Hello everyone,
I am currently working on a project of a hand cycle ergometer for therapy. The project aims to use a motor to either help the patient turn its arms or to give some resistance to the patient with the motor.
I control the motor with an odrive card.
The issue I am facing is that when the motor is used to create resistance, it generates electricity that cannot go back into the alimentation. I then use a braking resistor to dissipate the energy produced by the patient. In the specification, it is needed to dissipate 500w of power continuously.
I spoke with to the after-sales of the odrive and they advised me to use a resistor with a resistance ranging from 2.3ohm to 3.6ohm.
Regarding those two constraints, I choose the resistor :
- Digi-Key Part Number : A123917-ND
Manufacturer Product Number : TJT5003R3J
Resistance : 3.3 ohm
Power rating with std heat sink : 500w
Power rating in free air : 250w
I am not sure if this resistor meets my requirement. And I don’t understand what is an std heat sink, I thought that the heat sink was built in if it is not the case what heat sink do you recommend.
Also, the person I spoke with at Digikey told me about the TAP1000, but I do not know what are the benefits of this resistor regarding to the TJT500.
Some help would be greatly appreciated 
Greetings,
“std” is commonly used as an abbreviation for “standard,” and in this case would refer to any chunk of metal (or other thermally conductive material) that can be used to conduct heat away from the resistor. The housing of the TJT500 does have a substantial surface area and is capable of transferring quite a bit of heat to the surrounding air (roughly 250W before it gets hot enough to be damaged) but if additional heat sinking is provided to increase the rate at which heat is removed from the device, a larger amount of electrical power can be applied before the resistor’s temperature increases to the point of being damaged.
The TAP1000 is not designed to be able to dissipate a large amount of thermal energy by itself, and appears to need a liquid cooling system in order to reach its rated power dissipation. It is also quite expensive, and I would not recommend it for your application. The TJT500 in 3.3 ohm value also is not in stock currently, and for that reason may not be a fantastic choice either.
I would suggest that it might be worthwhile to re-evaluate your power dissipation requirement. 500W is more than most people can produce on a bicycle for any length of time, so unless you’re building a hand cycle for gorillas that number would seem higher than necessary.
An example of a solution that would seem like a reasonable starting point to me might be the use of (3) HSC10010RJ resistors connected in parallel on a heat sink like the 127735. If needed, it shouldn’t be to difficult to arrange for a fan powered by the voltage applied across the resistor to provide airflow.
Thank you very much for your quick answer,
I will look into that .
