Replacing a heater resistor pack with a DC PWM controller
Discussion
Hi all, I am hoping that an electronics guru can help me out here. My question relates to a Lotus, but it's general electronics info I am after so hopefully this is the best forum to choose? (It's not a good fit for any of the Technical forums either.)
My Lotus has a known fault, whereby the heater resistor pack (which is buried deep in the front of the car) fills up with water and fails. Getting to it to replace it means taking the front clam off, discharging and removing aircon components, and costs a lot of £ or hrs depending on how hands-on the owner is.
It is possible to access the last piece of fan wiring, which takes 12V from the resistor pack to the fan, and therefore to bypass the failed pack and provide simple on/off control. I saw a reference in an American forum which suggested using a DC 12V 30A pulse width modulated motor controller in place of the fan resistor pack. This idea is attractive, because using resistors to limit the motor speed generates a lot of heat (hence Lotus's decision to bury the resistor pack in the fan intake tract, so that it's always in airflow whilst in operation). I am hoping that a PWM controller will run a lot cooler, so that it could be mounted in a more accessible location for installation (and any required maintenance).
I know a bit about electronics but I am no expert, so I was hoping someone could answer the four questions I have:
1) Will a PWM controller run cool enough to avoid the need for constant airflow? I guess the answer is "maybe, depending on design and current draw" - if so, how can I ensure cool operation in the smallest possible package?
2) These units seem to do the job I'm after, any comments on their suitability or suggestions of better alternatives?
https://www.amazon.co.uk/300w-Motor-Speed-Control-...
https://www.4qd.co.uk/product/sst/ (too complicated for my needs?)
3) I would like to retain the three steps for speed control, rather than having a constantly variable potentiometer. I think I just need a voltage divider circuit hung off the back of my existing four-position dashboard switch, is that right?
4) I can only access the +12V wire going to the fan, not its earth cable point. The PWM controllers above have terminals for both motor +ve and -ve. Should I connect the controller's motor -ve terminal to earth?
Although none of this is simple, I am hoping that I can find a working solution which provides benefit to other S2 Elise/Exige owners. Thanks in advance for any help!
My Lotus has a known fault, whereby the heater resistor pack (which is buried deep in the front of the car) fills up with water and fails. Getting to it to replace it means taking the front clam off, discharging and removing aircon components, and costs a lot of £ or hrs depending on how hands-on the owner is.
It is possible to access the last piece of fan wiring, which takes 12V from the resistor pack to the fan, and therefore to bypass the failed pack and provide simple on/off control. I saw a reference in an American forum which suggested using a DC 12V 30A pulse width modulated motor controller in place of the fan resistor pack. This idea is attractive, because using resistors to limit the motor speed generates a lot of heat (hence Lotus's decision to bury the resistor pack in the fan intake tract, so that it's always in airflow whilst in operation). I am hoping that a PWM controller will run a lot cooler, so that it could be mounted in a more accessible location for installation (and any required maintenance).
I know a bit about electronics but I am no expert, so I was hoping someone could answer the four questions I have:
1) Will a PWM controller run cool enough to avoid the need for constant airflow? I guess the answer is "maybe, depending on design and current draw" - if so, how can I ensure cool operation in the smallest possible package?
2) These units seem to do the job I'm after, any comments on their suitability or suggestions of better alternatives?
https://www.amazon.co.uk/300w-Motor-Speed-Control-...
https://www.4qd.co.uk/product/sst/ (too complicated for my needs?)
3) I would like to retain the three steps for speed control, rather than having a constantly variable potentiometer. I think I just need a voltage divider circuit hung off the back of my existing four-position dashboard switch, is that right?
4) I can only access the +12V wire going to the fan, not its earth cable point. The PWM controllers above have terminals for both motor +ve and -ve. Should I connect the controller's motor -ve terminal to earth?
Although none of this is simple, I am hoping that I can find a working solution which provides benefit to other S2 Elise/Exige owners. Thanks in advance for any help!
To the best of my knowledge, Penelope is right Kambites, the new resistor assembly solves the water ingress issue but is located in the same spot and requires the same cooling. Thank you everyone for all of the replies, it sounds like I am on the right track here, so I have ordered the Amazon PWM controller. There don't appear to be any UK stockists so I have a two week wait for it to come from China!
There are some good suggestions about using a separate fan to cool the resistor pack/controller, I hadn't thought of that but the issue is lack of space. There are no hiding places behind the dash for that kind of arrangement and I want a solution that keeps the car looking 100% factory.
My plan is (once the controller has arrived) to connect everything on flying leads, and then test temps etc during sustained running, and to establish what resistors I need for the voltage divider circuit. I will try to update this thread once I have made some progress, it may be of benefit to others.
There are some good suggestions about using a separate fan to cool the resistor pack/controller, I hadn't thought of that but the issue is lack of space. There are no hiding places behind the dash for that kind of arrangement and I want a solution that keeps the car looking 100% factory.
My plan is (once the controller has arrived) to connect everything on flying leads, and then test temps etc during sustained running, and to establish what resistors I need for the voltage divider circuit. I will try to update this thread once I have made some progress, it may be of benefit to others.
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