# Feeding power from two sources?

#### MB107

Joined Jul 24, 2016
290
I have a situation where I need to feed power to an automotive radiator fan from two power sources. First off this is signal power only, not motor power. So we are talking less than 0.5 Amp here. The first source is for power to be fed with the Key on. For Mercedes Benz vehicles this would be called circuit 15 power. In a second case the power would be fed with the key off directly from the car battery through a thermal switch that would allow power to flow whenever the radiator temperature exceeds 110°C. For Mercedes Benz this is called circuit 30 power. The problem is I don't want circuit 30 power to activate circuit 15 when the thermal switch activates. Since these two circuits are not carrying fan power I was thinking a diode or similar device put in line with circuit 15 might do the job. I could do this with a relay that switches off circuit 30 and switches on circuit 15 whenever the key is on except if the temperature was above 110°, which could happen while driving then circuit 30 would be connected to circuit 15 which seams like it would be OK until some high power device on circuit 15 decided to feed from the tiny wires going to the radiator fan controller. So my question is what is the best way to accomplish this and if it's a diode what size and type.

#### LowQCab

Joined Nov 6, 2012
2,514
First You need to state what the problem is.

Then You need to state, step-by-step, how the manufacturer intended the system in question to work,
and a Schematic, or at least a flow-chart or block-diagram
showing exactly how the manufacturer's system was intended to work.

Then You need to explain how and why this is not ideal in your estimation,
and what You think may be a viable solution for making
this system "more" ideal for your particular purposes.

Supply any proposed ideas that You have in the form of a Schematic, Block-Diagram, etc.,
not a "word-salad" that leaves everyone else with their vision getting blurry, and a headache coming-on.

Schematics are the language of Electronics.
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#### DickCappels

Joined Aug 21, 2008
9,347
You can probably do this with just a DPDT relay and your temperature sensing circuit to select the power source. The diodes are a clever idea but you may not like the voltage drop across them,\.

#### LowQCab

Joined Nov 6, 2012
2,514
The chances that the Mercedes-Benz Engineers did something stupid is extremely remote.
Why do You need to cut into a ~$3000.oo Wiring-Harness ? . . . #### MisterBill2 Joined Jan 23, 2018 13,197 I was also going to ask the question of why do you want to do what it seems that the OEM would have done. M-B is not noted for cutting corners in their products. OR is this adding an electric fan to a much older vehicle that had a belt driven fan? I am wondering why you are wanting to modify what is already there. Really puzzled, in fact. Thread Starter #### MB107 Joined Jul 24, 2016 290 OK some background. Mercedes Benz made a beautiful car back in the 80's called the 560SL. It had a mechanical fan and worked perfectly. In its day with 220 HP it was considered a very powerful car. Today not so much. So I have since corrected that situation by installing a V12 engine from a newer Mercedes SL600 with 389HP. Problem is there was no room for the mechanical fan that came with the engine. So an electric fan became necessary. I was running a small un-shrouded 16", 18 Amp fan for 13 years without issue. But last year I installed AC into the car and I noticed every thing was fine but it was running a little on the hot side. I decided to do the ultimate test and try to climb Mt Mitchel NC on a 90° F day with AC on. Needless to say it did not make it. So now the approach is to install a shrouded 19" 50 amp fan removed form a later Mercedes. One of the options that may or may not be required is the ability to run the fan when the car is tunrned off until the radiator cools down to something tolerable. So the proposed circuit will run the fan off the battery, hopefully for a short time, in the event the coolant temperature is excessively hot. To do that, I proposed the attached circuit. The challenge is that when the temperature switch closes circuit 15 will become feedback through the green wire. I want to limit that back feed to the fan only. In the Schematic the solid red wires are circuit 30 power at all times. The red and white wires are Circuit 15 power with key on. I don't know how to characterize the green wire. Thread Starter #### MB107 Joined Jul 24, 2016 290 You can probably do this with just a DPDT relay and your temperature sensing circuit to select the power source. The diodes are a clever idea but you may not like the voltage drop across them,\. View attachment 255077 I actually thought about this. But I felt that once the thermal switch was activated it would also activate the relay because the relay would now be feed by circuit 30 as well as circuit 15. What kind of VD would you expect across the diode. The wire doing to the fan is only a signal to activate the fan, I may get away with a couple of volts drop, I would have to test it. Thread Starter #### MB107 Joined Jul 24, 2016 290 I was also going to ask the question of why do you want to do what it seems that the OEM would have done. M-B is not noted for cutting corners in their products. OR is this adding an electric fan to a much older vehicle that had a belt driven fan? I am wondering why you are wanting to modify what is already there. Really puzzled, in fact. The whole story of the fan starts here at post 1060. The rest of the car's story starts actually before post 1. https://www.benzworld.org/threads/theirs-a-new-one-coming.2876217/page-53 Thread Starter #### MB107 Joined Jul 24, 2016 290 The chances that the Mercedes-Benz Engineers did something stupid is extremely remote. Why do You need to cut into a ~$3000.oo Wiring-Harness ?
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It's not like I haven't been there before.

