I've been tempted to write a basic auto-electric book focusing on 6-12V conversions, and have even been encouraged to do so in the past. Too often, we get caught up in some of the more advanced electrical theories and it muddies the water with exacting precision and needless confusion. What works and why it works are often not as important as how it works. There are a lot of things that do work, but are less than ideal.

Sometimes, we know too much and get caught up in an emotional debate rather than a reasoned one - For example, I prefer true electronic CVRs to the OEM Ford "CVR" which is nothing more than a chopper. A chopper is an inelegant solution, but it is adequate for what we do. There are many advantages to using a true electronic CVR, but for many, the argument that "if it was good enough for Ford, it's good enough for me", I now acquiesce, make the case why the true CVR is better, and add that the choppers will work. There's more than one way to skin a cat.

For those who have been around this site and others for a long time, you may remember myself and an individual on Ford Barn (long before Ryan bought it) going round & round about the repro fuel tank senders. He insisted it could not possibly work and yet I had done it several times - as had others, obviously. In fact, I used to hold his view, but had to change in light of empirical evidence to the contrary and rethink what was really going on in the circuit. It makes perfect sense to me now why it works as well as it does.

In the grand scheme of things, our 6/12V DC automotive systems are very simplistic and usually easy to understand (although there are times even I scratch my head for a good long while). As long as we're keeping things relatively simple, with basic circuits, it's pretty straight-forward. Who knows...maybe I will write that little book one of these days.

 

Another good reason to use a relay is that some horns deteriorate rather quickly, depending upon the manufacturer. Sparton horns, used by Ford and others, were notorious for their problems. A relay in the circuit will protect other components like the horn button from damage.

 

To add my 2 cents about voltage, current and wattage. If I use a horn designed for 6V and it is set up to draw 17 amps and then run it on 12 volts, I expect the amps to stay at 17 amps. What will double is the wattage. 6V x 17 amps = 102 watts while 12V x 17 amps = 204 watts. I converted to 12V and my horn now sounds more like a 204 watt than a 102 watt. Think about that. I like the idea of a relay but i don't have one yet for the horn. Patrick

 

Watts muddies the issue considerably, because you're making assumptions about current remaining constant. However, the only two constants in this type of comparison are the voltage and the resistance. Here is a simple online calculator:

Watts/Volts/Amps/Ohms conversion calculator

Enter 6V and 17A and you get 0.352941176 Ohms and 100 Watts. So far, so good - now hit reset and enter 12V and 0.352941176 into the resistance and you get 408 Watts and 34A.

I love my louder, angrier horns, but you're quadrupling wattage by doubling the current in this case.

Yes, a relay is a good idea!