PWM — pulse-width modulation — is how an AC charger tells the car its current limit. The charger sends a repeating square wave down the control pilot wire, on and off in a steady rhythm, and the number travels in the proportion — the share of each cycle spent “on” is the advertised maximum current. The car measures that share and draws at or below it. No data packet, no login, no network — one wire, one wave, one number hidden in a ratio.
Key Takeaways
- PWM encodes the charger’s current limit as the “on” share of a repeating square wave on the control pilot.
- A ratio survives noise and wear better than a voltage level — hence the hidden share.
- The charger writes the wave and can revise it live; the car reads the current share.
- DC fast charging replaces the wave with a digital conversation, on the same fail-safe logic.
A Number Hidden in a Ratio
Think of a dimmer flicking a light too fast to follow: brightness follows the share of time spent on. PWM works the same way, except what is read is a limit, not brightness. The charger holds the rhythm constant and varies only the share: more “on” advertises a higher permitted current, less a lower one. The car times the wave, works out the proportion, and treats it as the ceiling — no knowledge of the charger needed.
Encoding the number as a share, rather than as a precise voltage level, is deliberate. A ratio refers to itself — the car compares the on-part with the whole cycle it just measured — so the message survives noise and ageing contacts outdoors. A level can drift; a ratio cannot drift without changing its meaning.
Why a Wave, Not a Data Network
| What AC charging needs | Why PWM fits |
|---|---|
| One cheap, rugged wire inside a plug handled outdoors | The whole message is a rhythm on one conductor — nothing to pair or configure |
| The same system in every car and charger brand | A share of a cycle is simple enough for basic electronics on either side |
| Failure must mean stop, never guess | If the wave disappears, there is no advertisement — and without one, no power flows |
| The limit must be live, not historical | The wave repeats continuously — the car reads the current offer, never a remembered one |
The pilot system sits inside the charging modes — Mode 2 vs Mode 3 charging shows where the signalling lives in each.
Who Writes the Wave, Who Reads It
The charger writes, the car reads. Change an adjustable charger’s setting and you rewrite the wave: the unit advertises its new ceiling by changing the share, and the car follows the wave it is receiving. The offer can be revised mid-session, too: the statement repeats continuously, so a new share is picked up within cycles. The car’s side rides the same wire: by how it loads the line, the car reports connected, ready or done.
Where PWM Stops: DC Fast Charging
AC needs exactly one number from the charger, and PWM carries it elegantly. DC fast charging needs a negotiation — voltage and current requests, battery state, insulation checks — and a single ratio cannot say all that, so on a DC inlet the pilot wire carries a digital data conversation instead, as the explainer on how J1772 works with CCS describes. One thing survives the change: the DC conversation sits on the same fail-safe logic — no valid signal, no power — that the simple wave established.
The Wave in EVDANCE Products
The EV-ACS4005T portable charger (9.6 kW, NEMA 14-50P, IP66) offers 16, 24, 32 and 40 A settings — each one a different advertised ceiling, which is to say a different share of the wave your car reads. The EV-ACS3210T (32 A NACS, about 7.7 kW, IP66) makes the same point with six steps between 10 and 32 A. Choosing a step for a smaller circuit works because the advertisement is rewritten — the car sees only your selected ceiling.
Frequently Asked Questions
Is PWM the same thing as the control pilot?
Not quite. The control pilot is the wire; PWM is the encoding the charger uses on it to state the limit. The same wire carries the car’s side — connected and ready states — by how the car loads the line.
Does PWM change how fast my car charges?
Indirectly: the wave states the ceiling, and your car decides the draw beneath it. A higher share raises speed only if the car’s own limit was not already binding.
Do all AC chargers use PWM signaling?
Yes — it is the standard AC approach for the Type 1 and Type 2 systems covered here, built into the connector standards, not a brand feature. Any compliant car can read any compliant charger.
Related Reading
- How Do Home EV Chargers Work?
- Mode 2 vs Mode 3 Charging: What IEC 61851 Means for Portable EV Chargers
- Does J1772 Work With CCS? CCS Connectors Explained
- What Is J1772? The North American EV Plug Explained