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#### MisterBill2

Joined Jan 23, 2018
13,197
Given the current draw of the fan iit does not seem reasonable to control it except by temperature, although it also makes sense to limit the run time when the engine is off. So how about using two thermostats to control the fan relay, each through a diode, so that the higher temperature thermostat will control the feed directly, even with the engine off, while the lower temperature thermostat is fed from the "engine on" circuit. Each diode should be rated for 5 amps. so that the drop will be less than a volt feeding the control relay. There will be no chance of either circuit feeding the other, and only a single expensive relay to mount and wire. There will need to be a switch in the lead from the two diodes to the relay to allow disabling the fan when doing service in that area.

#### MikeKulls

Joined Apr 4, 2016
60
First I would say, you need a bigger radiator. Fans don't really add much once the car is moving. If you need cooling after it's shut down then you have a different problem you need to solve. Only exception might be if you race it and shut it straight off but you really should run the engine at idle for a minute or 2. Once you shut off the engine the water stops circulating and you can get water boiling around the engine block.

If you still want this solutions then 2 diodes would work fine. An automotive relay won't be too fussed about a 0.6V drop. Just get 1A diodes from a local electronics shop, just because they have thicker legs.

#### LowQCab

Joined Nov 6, 2012
2,514
The Ideal arrangement is to have 2 identical Fans, ( 3 can also be done ),
that are normally running in series connection with each other.
This will cause the Fans to run at ~1/4-of their normal Power,
the Fans should run in this configuration anytime the Engine is running,
or, run when a Thermostatic Switch indicates that things are very hot with the Engine not running.

The Thermostatic-Switch can also be replaced with a 5-Minute-Timer that
keeps the Fans on "Low" for 5-Minutes after the Engine stops.

A separate Thermostatic-Switch should switch the Fans to "High", ( Fans connected in parallel ),
anytime the Coolant-Temps exceed ~100C.
In warmer climates,
the Thermostatic-Switch can simply monitor under-hood temps instead of Coolant.

Having 2 Fans running at ~25% at all times, generally means there will seldom be
any need to run the Fans after Engine shut-down.

I run 4-Fans on my Truck in this exact configuration, ( under-hood-temps, not the Timer option ),
with the exception that I take 3-phase-Power directly from the Alternator-Windings and
then run it through a stout 3-Phase-Bridge-Rectifier to power the Fans separately.
This way the Fans only receive Power with the Engine running,
and heavy Power switching is simplified.
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#### LowQCab

Joined Nov 6, 2012
2,514
Also,
pay close attention to the Air-Flow EVERYWHERE around the Radiator,
and the whole front-end of the Car.
Recirculation of HOT-AIR doesn't provide much cooling.

An Air-Dam, Spoiler, Splitter, ( call it what you like ), can make a huge difference.

Foam, thin-Rubber-Sheet, plastic filler-panels, etc., also make a noticeable difference.
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#### MB107

Joined Jul 24, 2016
290
Given the current draw of the fan iit does not seem reasonable to control it except by temperature, although it also makes sense to limit the run time when the engine is off. So how about using two thermostats to control the fan relay, each through a diode, so that the higher temperature thermostat will control the feed directly, even with the engine off, while the lower temperature thermostat is fed from the "engine on" circuit. Each diode should be rated for 5 amps. so that the drop will be less than a volt feeding the control relay. There will be no chance of either circuit feeding the other, and only a single expensive relay to mount and wire. There will need to be a switch in the lead from the two diodes to the relay to allow disabling the fan when doing service in that area.
This is not a relay controlled fan. It is a PWM controlled fan. The fan PWM controller uses its own separate thermister so the thermal switch is a secondary control only used when the engine is off and I do agree it may need to be time limited, which I should be able to do with a timer relay if need be.

