
Melbourn, South Cambridge
SG8 6DN, England, UK
Tel +44 (0) 1763 269554
Email info@sellenis.com
Web Sellenis.com