Hello I have a question regarding resistors. We are trying to create a draw to consume some additional power that is being generated.
Today we received our 2000w 10ohms resistor from digikey. We have it connected to our 48v lithium pack which has a resting voltage of 54.4v. We have one side of the resistor connected to the positive from the lithium pack, the other side is connected to a solenoid which is then connected to the negative terminal on the lithium pack. When we turn then toggle switch on we are only getting a 6.6 amp draw. We need a draw closer to 40-60amps. Would a 1 omh resistor work for that?
Amps = Volts / Ohms (in scientific form I=E/R)
54.4V \ 10 ohms = 5.44 Amps
54.4 / 1 ohm = 54.4 Amps
Although given the safety concerns of this circuit, I’m a little troubled that someone without trade/high school level electronics knowledge of ohms law is doing this. I strongly recommend you consult with someone local with a basic electronics education before something bad happens.
Thanks Paul, that’s what we figured just wanted double assurance. Appreciate your response.
Hello SimoloCustoms,
As Paul mentioned basic principle of Ohms law applies here.
Knowing the amp draw desired to a more specific number you can utilize basic algebraic equation to figure out the resistance needed.
For amp draw of between 40 to 60 at 54.4V you will need anywhere between 1.36 Ohms (which results in 40A draw) to 0.906 Ohms (which results in 60.04A draw).
Some Chassis mounted resistors that we have in stock at 2000W within those values are 1 or 1.2 ohms ones as seen in the links below.
A105579-ND
A131289-ND
Hope that helps. Thanks for visiting our tech forum and have a great day.
Thanks for the response, based on our testing anything over 40amps draw should suffice, we might actually be able to get away with less then that, but we will have to test in the real world. I just want to to be sure that we are sizing the resistor correctly. When I do the calculations using 54.4v and 1.2ohms I am getting 45.3amp draw at 2466 watts, again using the same calculations but using a 1ohm I am getting 54.4amp draw at 2959.36 watts. So I believe that I will need to size up to a 2500 watt resistor at 1.2ohms as a minimum? Would I be correct in assuming this or are the resistors you suggested sufficient. The circuit will only run for a maximum of 2 mins and will only do this once a day.
You’re going to need extensive testing at all ambient temperatures and variation in heat sinking effectiveness (e.g. simulated age corrosion in heat sink interface), to be certain that a the resistor does not over heat and end up with thermal runaway (when resistance drops due to heat, increasing current draw and heat, which cascades to total burnout and sometimes flames).
As a rule a of thumb to reduce the complexity of the testing most EE’s double the wattage rating of the resistor.
Thanks Paul,
Yes we will be giving this to our R and D department for real world testing in many conditions. We want to leave a large margin of room for safety, this is why we were wondering what sufficient watt resistor would work. We will will work as close to possible to give us double the wattage rating with our selection of resistor. Thank you once again.
