parallel thermistor programing

[Tpmoses]

Hello all,

I am building a multi heater print bed because of size. I've got the wiring I need, and there are four NTC 100K 3950 (one in each heater) wired in parallel. I know that I will need to reduce the T value, but I have also seen that for the 3950 I will need to put in specific heat parameters in the RRF config tool.

Any suggestions would be greatly appreciated.

Thanks,
-T

[Phaedrux]

Something about this setup makes me nervous.

See here for some details on using multiple sensors for a single heater.
https://duet3d.dozuki.com/Wiki/Connecting_and_configuring_a_chamber_heater#Section_Using_multiple_temp_sensors_for_a_single_heater

But I'm not too sure about using multiple sensors and multiple heaters as a single sensor and heater.

All I can say is proceed with caution.

[fcwilt]

Hi,

I have no idea how that can be made to work.

With only a single temperature sensor reading how can the firmware determine which heater(s) needs to heat more/less.

Is there a reason you cannot connect the temperature sensors individually?

Frederick

[alankilian]

I'm guessing all the heaters are connected in parallel.

So there's really no way to adjust an individual heater.

I'm also nervous about this.

It would be better to hook up each heater/thermister pair as one of four heaters and set the temperatures all the same.

[Tpmoses]

The heaters are two pairs in parallel controlled by two SSRs getting a single signal, end goal is to put a single sensor in the center of the bed, but waiting on parts for that. The reason for not having each heater individually controlled is these four are replacing a single custom 1000X1000mm mat. I had to split the single power source and SSR into two because of amperage draw. I get that I could control them individually however, because of the size of parts that are printed on it there isn't a need. This is a printer at my work that I did not build, and time is a factor on getting it running again. Then end goal is to put a single thermistor in the center of the bed and have that be the temp reading, however Time.

[alankilian]

I'm sure you are not going to get uniform temperature across the four quadrants of your plate using heaters connected in parallel and a single sensor.

Imagine air flowing across the bed. One heater will be cooler and the others will be hotter than your temperature sensor.

Depending on difference in the building of the heaters, one could be significantly hotter than the others.

It's just not going to work the way you want it to I'm sorry.

[Tpmoses]

@alankilian Alright so say I wanted to split the signal, and make the the pairs of heaters work separate, so one SSR with two heaters and two thermistors wired in parallel. That way the build plate has two zones, then there is less error between the heaters. how do I wire the signal for the second pair of heaters, run off of another tool? The first one is controlled by the bedheat output on the board, I only use one extruder heater on E0, so do I control the second half of the bed with E1 then assign it in config.g?

[alankilian]

I'm not sure this is safe, so I'm not going to encourage you to do this.

I'm sorry if I'm a worrywart and you know it's safe from your experience.

You'll be able to figure it out from the available documentation I bet.

[Phaedrux]

@Tpmoses said in parallel thermistor programing:

do I control the second half of the bed with E1 then assign it in config.g?

Basically yes. Since you're controlling an SSR you can use any PWM capable output to switch the SSR. E1 would make logical sense if it's free.

[dc42]

@Tpmoses said in parallel thermistor programing:

I am building a multi heater print bed because of size. I've got the wiring I need, and there are four NTC 100K 3950 (one in each heater) wired in parallel. I know that I will need to reduce the T value, but I have also seen that for the 3950 I will need to put in specific heat parameters in the RRF config tool.

If you connect identical thermistors in parallel, the R25 value (which is the RRF T parameter) of the combination reduces in proportion, but the B value is unchanged.

If the thermal coupling between the different parts of the bed is good (e.g. because all 4 heaters are heating the same thick aluminium bed plate), then it should be sufficient to connect the thermistors in parallel and the heaters in parallel, and drive them as one. Otherwise, the four heater/thermistor pairs should be controlled independently.