Retro Modding a vintage Beetle - electrical question

ManglerYJ

ManglerYJ

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Joined
Mar 20, 2005
Location
Lexington, NC
So I have a 1972 Super Beetle that I'm hoping to get a little more ventilation out of at low speeds. It came from the factory with a 2 speed "fresh air" fan that no longer works (they were crap from the factory from what I understand). I have two PC case fans that fit nicely 1 where the busted dash speaker was and the other behind the screen on the other side of the steering wheel. The wiring on the beetle comes from a control knob on the dash and has two detents, one for each lead that comes from the switch and to a lead on the fan. There is also a ground wire. My hope is to be able to use the first wire to power one fan and the second wire to power both of them, sort of a "low-high" setting. Basically, the switch has 12 volts in and two 12 volts out as opposed to a rheostat (I think that's what it's called) how most other fan motors will run.

Is there an easy way to run one fan on the low speed wire and both fans off the high speed wire? Am I making this more complicated than it needs to be?
 
You already need a relay for each fan just like normal, and all you need to add is a diode, or two diodes for best practice.

I'll draw something up in a minute.
 
Okay, here's my 2 minute drawing. I'm watching the race, I can spare 2 minutes.

Switch LOW position turns on RELAY1 for FAN1. Switch HIGH position turns on RELAY2, and also RELAY1 through DIODE1, so both fans are now on.

DIODE2 is an extra layer of protection for unexpected switch behavior if you don't know exactly how that switch works. It's just to prevent the low terminal from getting fed with voltage when the switch is in the HIGH position, and will also prevent the wrong indicator if there is a LO/HI indicator light or whatever. If it's just a switch, with no indicator and no overlap during switching, DIODE2 is totally optional.

There's nothing special about anything else, it's all just normal relay contact and relay coil wiring. The diodes are the only thing different.

DIODE.jpg
 
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Fabrik8 said:
You already need a relay for each fan just like normal, and all you need to add is a diode, or two diodes for best practice.

I'll draw something up in a minute.
Click to expand...


They didn't use a relay in the original - probably the reason why the oem fan sucked. I'm surprised that these little PC fans would require a relay, though since they are only like 20 gauge wire (or even smaller). So the diode will act as like a one way "valve" for the electricity?
 
Yes, a diode acts like a one-way valve. If it's just a PC fan, and is able to be directly powered from the switch, you can get rid of the relays and just use the diodes. Or, you can use a small relay instead of a big Bosch automotive relay.


BUT


A diode has a voltage drop across it (it's a fixed drop and varies with the type of semiconducting material, it's just how a diode works) and you'll lose that amount of voltage to the fan. Different types of diodes have different Vf (forward voltage drop) from maybe 0.3V to almost a volt, so diode selection becomes a little bit of a headache if voltage to the fan is important. If you don't use a relay, the diode also has to be sized for the fan load, but that's pretty easy for a tiny fan of that size.

None of that stuff matters with a relay. Almost any diode will work fine for a relay coil, because the relay coil has a fairly wide voltage range and the coil current is low.
 
Side thought, Rather than wire it to run the two fans separately, you could wire it to run both fans on either setting, but use a resistor off the one switch setting to reduce the speed for a low setting...
 
Like so;

resist.jpg


If you happen to have an old heater blower motor resistor sitting around, you could try the different legs on it, to see what might be a good speed for your low setting...
 
Above is series, you could also wire it parallel, which in theory would yield the fastest high speed setting, And would require a diode, but in reality, because PC fans are such low resistance anyway, probably won't be a noticeable difference.
parellel_resist.jpg
 
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If you wire the fans like that (in series) each fan will get 6V. If they're 12V fans, they probably won't operate at that low of a voltage. If they do operate down to 6V, you'll get very, very slow fans. There's also the matter of the badly tracking ground reference for the first fan, because the negative terminal will change voltage based on what the second fan is doing. Computer fans have circuitry inside for speed control and motor commutation, and they don't like to track all over the place when held above ground by 6V that floats all over the place. Basically it doesn't like its ground terminal to be at 6V instead of ground.

And if you used a diode, (along with the compromises that go along with it), you would have no need for the resistor.
If you wanted to use a diode, the proper way to do it is per my drawing above, but the fans would take the place of the relay coils in my drawing above, and only D1 is needed.
 
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Hmmm, Thinking about it, I don't think the diode in the parallel diagram is really even necessary, but I'm honestly not sure...
 
zuke said:
Hmmm, Thinking about it, I don't think the diode in the parallel diagram is really even necessary, but I'm honestly not sure...
Click to expand...

It's not necessary, because you're bypassing it with the other wiring. You've got two connections to each positive fan terminal, so the diode is in parallel with the other circuit branch between the two fans. It will be much more obvious if you re-draw it.

The two fans are in parallel no matter what, the diode is just hanging out in the wind.
 
Fabrik8 said:
It's not necessary, because you're bypassing it with the other wiring. You've got two connections to each positive fan terminal, so the diode is in parallel with the other circuit branch between the two fans. It will be much more obvious if you re-draw it.

The two fans are in parallel no matter what, the diode is just hanging out in the wind.
Click to expand...

Yup, You're absolutely correct, The diode is doing nothing! And I'm trying to figure out what I was even putting it in there for... Been too long since I even screwed with this kind of basic electronics....

So, I defer to @Fabrik8 for any advice on this matter, @ManglerYJ , Take his word first!
 
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