[Sussex] Outcome Re: OT: another hardware question

[Desmond Armstrong]

Antony wrote:
> Desmond Armstrong wrote:
>   
>>> The circuits you found got me wondering: could a simple triac controller
>>> like [3] be run in DC mode as in [4] circuit 1c, and fitted with an RC
>>> on the gate so as to ramp up when power is applied?  It's beyond my
>>> skills though - any offers?
>>>       
>> No.
>>
>>     
> <snip>
>   
>> The triac is an on-off avalanche device <...snip...>
>> Philips did take up this proposal and make triacs 
>> with only the 3 quadrants of usability. The benefit is vastly improved 
>> dv/dt capabilities.
>>
>> It would be interesting to do more motor speed control work using this 
>> new generation of triacs. But that would require much time.
>>     
>
> Do you mean this high dv/dt device would be essential for use on
> rectified AC, but be hard to design for properly?
>
> I hoped that DC mode would make it easier to provide a ramping (fixed at
> 2 seconds) gate voltage, leading to a simple DIY circuit adequate for
> amateur use.  If the current never drops sufficiently for a triac or SCR
> to turn off, how about controlling AC using two SCRs back to back with
> RC circuits on their gates?
>   

No, you can't do that either, as the thyristor is simply 1/4 of a triac 
(or 1/3 of a modern triac).

I can see that I will have to talk to you on the elements of phase 
control of these devices. And a bit about controlling AC as well.

It's a field which is very interesting and was certainly possible to 
work in with the resources of a large company like Philips.

As for the hard to design bit, not really, it just that one needs to 
understand things like phase angles and the functioning of these 
switching devices.

As an example of design, I designed a power supply where I required the 
use of a linear voltage regulator but could not afford the high power 
dissipation that would result. Therefore I used a linear circuit which 
would typically have just 2 volts across it and then used a thyristor 
controlled pre-regulator to drop down from some 60 volts.
The result is a power supply going from 0 - 50 volts which is totally 
short circuit proof.
This application was designed as a replacement for a totally linear 
design with the problems of heat dissipation which was not s/c proof and 
in a laboratory power supply you will understand what this means.

If there is interest in this OT topic perhaps I should do a presentation 
(on this power supply), totally not computers, but then some 
understanding might be useful??