1 Topic by RavensCrest

  • RavensCrest
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Topic: [HELP] I need two power supply, one for bed and one for printer

I am reading this build. i like the way he uses two power supplies to separate bed and printer.
http://reprap.org/wiki/Adrians_Prusa_Notes#Heated_Bed


so here is my question
http://reprap.org/mediawiki/images/9/94/Prusa-heated-bed-schematic.png
can someone explain why is R12 using 1M resistor?

Solidoodle2 with Ceramic tile heated bed http://www.soliforum.com/topic/2544/my- … eated-bed/
"1kg should last for an while" is a lie!

2 Reply by Rincewind

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Re: [HELP] I need two power supply, one for bed and one for printer

It's a pull-down resistor for the mosfet in case the board is not connected/high impedance/faulty, to avoid partial or otherwise unwanted conduction. The resistance doesn't need to be low, as a relatively slow discharge should be sufficient. Does this answer your question?

3 Reply by adrian

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Re: [HELP] I need two power supply, one for bed and one for printer

http://en.wikipedia.org/wiki/Pull-down_resistor

4 Reply by RavensCrest

  • RavensCrest
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Re: [HELP] I need two power supply, one for bed and one for printer

@rincewind, thank you

adrian wrote:

http://en.wikipedia.org/wiki/Pull-down_resistor

Are you "the" Adrian who wrote the prusa note!?

And does the resistance really have to be 1M? is there a range?
I am just curious how you come up with 1M

Solidoodle2 with Ceramic tile heated bed http://www.soliforum.com/topic/2544/my- … eated-bed/
"1kg should last for an while" is a lie!

5 Reply by adrian (edited by adrian 2013-05-31 15:00:44)

  • adrian
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Re: [HELP] I need two power supply, one for bed and one for printer

the higher the value of the resistor, the less power it needs to dissipate.

You could use 'any old resistor' in a drawer for the job assuming that its acceptable in the circuit (since the resistor will end up being the top half of a voltage divider).

Traditionally in 5V microcontrollers, you use 1k or 10k or 100k depending on your mood/needs/circuit/persuasion. Due to higher voltages and current, you need to go higher to allow the resistor to have to dissipate less heat... like in say things like a guitar pedal, where you would often see 1.2M etc etc used on inputs to prevent 'popping' as it turns on/off...

Anyway.. nutshell - its a value thats a compromise and you can use anything that would otherwise work within the circuit/power constraints but ultimately its a tradeoff in so far as lower values need to burn off higher power values.... and also consume more power etc etc...

And no, I'm not that Adrian smile