Re: Motors comparison page updated

In lugnet.robotics, Brian Davis wrote:
> In lugnet.robotics, Philippe Hurbain wrote:
>
>> Good thing, the load is not inductive.
>
> No, purely resistive. Although it does have a "boot up" time, I think it's just
> due to heating up the element. I'm not completely sure if the element will
> respond better to a "dimmer" type set-up (running it at less than normal
> voltage) or a "pulsed" set-up with a relay... but with the relay I'd have
> all-electronic NXT-based control, which I really like.
>
>> Looks like some people do:
>> http://www.amazon.com/Leviton-61500-I-Single-Pole-Electro-Mechanical-Incandescent/dp/B003AUDF0Y/ref=sr_1_1?ie=UTF8&s=hi&qid=1294925232&sr=1-1
>
> Wow! Thank you, I didn't realize they made those (although they aren't cheap...
> but it's at least viable).
>
>> Fortunately no, they are not dissipating all the power not going
>> in the load!
>
> Thanks for that tidbit as well... I wasn't aware of that, and was worried about
> it.

I have come across some industrial heater controls.  To get the heaters at part
power (to run them in a closed-loop temperature control), a method of burst
firing is used.

The circuit measures the zero crossings of the AC supply and turns the heater on
for a number of cycles at a time.

If you don't measure the zero crossings on something that's switched often, it
could cause a lot of electrical noise and break the FCC rules!  I guess the
rules specify the maximum amount of noise in a given frequency range.

Ideally you could get the NXT to supply a PWM motor power level and turn that
into a number of cycles.  You might have to smooth it to a variable voltage
(diode in series, capacitor and resistor in parallel to ground) before feeding
it into the burst firing circuit because the NXT PWM frequency is higher than
the mains frequency.

I don't have a definitive circuit but there must be lots out there!  I merely
suggest this as a line of enquiry.  Hope it helps.

Mark