MakerGear Forum

Hi,

It looks like my HBP isn't working anymore. My prints won't start cause the HBP doesn't warm up so it cannot reach the right temp. I checked the seperate12V power input on the Rambo and that is OK.
At the screw connector (on the Rambo board, at the right side of the Fan 1 connector) I don't see any voltage if I gave something like a M140 S50 gcode command.

What are the steps to test which part (HPB, thermistor, Rambo board) is broken?

thanks.
Marco

DIY3D wrote:At the screw connector (on the Rambo board, at the right side of the Fan 1 connector) I don't see any voltage

The MOSFET acts as switch connecting one side of the heater to common (0 V), with the other side of the heater connected to the power supply (+24 V for new M2s, +12 V for old ones with two power bricks). When you measure at the MOSFET terminals:

When the MOSFET is off, you'll measure 0 V across the terminals: the MOSFET side will float high to match the power supply on the other terminal.

When the MOSFET is on, you'll measure 24 V (or 12 V on old M2s) across the terminals: the MOSFET side is pulled low, with the entire power supply voltage across the heater.

It's the other way around when you measure from the MOSFET terminal to the common (0 V) power supply terminal:

When the MOSFET is off, you'll measure 24 V (or 12 V on old M2s) to common: the MOSFET side will float high to match the power supply.

When the MOSFET is on, you'll measure 0 V to common: the MOSFET side is pulled low.

So you must know what you're measuring: the voltage across the heater at the MOSFET terminals or the voltage from the MOSFET to ground!

Hi Ed,

Thank you for your explanation.

The MOSFET acts as switch ...

, that means I can connect both wires to check if the HBP is still working?

Kind regards,
Marco

DIY3D wrote:I can connect both wires to check if the HBP is still working?

The two wires come directly from the heater; the board supplies all the power & switching. The connector on the board puts +24 V on one wire and the MOSFET drain on the other.

You can measure the resistance between the heater wires, after you turn off the power and disconnect the wires from the board, which will tell you if there's an open circuit (a.k.a. broken wire) along the way to the heater. An "infinite" resistance = broken wire, a "low" resistance = probably OK. If the resistance changes as you wiggle any part of the cable, then you've found the bad / broken section.

Historically, the wire-to-heater joints at the platform have been the most troublesome, closely followed by the most-flexed section of cable just beyond that. Absent any other information, that's where to look.

Note: shorting the two connector terminals with the power on will destroy the MOSFET.

I just checked the resistance of the HBP (closed circuit) and the thermister (+/-97 K) at both connectors and that seems OK.

After a M140 S30 command I see the red leds burning at the Rambo board and I measure 0V between the HBP mosfet output connector and a GND contact point TP24 near the HBP power input connector. M140 S0 gives 12 Volt between those points and red led is off. Guess the Rambo board (or the mosfet) is still working.

So, it looks like I have to check the wires.

Hi,

Problem found . It was the Molex connector, or rather one of the wires had too little copper. The connector was molten inside. I can not find easily the Molex 3953x so I switched the one which is used for the fan. The fan is connected with 2 small single connectors. I've crimped 2 wire-end ferulles at the end of the HBP wires and I' m up and running again

Happy printing