Lowering voltage to a 6V wiper motor

[Dan Ruddock]

Being an electronics guy thought I would pass along a better way to lower your voltage to the wiper motor than using a resistor.

The problem with a resistor is that the voltage drop across a resistor is dependent on load, more load less voltage output. Semi dry window will wipe too slow, very wet will be too fast.

Diodes load-regulate, and resistors do not. Although it is not a voltage regulator. System voltage goes up, diode output voltage goes up.

The better and cheap way is to use a string of 6-amp barrel rectifier diodes soldered end for end and you have to connect them all with the polarity the right/same way. They will get hot so glueing them to a small heat sink is a good idea. The more diodes the more voltage drop. You could even use a switch to create more speeds. If you connect the string backwards it will just not work, reverse it. Be careful about your setup shorting to ground and use an 8-amp fuse.

Diodes will have about close to 1 volt of drop per diode so you will get close to what you want from calculating. I would just make a string and then connecting with a alligator clip on it to move it up and down to find the sweet spot, you can then solder the wire.

https://www.mouser.com/ProductDetail/Rectron/6A05-...gJdP_D_BwE

Dan

[TomVaughan64]

Thank you for posting this. This is a great and timely post. I am working on this right now on my 55. Do you know where I would find a sample picture of this setup?
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Tom

"Begin with the end in mind"
Stephen Covey

"Keep focused and seeking knowledge, you will never find disappointed, only small hiccups!"
Jimbo

56 Old Speed Euro Deville Influenced Build

[Dan Ruddock]

I will be doing this to my 61 bug very soon. I am installing a 67 switch which has two speeds and will incorporate it into my setup, will take pics when done.

Dan

Here is the basic idea.

[mdege]

[Zwitterkafer]

If the diodes are getting hot, they are not adequately rated for the circuit. The current ratings should be high enough to offer a wide safety margin beyond what a wiper motor draws when first switched on. Connecting pairs of diodes in parallel for each segment of dropped voltage (0.6 to 0.7 volts each typically) should theoretically reduce their heating, since each would see about half the current. But having been down this path I advise caution and adequate duration bench testing.
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"Criticism comes easier than Craftsmanship"
- Zeuxius, 400 BC

[Dan Ruddock]

[Zwitterkafer]

Suggestion: Test the motor with wiper blades attached and on a dry windshield, grease inside the motor not warmed from previous running. Then add safety factor to the current value. Whatever the diode max continuous rating is, it is poor practice to design something that runs at the limit. Designers are typically comfortable with 50 to 70%, similar for thermal stresses. A diode hot enough to burn fingers will fail, it is just a matter of time. Of course, semiconductors can generate heat, but it is destructive. Engineers do what is necessary to keep the warming to a safe level. A fun experiment is to run a CPU chip with heatsink removed: Classic Thermal Runaway and smoking failure within a minute.

I apologize, I was not clear, I am not advocating parallel diodes in a final product. But in the testing phase, it can be useful to check heating reduction when halving the current. I've found it helpful in ballparking what "bigger diode" might be needed in the real world, before going out and buying them.

Go for it, a robust solution is surely possible. As long as the diodes are not overheated like in some of those solid state regulator failure threads on this site.
_________________
"Criticism comes easier than Craftsmanship"
- Zeuxius, 400 BC

[Dan Ruddock]

Here is my progress report. Takes about 9 diodes to lower to 7 volts on a 14 volt supply. The amp draw is 1.7 amps unloaded. You can see the 67 bug two speed switch which adds three more diodes for a 5 volt output second speed. I can fine tune once I get it in the car by moving wires to add or subtract diodes. JB'ed them to a aluminum bar. I will get rid of all the alligator clips.

Dan