Resistive load for discharging

[Hackentosher]

I love my hitec charger, but I can only dissipate energy at like 10w. I have an old prusa i3 pcb heatbed lying around, a bucket, and an abundant supply of water. I've heard of people running wire through some water for discharging, but I'm trying to use materials I have at hand. Anyone know if I would blow up the pcb heatbed, and what the anticipated load would be? Thanks guys. 

[Hackentosher]

Oh, @manikyath probably knows about these beds. I realize they're ancient, but it's what I have. 

[manikyath]

well.. the math on heated beds is easy.. they are a resistive load, they have a rated voltage, and wattage.

 

for example, a 40 watt heated bed running off 24 votls. => 1.67A; from that you can apply ohm's law and conclude the bed has a resistance of roughly 14 ohms.

 

essentially, a heated bed is just a very big resistor with some thermal mass stuck to it.

EDIT: also, measure it out with a multimeter if you wanna be sure

[unknownmiscreant]

9 minutes ago, manikyath said:

well.. the math on heated beds is easy.. they are a resistive load, they have a rated voltage, and wattage.

 

for example, a 40 watt heated bed running off 24 votls. => 1.67A; from that you can apply ohm's law and conclude the bed has a resistance of roughly 14 ohms.

 

essentially, a heated bed is just a very big resistor with some thermal mass stuck to it.

EDIT: also, measure it out with a multimeter if you wanna be sure

Be careful measuring the resistance of heaters with multimeters, as the resistance changes with temperature. (I can explain in more detail if you wish)

[Hackentosher]

1 hour ago, unknownmiscreant said:

Be careful measuring the resistance of heaters with multimeters, as the resistance changes with temperature. (I can explain in more detail if you wish)

With any kind of heater? I mean this pcb thing is just a bunch of thicc traces across the whole board. 

[unknownmiscreant]

Just now, Hackentosher said:

With any kind of heater? I mean this pcb thing is just a bunch of thicc traces across the whole board. 

Resistance of anything increases with temperature, be it heaters, wires, resistors, light bulb filaments.

[Hackentosher]

Just now, unknownmiscreant said:

Resistance of anything increases with temperature, be it heaters, wires, resistors, light bulb filaments.

Huh, I didn't know that. 

[Pixel5]

you can buy power resistors for a few bucks on aliexpress, i just got one yesterday that is 1 ohm and rated at up to 50W before it gets way too hot.

 

i used it to discharge my super capacitor bank yesterday but i guess ill need more of them to keep the heat under control.

[Hackentosher]

1 hour ago, Pixel5 said:

you can buy power resistors for a few bucks on aliexpress, i just got one yesterday that is 1 ohm and rated at up to 50W before it gets way too hot.

 

i used it to discharge my super capacitor bank yesterday but i guess ill need more of them to keep the heat under control.

Yes the thing is here I don't want to spend any money. I have the heat bed, I have a bucket, I can add water. 

[Hackentosher]

Alright, I soldered an XT60 to the heated bed, it measured in at 1.6ohms, then with my input voltage of 15v, it should be around 150w. Not too bad.

 

Update: works like a hot damn! Drains about 200mah from a 4s 1300mah lipo in 1 minute, so what's the math there for discharge capacity and current? Running this in a bucket of water btw.

Edited September 13, 2017 by Hackentosher