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Hackentosher

Resistive load for discharging

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Posted · Original PosterOP

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. 

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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 ;)

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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)


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since I have to make my own discharge loads all the time with my ebike battery, I have a few words to say. 

 

At first what I did was I actually used a giant load of enamaled copper wire. Regular copper wire works too, but it needs to be wrapped around something non-conductive (like wood) in order to work. I used a resistance calculator for 22AWG copper wire online to get to the target resistance for the dummy load. 

 

 

After the wires kept breaking (after multiple uses, once is okay) I used a 100ft roll of steel wire I had laying around and wrapped that around a wooden pole I submerged in a cooler. The steel wire acts as a resistor and serves as a one-time use dummy load. You can find resistance calculators for various gauge steel online by foot (google this one). Since I was load testing a 36v battery I think I had to use something like 2x 50ft lengths of steel wire for my application. But yours WILL vary since all we are doing is creating a giant resistor and the amount of current you want your resistor to draw will be dependent on the calculated ohms. Since you are working with lower voltage RC lipo batteries, you will need an even lower resistance rating (in ohms) than me. 

 

Now I use 12 stove heating elements strung together in parallel to give me a roughly 2.5 ohms resistor. This means my giant resistor I have made out of stove heating elements draws 1000w with a 42v battery under load. 

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Posted · Original PosterOP
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. 

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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.


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Posted · Original PosterOP
Just now, unknownmiscreant said:

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

Huh, I didn't know that. 

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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.

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Posted · Original PosterOP
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. 

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Posted (edited) · Original PosterOP

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 by Hackentosher
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