#### MB107

Joined Jul 24, 2016
290
First I would say, you need a bigger radiator. Fans don't really add much once the car is moving. If you need cooling after it's shut down then you have a different problem you need to solve. Only exception might be if you race it and shut it straight off but you really should run the engine at idle for a minute or 2. Once you shut off the engine the water stops circulating and you can get water boiling around the engine block.

If you still want this solutions then 2 diodes would work fine. An automotive relay won't be too fussed about a 0.6V drop. Just get 1A diodes from a local electronics shop, just because they have thicker legs.
Ahh Mercedes Benz and myself will disagree. In the last 20 years Mercedes and other car companies have thinned up the radiators and changed to the high power electric fans. Thats the reason I was able to find a fan that powerful in such a compact design. My problem occered at low speeds following slow cars up the mountain.. I dont have any issues once on the highway.

So you feel a second diode would be appropriate I'm assuming on the circuit 30 where it goes into the temperature switch.

#### MB107

Joined Jul 24, 2016
290
The Ideal arrangement is to have 2 identical Fans, ( 3 can also be done ),
that are normally running in series connection with each other.
This will cause the Fans to run at ~1/4-of their normal Power,
the Fans should run in this configuration anytime the Engine is running,
or, run when a Thermostatic Switch indicates that things are very hot with the Engine not running.

The Thermostatic-Switch can also be replaced with a 5-Minute-Timer that
keeps the Fans on "Low" for 5-Minutes after the Engine stops.

A separate Thermostatic-Switch should switch the Fans to "High", ( Fans connected in parallel ),
anytime the Coolant-Temps exceed ~100C.
In warmer climates,
the Thermostatic-Switch can simply monitor under-hood temps instead of Coolant.

Having 2 Fans running at ~25% at all times, generally means there will seldom be
any need to run the Fans after Engine shut-down.

I run 4-Fans on my Truck in this exact configuration, ( under-hood-temps, not the Timer option ),
with the exception that I take 3-phase-Power directly from the Alternator-Windings and
then run it through a stout 3-Phase-Bridge-Rectifier to power the Fans separately.
This way the Fans only receive Power with the Engine running,
and heavy Power switching is simplified.
.
.
.
Unfortunately I am constrained by real estate for any more fans, I currently have one fan up front which Mercedes uses only with AC on. To install the V12 engine I had to move the radiator and condenser forward an inch and consequent had reshape the front cross member to allow the front fan to fit. Finding a fan to fit on the back side of the radiator was a major miracle. The current fan is now 5/16" away form the engine pulley, and it cant be moved left, right or up or down without hitting something else. The new fan I'm putting in place requires a custom made low profile shroud. The fan motor is supported by struts placed in strategic locations to keep it from hitting engine pulleys etc.

The PWM fan will control fan speed based on thermister output. The fan will run at 20 to 90% based on engine and AC condenser temp. Unfortunate without the PWM signal the fan runs in emergency mode at 100% unless I build another PWM controller or tap into the first one. Trying to avoid that level of complexity and in fact I'm actually hoping I don't even need the thermal switch once I have the big fan installed. And these larger more powerful fans have worked out well for people who have adapted them to there older mechanical fan error cars. They were able to remove the second aux fan in front of the radiator and still have superior performance.

I do like the idea of monitoring under hood temps instead of radiator temps to run the fans after shut down in my situation. Fortunately I was smart enough to put two ports on my radiator for temperature taps but I may try the under hood temps for after shut down.

#### MB107

Joined Jul 24, 2016
290
Also,
pay close attention to the Air-Flow EVERYWHERE around the Radiator,
and the whole front-end of the Car.
Recirculation of HOT-AIR doesn't provide much cooling.

An Air-Dam, Spoiler, Splitter, ( call it what you like ), can make a huge difference.

Foam, thin-Rubber-Sheet, plastic filler-panels, etc., also make a noticeable difference.
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Yes I'm working that angle as well. I have room to do some filling below the radiator and between the condenser and radiator.

#### MikeKulls

Joined Apr 4, 2016
60
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.

#### MikeKulls

Joined Apr 4, 2016
60
BTW, can you post a pic of the the car? My first car was a 1964 mercedes 190c or 190b. It's been a while but I still do like the older mercs

#### LowQCab

Joined Nov 6, 2012
2,514
BTW, can you post a pic of the the car? My first car was a 1964 mercedes 190c or 190b. It's been a while but I still do like the older mercs