HP LaserJet 5000 — Changing Fuser Heat
- By Don Thompson
- Mar 01, 1999
Our interest in the HP 5000 fuser technology sidelined our plans to continue with other aspects of this printer in this issue. Instead, this article covers how changing fuser heating for various kinds
of paper stock works.
HP LaserJet 5000 Fuser
The fuser system involves not only the fuser, but the low voltage power supply, DC controller and wiring between them. Users can adjust fuser temperature through the control panel,
within limits, to better fuse toner to a variety of media. By the list of media in Paper Handling menu, it would seem that fine tuning for fusing heat is possible for each selection. This apparently is not the
case. Could this aspect of machine configuration create new problems in printing? Perhaps it will.
Selecting Variable Fusing Temperature
HP provides an interesting option in its LaserJet 5000 Paper Handling Menu: "CONFIGURE FUSER MODE=NO [YES] (Figure 1, A). When you first see this, you might ask, "Why would
I need to configure a fuser"? The fuser melts toner onto the page in the image forming process. Maybe this is where you set the fuser for more heat for thicker paper stock. Or, drop the heat for
Figure 1. The Paper handling Menu on an HP LaserJet 5000. CONFIGURE FUSER MODE (a) suggests that the user can "fine tune" fuser heating by entering "YES" following the equal sign. The primary means of changing fuser heat involves selecting media type in tray 2 (b).
Actually, there are two items in the Paper Handling Menu which govern fuser heat. The primary item, as seemingly unrelated as it is, lies in "TRAY 2 TYPE=PLAIN [11 other options]"
(Figure 1, B).
In the Paper Handling Menu under TRAY 2 TYPE=PLAIN, selecting one of 11 other options for type of media supposedly automatically adjusts the fuser heat. One assumes that this means
that for any media selection, the heat is magically adjusted in increments, customizing the heat for that paper type. Based on the number of choices, one would think that each selects a different heat. That
is not the case, as we will see.
Sensing Heat Variation in the HP 5000 Fuser
The DC controller circuit electrically "feels" the fuser heating element turned on during printing and turned off afterward. If heat is not sensed when a print cycle begins, the DC controller
shuts down the printer, signaling the formatter to issue the fuser heating error code "50 SERVICE" on the control panel.
The thermistor, built into the end of the ceramic heating element assembly of the 5000 fuser (Figure 2), reacts to changing heat generated by the heating element. Electrically speaking, it
leaks current according to the heat applied to it by the heating element. The hotter the element, the more current flows through the thermistor and vice versa.
Figure 2. The LaserJet 5000 fuser heating element (above) with fixing film removed. The thermistor sensing occurs at the right end of the fixing film assembly in this view. The illustration below the photo shows the two heating connectors at the left. Each of these connects to a common (neutral) connector wire shown in the illustration. Each loop of the separate line contacts operates separately, connected to the neutral element.
How much fuser heating variation is possible in the HP 5000 fuser? To test this, we soldered a fine wire to the output side of the
thermistor low voltage power supply connector at J203, pin
1 (arrow in Figure 3).
Figure 3. The LaserJet 5000 low voltage power supply showing thermistor connector J203 (arrow) from the fuser tapped for testing fuser heat.
Since the thermistor senses heat, monitoring it on a chart recorder offers a permanent record of changing heat from changing current flow through it.
Setting Up the Chart Recorder
Figure 4 shows the chart recorder used for monitoring thermistor readings from the HP 5000. The positive contact was attached to J203, Pin 1 and the negative contact was attached to frame
ground on the machine.
Figure 4. The chart recorder used to study the thermistor output of the HP LaserJet 5000 fuser.
Chart recorder settings:
No absolute temperatures were recorded since a temperature probe was not used. We only observed heating at different levels, reported by the thermistor circuit.
Setting Up the Printer for the Tests
We ran a series of tests to confirm repeatability using the PRINT MENU MAP print test. Over 150 test runs were made, using all combinations of paper type in Tray 2 and
selecting various states (LOW, NORMAL, HIGH, and VELLUM) within the CONFIGURE FUSER MODE menu.
NOTE: To manipulate the menu within the CONFIGURE FUSER MODE from the Ready status, press the Menu Key on the right side twice, then the Item Key on the right, five
times. This brings you to the CONFIGURE FUSER MODE, which is usually set to "=NO." Press the + / _ key to switch to "=YES." Press Select Key. Go back to the Item key to get to paper type
for which you want to change fuser heating. Within the "CONFIGURE FUSER MODE=YES" of the Paper handling Menu, each type of paper can be configured for HIGH, NORMAL, LOW,
or VELLUM. Running a Menu Map shows the paper types with what heat has been selected for each. The default setting is "Normal" for all but
vellum (set to "VELLUM") and rough
(set to "HIGH."
Results of the Heat Tests
The heating profiles from these tests reveal that there are only three heating levels for the HP 5000 fuser. Call them high, normal, and low if you want, since we have no temperatures
to attach to them. Clearly, selecting "TRAY 2 TYPE=VELLUM" results in the low temperature level, "TRAY 2 TYPE=ROUGH" results in the high temperature level, and all the other choices
run about midway between. Figure 5 shows typical profiles, using "PLAIN" to represent all the others of the list, except "VELLUM" and "ROUGH," from left to right.
Figure 5. Heating profiles from the HP LaserJet 5000 fuser. On the left, (a) shows the average heat generated by selecting an media type for Tray 2 other than vellum or rough. This one happens to be plain paper. The middle profile (b) shows a marked drop to lower heat produced by selecting "vellum." The profile on the right (c) represents greater heat generated by selecting "rough" media from the Paper Handling Menu.
So, there are only three temperatures to select from on this printer, high, normal, or low. The factory set all but the rough and vellum to the same temperature. Although there are
separate settings in the CONFIGURE FUSER MODE=YES mode for low and vellum, we see no difference in heating profiles.
Questions and More Questions
The big question concerns how fuser heating is changed in the printer. So far, we have part of the answer to that, and a few unanswered questions.
Figure 6 shows the two triacs (CR1 and CR2) used to turn on and off the two heating element loops shown in Figure 3. Each triac is triggered by separate solid state relays
which receive independent signals (FSRDRV1 and FSRDRV2) from the DC control logic at printing time. Having two independent loops presumably means that during low heat only one loop is
turned on. During normal heat, the other (hotter than the other) is powered. During high heat, both loops heat. Sounds good, but if one loop (Figure 2) runs hotter than the other, shouldn't that
loop have higher electrical resistance than the other? We thought so, but measured little difference. Do they do it by varying the current? This remains to be tested.
Figure 6. Location of two fuser heating triacs (arrows) on the HP LaserJet 5000 power supply.
What this means to you
Given controls to change something, people will often misadjust them! Not only can uninformed people select the wrong type of paper in Tray 2 from the Paper Handling Menu, they
could improperly adjust the fuser heating through the CONFIGURE FUSER MODE. You might see the results of incorrect settings affecting toner cartridge performance, fuser offsetting, paper curl, or
other phenomena. Be aware of the heating system and how it works on this and other machines, such as the HP LaserJet 4000. We will present more on this machine
in a later article.
For further information, contact Don Thompson at 818-709-1234, ext.2537.
This article originally appeared in the March 1999 issue of Recharger.