Jay Summet via EV wrote:
I think the hardest "feature" that I want (w.r.t. building a simple "bad boy" style charger) is the ability to automatically work with both 120v and 240v input, and switch between them without user intervention.
Most power supplies without PFC have a circuit that switches the input rectifier to a voltage doubler with a 120vac input, and a full wave bridge with 240vac input. 4 diodes, 2 capacitors, and a SPST switch. Often, the switch is a triac, and there's a simple circuit to sense the AC input voltage and switch it automatically.
That said... You will need two different cords anyway; one with a 120vac plug on the end, and one with a 240vac plug on the end. The cord itself can have a connector at the other end that plugs onto the charger with whatever wiring is needed to change voltages. For example, on products with a transformer having a dual primary, the 120vac cord can wire them in parallel, and the 240vac cord can wire them in series.
With a simple phase control system, I imagine that this would mean I would have to only have the SCR's turned on 50% of the time (or less) when running at 240 volts, while using most/all of the phase at 120 volts. (I believe this would result in the need for serious PFC?)
That's true, though if you're in the USA, power factor is more of a convenience than a requirement. A 0.5 power factor just means that the peak AC current is higher, though the average is still the same. It doesn't affect efficiency; it just limits the power you can get from a given outlet without tripping its breaker on the peaks.
There are also tricks for improving the power factor without the complexity of a full-blown high-power PFC circuit:
- As Cor noted, constant-voltage transformers have a pretty good PFC all by themselves. Standard ones are around 0.8, and harmonically-neutralized ones are over 0.9. - If you have a standard transformer input, they are inductive. You can add a compensation capacitor across the AC line, chosen to maximize the power factor at full load. - The "valley-fill" circuit dynamically switches between voltage doubler and bridge rectifier within each cycle. Instead of a big current spike at the peak of the AC line, you get a smaller peak in the middle and two smaller peaks on each side of it. -- Excellence does not require perfection. -- Henry James -- Lee Hart, 814 8th Ave N, Sartell MN 56377, [email protected] _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
