DC =/= AC

1/13/2025

I spent some real time today trying to figure out how the heater in this car works, and before we launch, I want to tell you –

1. It’s not what you think
2. I’m still not sure I get it, and I’m not sure anyone but the engineer does either

Let’s start with a favorite drawing of mine…

Obviously related, but probably won’t help you understand the issue – but damn it, it’s cute, right?

Alternating Current (AC)

Emma does not use Alternating Current (AC) power. She uses Direct Current (DC), coming from the battery, which is a 12 volt cell.

Your house uses AC power – 120 volts for the regular plugs and 240 volts for the big things like a clothes dryer.

It’s called alternating current (we’re going to distill this down – it’s much more complex than I’m making it) because it literally changes direction over and over.

Over and over – 60 times per second, as a matter of fact. That equates to 60Hz.

Since the power goes back and forth over and over, a ground in AC power is an off-ramp for faults (literally it eventually directs it to the earth).

I need you to remove that concept of ground from your brain for the remainder of this blog post.

Direct Current (DC)

I want you to think of direct current (for this context) as… well, this:

The faucet is the battery, the muck in the pipe is some resistance.

That’s how power in Emma works, with the caveat that a lot of the return pipe is actually the chasis of the car. We’ll get to that.

Emma and the Not-So-Great Heater

Right, so up front I’m going to drop this diagram I made, and we’ll discuss.

Don’t get overhwelmed, the steps are numbered and we’ll walk through that.

1. BATTERY to FUSE box – I think everyone know this is how it works. It’s how it works in your house, too – just on a much larger scale
2. FUSE box to BLOWER motor. What I need you to plant in your brain for this is that the FUSE box provides a 12v supply to the BLOWER motor.
3. Barring all else, BLOWER motor will do BLOWER motor things, meaning it’ll go full blast – like if we remove the rest of this chart, that’s what would happen
4. Current flows through the BLOWER motor and exits via a return lead to the SWITCH
   • 4.5a & 5a – If the SWITCH is on HIGH – the SWITCH just lets the current go to GROUND, creating a loop that powers the BLOWER motor at full blast
   • 4.5b & 5b – If the SWITCH is on MEDIUM or LOW, the SWITCH sends that live load to the RESISTOR, which expends a portion of that energy into the HEATER BOX as heat. The RESISTOR is grounded, and the completed circuit now allows reduced current
     • This in turn reduces the speed of the blower motor!
5. This step is technically articulated in the previous bullets, but what wasn’t said here was that when you look at the drawing above, 5 is technically Emma’s chasis.
   • WHY? – In this model, the whole point of GROUND is to find a path back to the battery. That’s it. It completes the circuit. The negative terminal of the battery (also a ground) terminates to the chasis of the car, right? Well, that becomes a power return path to the battery – like the pipe illustration above

All that to say – we’re missing the SWITCH > GROUND connection on Emma right now, which is why no matter what we set the SWITCH to, the BLOWER motor runs at 100%.

If you made it this far, thank you for being a loyal reader, and know that this blog post was more for future-Brian than anything else.