Diode help
01-24-2006, 10:54 AM
#16
Electron Wrangler
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Bob,
You are right about the total load (I really have no idea what the load of the vacuum actuator is?). However for the battery alone I think the resistor is probably still a better limiter (maybe in series with the thermal fuse idea - good one).
I'm now thinking a relay is the way to go. Have battery charging only when the ignition is on and supply current to the actuator from the battery only when the ignition is off. The reason is: if this circuit drives the actuator all the time its going to conflict with the AC controller at least some of the time, (say in the winter when you have heating on...).
I think you mentioned having a switch.... but this is not really a very good solution to this problem.
Have a relay with coil activated by the ignition, term 30 goes to your new battery+ , term 87 (normally open) contacts go via charge current limiting resistor and/or thermal fuse & diode to the car battery or ignition connection. The 87a (normally closed contacts) go to your vacuum actuator. This way its only ever driving the actuator with car off. I'd still include the diode so your battery isn't trying to help start the car.
In that case calculate the charging resistor as ~13.8v - ~12v- ~0.7v = 1.1v
Charging current of ~ 1A => resisitance ~ 1.1ohms
Power = 1.1v x 1A = 1.1W So even for charging you are right that probably 2W would be better. I was assuming the actuator was never powered by the battery circuit when the car was on - which is still the idea I think - but only achieved with the manual switch - so no protection = bad assumption.
Brian I really think you need the relay here... clean functional install - no switch to remember.
OR.... I still think the ideal solution here really is an actuator that defaults in the other direction - its not really so much an electrical issue although we can provide a solution....
Alan
You are right about the total load (I really have no idea what the load of the vacuum actuator is?). However for the battery alone I think the resistor is probably still a better limiter (maybe in series with the thermal fuse idea - good one).
I'm now thinking a relay is the way to go. Have battery charging only when the ignition is on and supply current to the actuator from the battery only when the ignition is off. The reason is: if this circuit drives the actuator all the time its going to conflict with the AC controller at least some of the time, (say in the winter when you have heating on...).
I think you mentioned having a switch.... but this is not really a very good solution to this problem.
Have a relay with coil activated by the ignition, term 30 goes to your new battery+ , term 87 (normally open) contacts go via charge current limiting resistor and/or thermal fuse & diode to the car battery or ignition connection. The 87a (normally closed contacts) go to your vacuum actuator. This way its only ever driving the actuator with car off. I'd still include the diode so your battery isn't trying to help start the car.
In that case calculate the charging resistor as ~13.8v - ~12v- ~0.7v = 1.1v
Charging current of ~ 1A => resisitance ~ 1.1ohms
Power = 1.1v x 1A = 1.1W So even for charging you are right that probably 2W would be better. I was assuming the actuator was never powered by the battery circuit when the car was on - which is still the idea I think - but only achieved with the manual switch - so no protection = bad assumption.
Brian I really think you need the relay here... clean functional install - no switch to remember.
OR.... I still think the ideal solution here really is an actuator that defaults in the other direction - its not really so much an electrical issue although we can provide a solution....
Alan
01-24-2006, 03:11 PM
#17
Three Wheelin'
Thread Starter
By "actuator" I'm not sure whether you mean heater control valve or solenoid, but the problem, it seems to me, is that the mixing motor sends an electrical signal when it wants the heater valve shut, which currently requires both power and vacuum.
If you had either a heater control valve that is normally closed, or a solenoid that when energized breaks the vacuum circuit (each opposite of its current function), wouldn't the mixing motor/climate control get the opposite of what it was expecting under normal operation? And if you change both of them, aren't you back where you started? I'm trying to think of this like algebra. Same x Different = Different, Different X Same = Different, Different x Different = Same, or (+) x (-) = (-) [and vice versa], (-) x (-) = (+)
Wouldn't you need to change the logic the climate control is using to energize the heater valve solenoid if you were going to change either the solenoid or the heater control valve to "opposite" default? I guess you could use a relay to do this, and maybe this was your original point.
It seems like I could also put a diode on the climate control lead to the heater solenoid, which would keep power from running back up into the climate control circuit when my switch was on.
There are specific times that I want to activate this feature, and times that I don't care to, which is why I chose a switch. If it's cold out, or if I'm done driving the car for a few hours, I don't need the heater valve to automatically closed if I turn off the car. Plus I've already got the switch mounted in the console, so the switch stays whether its used or not.
If you had either a heater control valve that is normally closed, or a solenoid that when energized breaks the vacuum circuit (each opposite of its current function), wouldn't the mixing motor/climate control get the opposite of what it was expecting under normal operation? And if you change both of them, aren't you back where you started? I'm trying to think of this like algebra. Same x Different = Different, Different X Same = Different, Different x Different = Same, or (+) x (-) = (-) [and vice versa], (-) x (-) = (+)
Wouldn't you need to change the logic the climate control is using to energize the heater valve solenoid if you were going to change either the solenoid or the heater control valve to "opposite" default? I guess you could use a relay to do this, and maybe this was your original point.
It seems like I could also put a diode on the climate control lead to the heater solenoid, which would keep power from running back up into the climate control circuit when my switch was on.
There are specific times that I want to activate this feature, and times that I don't care to, which is why I chose a switch. If it's cold out, or if I'm done driving the car for a few hours, I don't need the heater valve to automatically closed if I turn off the car. Plus I've already got the switch mounted in the console, so the switch stays whether its used or not.
01-24-2006, 05:27 PM
#18
Three Wheelin'
I was always taught not to use diodes unless it is the only solution to the problem at hand. They are inefficient, their failure mode is undesirable and work best at lower power applications.
01-24-2006, 10:12 PM
#19
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Brian,
Yes the whole sense of the AC drive circuit would be different too. I wans't proposing that this is a solution you could implement - but the car would work better for hot climates if Porsche had engineered it the other way around.... (but of course for most of the year AC is a low priority in Germany so easy to see what its the way it is).
As for the switch - since you don't always want to use the feature I'd say keep the switch - but add the relay too, then you need to flip the switch less often... and there is no risk you have it on when you are driving.
Assume that your vacuum holds as long as you want this to run with no issues?
Dave,
Diodes inefficient ? compared to a relay...? umm
You tend to be relying on some big diodes in the alternator everytime you drive...
Given a sufficient spec for the application they are quite robust and ultra reliable - but they won't tolerate overcurrent conditions so it has to be well designed to limit such cases.
Alan
Yes the whole sense of the AC drive circuit would be different too. I wans't proposing that this is a solution you could implement - but the car would work better for hot climates if Porsche had engineered it the other way around.... (but of course for most of the year AC is a low priority in Germany so easy to see what its the way it is).
As for the switch - since you don't always want to use the feature I'd say keep the switch - but add the relay too, then you need to flip the switch less often... and there is no risk you have it on when you are driving.
Assume that your vacuum holds as long as you want this to run with no issues?
Dave,
Diodes inefficient ? compared to a relay...? umm
You tend to be relying on some big diodes in the alternator everytime you drive...
Given a sufficient spec for the application they are quite robust and ultra reliable - but they won't tolerate overcurrent conditions so it has to be well designed to limit such cases.
Alan
