Feeding power from two sources?

Thread Starter

MB107

Joined Jul 24, 2016
290
I think though mercedes would have everything spec'd so it doesn't overheat, from my understanding you currently don't have that. From my understanding you're not planning to fit bigger fans, so I'm not sure I understand your reply. Trying to cool things after you've shut the engine down isn't going to help much as the water has stopped moving, unless you go electric water pump also.
Read post #6. I am doing with bigger fans. A lot bigger. It has been a common practice among many auto makers to run the fan after shut down. Usually on cars that have marginal cooling. But it will cool the under hood area as well as the radiator coolant.
 

Thread Starter

MB107

Joined Jul 24, 2016
290
OK so now getting back to the original question. Is everyone on board with the diode as a solution to this circuit? and should I use 1 or 2 diodes. And if so what size and how much wattage. At this time I am very limited on diodes. I have a complete set of 1 W zener diodes from 3 to 47 V and about 3 general purpose diodes. I haven't measures current yet on the signal wire in question but I do believe it will be well under 200 ma. Unfortunatly I ordered a 0.02 Ohm shunt resistor but it wont be in until after the holidays.
 

MisterBill2

Joined Jan 23, 2018
13,824
If the diodes are feeding a control signal to a PWM motor controller then two of the one amp,100 volt, diodes will work well. If those diodes are feeding a relay with 50 amp rated contacts then I suggest using 5 amp rated diodes with a 200 volt rating, because the relay will draw more current and the transient voltage may be significant. And he added cost will not be much.
 

Thread Starter

MB107

Joined Jul 24, 2016
290
If the diodes are feeding a control signal to a PWM motor controller then two of the one amp,100 volt, diodes will work well. If those diodes are feeding a relay with 50 amp rated contacts then I suggest using 5 amp rated diodes with a 200 volt rating, because the relay will draw more current and the transient voltage may be significant. And he added cost will not be much.
The diode as shown feeds 14V straight DC to the PWM motor just to turn it on. Its about an 18ga wire but caries probably less than 200 ma. I am assuming it operates an opticoupler or some other ingenious electronics within the motor as I dont think there is room for a relay especially of the 50 Amp size. The actual PWM signal comes through the wire shown in the diagram not connected to anything. The older versions of this motor had the relay portion in a box outside the motor and there would have been room in the box for a relay. But I really don't see the possibility of them packing a 50 amp relay in a 3/4HP motor that's 6" in diameter and 2" high.

I actually intend to measure the current doing to the controller but I have to wait until my shunt resistors come in. So the way I understand this is the 100V or 200V requirement on the diode is to cover transient spikes, most likely to occur when switching on the ignition key. The 1 amp or 5 amp requirement is to have some margin above the measures current going into the controller. If I only measure 200ma on that circuit should I use a lower amp rated diode. Is there a percentage above measured current I should use. Or should I try to measure the transient current spikes when turning the key on and off. Unfortunately I only have an 8 bit digital scope which make measuring transients kind of like guess work.

In any case it is a diode that I cant afford to have fail, so good margin of protection is required. But are there any down sides to going with overly high current or high voltage diodes.
 

MisterBill2

Joined Jan 23, 2018
13,824
OK, not sure where I got that 50 amp relay from, possibly somebody else's comment. And yes, the 200 volt diode was to avoid inductive transient problems. So it seems there will be no need for an external control relay. That is good. And for a pilot signal a one amp diode in each of the legs will be quite adequate. And at this point mechanical considerations come in, because diodes vary in size and ruggedness, and even a carefully driven car is a rather harsh mechanical environment for small components. And certainly every bit of the cooling system is a rather critical component and have adequate mechanical durability. That has been mentioned once already and it is true. Thus the recommendation of a somewhat heavier diode.
And with dual controlling inputs, disabling the fan to safely work in that area becomes more important. (But you know that, I am just obligated to repeat the caution.)
 

Thread Starter

MB107

Joined Jul 24, 2016
290
OK, not sure where I got that 50 amp relay from, possibly somebody else's comment. And yes, the 200 volt diode was to avoid inductive transient problems. So it seems there will be no need for an external control relay. That is good. And for a pilot signal a one amp diode in each of the legs will be quite adequate. And at this point mechanical considerations come in, because diodes vary in size and ruggedness, and even a carefully driven car is a rather harsh mechanical environment for small components. And certainly every bit of the cooling system is a rather critical component and have adequate mechanical durability. That has been mentioned once already and it is true. Thus the recommendation of a somewhat heavier diode.
And with dual controlling inputs, disabling the fan to safely work in that area becomes more important. (But you know that, I am just obligated to repeat the caution.)
Ahh there has to be a 50 amp relay of some sorts in the motor. The main power cable and ground to the motor are like 6ga. I should have drawn them a lot thicker in the diagram. But thats all taken care of by the wizards of Bosch, so its just the pilot signal I am working with.

Good point on the safety issue. It's actually something I didn't think of. The possibility does exist for the a minute or so after the car is shut off that the radiator could heat soak and the fan could come on powered by circuit 30. Another good reason to only use this circuit if absolutely necessary. Maybe its the same reason I haven't seen this feature for many years on production cars. I could put a hood switch on that would disable it but I could picture driving around to years not know that the hood switch is broken until I stuck my fingers in it.

Many Thanks
 
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