> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Kathy Quinlan
> Sent: Tuesday, January 22, 2002 2:05 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]: Calculating DC voltage from a sine wave
>
> Ok I am not using a PIC (the client specified an AVR) but I am using a
> Microchip A2D that uses SPI ;o)
>
> I have to opto Isolate the A2D as it is very expensive to provide opto
> Isolated DC to DC supplies to run the external SPI port for
> programming and
> control and the external RS232 / RS 485 :o(
>
> Ok now my problem, using one full cycle and doing 1 cycle on 9
> cycles off, I
> get 620Vdc and it does not move :o( I have checked my SCR firing
> pulses, and
> yes they happen every 200ms (I am in Au and we use 50Hz 240Vac (nominal))
>
>
> So it looks like I have 2 options
>
> #1 heavy loading of the storage caps (atm I have a ~64ma loading from the
> opto isolated SPI)
>
> #2 move to phase control and ignore the electrical noise ;o)
>
> Will need to find out what the customer wants to do, as these
> units are used
> in the middle of nowhere (up in mining country in the top of AU ;o)
>

Kat,

Are you saying that you have a fixed 1:9 drive to the charge control?  Your
software only needs to pulse the charge circuit for one complete cycle
whenever necessary to keep the storage caps at the required voltage.  This
is likely to be at a much slower rate than 1:9 when regulating.  If the
voltage is above your require level then the software should stop driving
the triac. Leakage current will normally be enough to cause the voltage to
drop naturally but a bleed of 64mA sounds high and should certainly be
enough to cause discharge.

If your software can switch the charge in single cycles anywhere from
continuously on to continuously off then it must be able to control the cap
voltage. Check that the A-D is working correctly and you have the correct
polarity output.  Using SPI is going to slow the loop response down somewhat
so I cannot guess what will happen.  You really need to be switching in near
real time for effective control.  It would be better to put the isolation in
the external RS232 and have the micro connected directly to the cap 0V. A
built-in A-D is even better.  Check your software open loop with a suitable
low voltage and check that the drive pulse rate varies in the correct
direction and is in sychronism with the line input.  If not then you are
likely to have problems.

Without knowing your circuit configuration it is difficult to work out what
is happening.  Do you have a split rail simply to increase the effective
capacitor voltage or is the centre point connected to neutral and used as
part of the multiplier?  What multiplier capacitors are you using?  You are
using multipliers rather than just rectifying the line aren't you?

You say that you are using an SCR, but if you are controlling the input (AC)
to the multiplier then you need either a triac or two SCRs back to back.  If
you are trying to control the output (half wave DC) of the multiplier then
that may well be your problem.  It could be that your capacitors are just
charging to the peak of the input.  It is difficult to guess without knowing
the detail and there are so many possible circuit configurations.

If you need the capacitor voltage to respond reasonably quickly to a control
input over a wide range then the traditional method is to add a resistive
dump circuit that works in opposition to the charge circuit to discharge the
capacitors.  Use a MOSFET (Opto-coupled in your case) to switch a resistor
across directly across your caps.  The software needs to have upper and
lower limits which cause the circuit to either charge or dump (but not both
at the same time).

If your circuit is to be used away from civilisation ;-) without a
reasonable mains supply then be aware of the characteristics of small
generators with varying load - usually poor frequency control and potential
under and overvolting.  This can play havoc with phase control circuits.

David

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