Power circuit

Archiver

Hi Gang:

Just when I thought I knew it all, Murphy shows up and goes, "Gotcha!"

I am looking for an SCR firing circuit for four SCR's. The SCR's are going to
allow a full wave of AC to flow to the load when they are turned on (0 degree
firing only). I was going to use a triac circuit complete with a triac based
opto-coupler but my voltages and currents are too high to allow this (especially
since I am not switching with respect to a neutral (0V) connection). I also
cannot use solid state relays unless I "roll my own." Therefore I need to go
back to a conventional power bridge design.

What will fire this circuit will be an output signal from the stamp (go or
no-go). And I have all of that planned out and working - I am just missing this
one circuit.

Any ideas or suggestions would be welcome.

Thanks and regards,

Gerry Shand
gshand@h...

With enough time, energy and/or money, anything is possible



[noparse][[/noparse]Non-text portions of this message have been removed]

Archiver

Gerry:

It sounds like you should be considering inversely paired SCR modules, or
back-to-back switches, as they are sometimes called. Try www.eupec.com where
these are listed as Type TT devices, for thyristor/thyristor. These are
typically used for full wave control. Firing at the zero-crossing is also a
bit unique in your scheme, since the voltage to N or Gnd will only be zero
if your Ac line pairs 180 degrees out of phase with each other, such as with
240V single-phase lines. 208V lines are the same sine waves that will not
cross at zero. They will, however be at zero potential to each other at the
crossing. In either case, there are many devices rated for the voltage I
think you are talking about.

An easy way to gate the thyristors at the ZC is to use opto's that will only
output at zero. You might check the MOC306x series for starters at
www.fairchildsemi.com, these are a good starting point. You'll only need to
time your Stamp output to occur across the zero-crossings on your AC time
line. This also means that you will have to know at least approximately
where they are, so you'll need to design a ZC detector to input a timing
reference to your process.

I have used similar schemes to control many hundreds of Amps at conventional
line voltages up to 460. Theoretically these devices can be series connected
to increase voltage ratings, such as what Siemens did to create the first
large-scale software-controlled power-grid switcher featured in MIT's
technology review a few months ago.

I don't know why SSR's won't work for you. How big is your load, and why
four AC lines?
It would be nice to know that I am not giving the store away to a
competitor. If not, I can share much more....

Chris


> Hi Gang:
>
> Just when I thought I knew it all, Murphy shows up and goes, "Gotcha!"
>
> I am looking for an SCR firing circuit for four SCR's. The
> SCR's are going to allow a full wave of AC to flow to the
> load when they are turned on (0 degree firing only). I was
> going to use a triac circuit complete with a triac based
> opto-coupler but my voltages and currents are too high to
> allow this (especially since I am not switching with respect
> to a neutral (0V) connection). I also cannot use solid state
> relays unless I "roll my own." Therefore I need to go back to
> a conventional power bridge design.
>
> What will fire this circuit will be an output signal from the
> stamp (go or no-go). And I have all of that planned out and
> working - I am just missing this one circuit.
>
> Any ideas or suggestions would be welcome.
>
> Thanks and regards,
>
> Gerry Shand
> gshand@h...
>
> With enough time, energy and/or money, anything is possible
>
>
>
> [noparse][[/noparse]Non-text portions of this message have been removed]
>
>
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