REFLECTOR: PWM Controller

[Joel Smith]

I was a little surprised to that you might be using an SCR controller to 
vary the speed of a DC motor. In my opinion, you have to do some 
unreliable things in electronics (none of which are RF quiet) to de-gate 
an SCR under a DC load. I would have thought that with today's 
technology, a MOSFET would have been utilized. They do not have some of 
the bad characteristics of an SCR. They are more reliable and lower on 
resistance which results in lower power dissipation.

So I went to the web site suggested and the controller I found did 
indeed use MOSFET. If you have a choice, I would recommend the MOSFET 
design. It is likely to be noisy since they are using pulse width 
modulation via square wave to control the power. While this reduces the 
load on the controller, it will produce a lot of high frequency 
harmonics in the square wave. It could be that their design is doing a 
slow switch on the MOSFET to reduce RF noise. This might be reflected in 
the specs by observing a higher dissipation in the controller. If so, 
you would want to mount it in a location where it gets reasonable cooling.

The lighting circuit is probably not a problem. If it is, shielding the 
controller and a low inductance capacitor across the controller would 
probably solve the problem. Higher motor currents may take more work.

You should be able to get an idea via ground testing by turning on your 
av radio, antenna connected, unused frequency, and open squelch. Listen 
for noise as you vary the controller across the full range.

I am surprised that they would say their controller emit no noise unless 
they are linear. The say they are PW controlled.

Joel

Hiroo Umeno wrote:

> I have built a MCU based annunciator panel that incorporates PWM based 
> panel light dimming mechanism.  It also incorporates a slaved cabin 
> light dimming solution based on its own logic.  (Each PAX has control 
> over the brightness of their map lights but the PIC can set the 
> maximum brightness up front).
>
>  
>
> I have not flown it and it has so far seen a fairly limited ground 
> testing but so far, I have not seen any RF or audible interferences.  
> The circuit only average a couple mA with 20mA peak current.
>
>  
>
> I will also be carrying PWM based camera servo control eventually.  I 
> am more worried about that one since there is a motor connected to it.
>
>  
>
> Hiroo
>
>  
>
> ------------------------------------------------------------------------
>
> From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] 
> On Behalf Of NMFlyer1 at aol.com
> Sent: Thursday, June 09, 2005 6:36 AM
> To: reflector at tvbf.org
> Subject: Re: REFLECTOR: PWM Controller
>
>  
>
> Scott,
>
>  
>
>  I am using a transistorized DC Motor controller for my fan speed. It 
> is the SCR type.
>
>  
>
> I talked to the manufacturer of mine, and they said that there is no 
> RF interference from the controller. There might be some from the fan 
> motor itself they warned.
>
>  
>
> Just to be safe(er), I wired the switch/pot to the unit using shielded 
> wire.
>
>  
>
> Since I still have my panel out for final wiring, I haven't been able 
> to test the RF on my installation. I'll let you know when it is done.
>
>  
>
> I bought my controller from www.winland.com <http://www.winland.com>  
>
>  
>
> They were very helpful. They removed the board mounted pot for me and 
> provided tabs for my own remote mounted one.
>
>  
>
> Hope this helps.   Kurt
>
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