Hacking the HP45

[ezrec]

I really like that 'warm bath' solution! Quite innovative!

It will also allow you to easily see when the jets fire, and if the bath is still instead of circulating, you should even be able to see which jet fired.

[DigitalShadow]

Thanks I originally thought about using hot air, but I didn't want to dry the existing ink. Worst case scenario the ink will seep out. I was going to end up replacing the ink anyhow so I'm not too worried. Plus air would be much harder to maintain a stable and reliable measurement. At least with this method I should have some easy to obtain baseline to compare against.

I'm starting to wonder how applicable the patent referenced in my last post is. The number of heat sense pins is off, as well as some electrostatic protection grounds. The second patent from the beginning of this thread at least mentions this particular cartridge ("51645A")

[ezrec]

I was thinking - if you used a mineral oil bath, you wouldn't even need to worry about potential short circuits, and could immerse the whole cartridge.

[DigitalShadow]

Ah hah, here are some patents to add to the list.

Ink jet printer thermal control system
Stable substrate structure for a wide swath nozzle array in a high resolution inkjet printer (This appears to be specific to the hp45)

[DigitalShadow]

For whatever reason I've put off actually buying the print cartridge itself. I'll make that a priority to pick that up when it's not night time here. I think I've got a pretty good handle on how they utilize both the thermal sense and the 10x resistors. There is only one thing I cannot figure out by reading up on them is how those two pins are grounded. I'm pretty sure they both get a positive voltage pulse tuned with a trim pot, but I cannot figure out where that current would drain to until I scope the thing. If anyone has any information in that area i'd appreciate it.

I notice this model lacks a pin dedicated to ESD protection, yet the traces are still there. They seem to connect to the copper circles (Pictured here and here). I wonder if any of those serve any other function other than just ESD grounding. In any case, it may be worth adding a connection to ground them in future revisions.

In any case, I'll pick up one of the cartridges tomorrow and start my testing. I don't have what I need to fire ink yet, but I should be able to start the thermal tests.

Something I thought of that may possibly be important when you do your next ink test; the address lines are meant to be connected to a pull down resistor. What I would recommend is grounding all address lines you don't plan on using for the time being

[dragonator]

The whole thing is common ground for the printhead. Wouldn't the thermal sense and 10x go to the same common ground?

My printhead lacks the copper circles, bit it does have one hole in it. I took the liberty to measure the hole against the common ground. There is about 2ohms of resistance between contact number 5 and the hole. The same 2ohms is also between 2 random ground contacts

I think there are already pullups and pulldowns in the printhead. I measured the printhead with my multimeter. Ground (pin 5) gives 24kohm with an address and 22kohm with a primitive quite consistently, but only with the positive against to ground. With the ground of the multimeter on the ground pin of the printhead, the addresses are 63kohm, and the primitives behave weird (giving 1.5Mohm for half a second, and then going out of range). These could also be the identification lines you were talking about.

I already had the multimeter and the printhead. The 10x gives about 303ohm regardless of polarity, the thermal sense gives 307ohm, also regardless of polarity. After measuring all pins against pin 5 (ground), I am convinced that the pinout is as I stated on page 1. All addresses are pulled down with 63kohms, all primitives are high and weird, all grounds give 2ohm.

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[dragonator]

A quick followup, by heating the printhead (over a cup of hot coffee) the thermal sense rose to 321ohm, while the 10x remained mostly 303ohm.

My only hope now is that there is no ink in my coffee, both are black.

[DigitalShadow]

Ahh I didn't realize there is a common ground. That simplifies things greatly. And awesome results with the thermal sense and 10x, they behaved pretty much as I expected. They did say the purpose of the 10x was to represent the resistance of the primitives (times ten), as the primitives could not be reliably measured from pad to pad due to the nature of the circuit. I would guess that is the phenomena you are running into there.

[dragonator]

Hacking report, day 2

There is continuity. I connected the ground of the printhead to a 10kohm resistor (to ground) and a 1uF capacitor. I then ran a series of programs. First single pulses, then A burst of quick pulses. I am not sure all resistors are functioning optimally, but at least there is continuity.

I wanted to see if a single nozzle would trigger. I started with 1us of trigger time, and worked my way up to 6us. Between every 1us, increment, I measured continuity and it stayed the same, but no ink. After 6us I stopped.

I measured another address while I was running the program to be sure that no other address was triggered. On a random address I found a 2V peak every time the nozzle was triggered. This could be the pull down being too small, but I need to do more testing. I also don't know if 2V is enough to open the gate.

I then did some rough calculations. The first pulse of a burst charges the capacitor from 0V to 1.5V. At 1uF, this gives 1.125x10^-6J (formula: E = 0.5*C*V^2). The expected energy for a 2us trigger at 12V and 300mA is 7.2x10^-6. About 6 times less than expected.

Also, if the 10x resistor is really 10x as big as a printhead resistor, the printhead should be 30ohm. This should give 400mA in stead of 300mA at 12V. I am missing energy. and I am still not triggering.

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I do not know if preheating the head will make it trigger and I do not know if the other addresses are leaking. I still have not directly measured the resistance of a fresh nozzle and I do not know if I have properly opened the FET with the address. Things for the next series of tests.

[ezrec]

I would suggest tying all the 'unused' addresses and primitives to their off state (Vcc, right?)

They could be leaking to ground if untied.