Homebrew PCBs

Richard Mcfarlane wrote:

>Hi Curt,
>I like to see more about your CNC- EDM experiments.
>I carefully observed THE GARDEN OF EDM - PWM power
>supply is much better than the crude old RC-Generator.
>For efficiency reasons I have to sacrifice about IGBT
>costs.
>I wish to learn from you about the UP/DOWM motor
>control quill movement to maintain the constant gap
>voltage across the tool electrode.
>Presently I am experimenting with an Opto-Electronic
>comparater design to control the arc voltage (Based on
>a Japanese Patent).
>
>Regards.
>Richard
>
>  
>
Hi Richard,

Sorry about the delay in my PCB EDM work but I've been rebuilding a 1954 
Willys Pickup!   The town I live in does not like unlicensed vehicles on 
private property ( might be a REDNECK in town   :-)    gotta' protect 
property values   >:o    ).  I've been hip deep in truck parts for the 
last few months.  I've only been able to do a small amount of work on 
the PCB EDM project, but I've summarized it below.

I'm working on reconfiguring the Garden of EDM to the International 
Rectifiers  IPS5451 "intelligent power switch" instead of IGBTs or 
FETs.  The IPS5451 chips have a 50 volt and 45 amp pulse limit. and 
built in thermal and over current protection.  I'm using a 556 dual 
timer to control the pulse repetition rate and gap breakdown voltage.  
The 555 allows pulse rates up to the 1 mHz  range and the 30 amp rating 
of the IPS is about twice the needed pulse current.  I would like a 
higher voltage rating, but my initial experiments show clean traces with 
.003 inch brass wire as the electrode with pulse rates of 30,000 per 
second.  Since the IPS handles all the proteection circuitry internally 
( the chips are meant for automotive uses ) the Garden of EDM circuit is 
reduced to a complexity level that matches the RC EDM circuitry. 

My breadboarded circuit consists of  a 48 volt DC-DC converter ( too 
close to the IPS limits, but it's what I've got at hand ) feeding a   
~10-20 uf polypropylene cap with a SCR triggering the pulse when the IPS 
has switched off.  One half of the 556 controls the discharge rate and 
the other half blocks the IPS while the gap is conductiong.  I'm using 
the servo circuit from Ben Fleming's _The EDM How-To Book_  ( based on a 
SN754410NE and LM-339 ) to drive an electrode of .003 brass wire held in 
a modified pin vise over a bath of deionized water.  My initial runs 
have burned clean .003 lines in 1 ounce copper glass epoxy board with 
out burning the substrate at 60 inches per second.  Not too bad!

I'm thinking about an electrode  design using .003" ( or finer ) brass 
wire feed through a plastic shield/electrolyte feed tube and the gap 
controlled by a stepper driving a worm on a gear which would drive a 
wire spool.  The gap could then be controlled by a simple step in step 
out  sequence while the electrode chuck could be kept compact and light 
in weight.

Finally a new area of inquiry.  Using microwaves to burn the copper ( 
and possibly the substrate for through holes ).  See the October 18 2002 
edition of Science magazine for an article on a microwave drill!

I'll try to put more time into the EDM project now that I can see the 
light at the end of the tunnel on my Willys restoration.

-- 
Curtis W Richards

--
"Law and liberty cannot rationally become the objects of our love,
unless they first become the objects of our knowledge." 
--James Wilson



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

--- In Homebrew_PCBs@yahoogroups.com, Curtis Richards <cwrxr@e...> 
wrote:
  My initial runs 
> have burned clean .003 lines in 1 ounce copper glass epoxy board with 
> out burning the substrate at 60 inches per second. 

Curtis,

That is very impressive. I am going to breadboard this on a pcb and try 
it this w/e.

BTW, can I ask what spec of SCR you used?

TIA

Roger

Curtis, I would be very interested in trying this also, do you have a 
schematic of your circuit available?

How fast is brass wire used up as part of the process?

Thanks!
Dave Miller

> 
> I'm working on reconfiguring the Garden of EDM to the International 
> Rectifiers  IPS5451 "intelligent power switch" instead of IGBTs or 
> FETs.  The IPS5451 chips have a 50 volt and 45 amp pulse limit. and 
> built in thermal and over current protection.  I'm using a 556 dual 
> timer to control the pulse repetition rate and gap breakdown 
voltage.  
> The 555 allows pulse rates up to the 1 mHz  range and the 30 amp 
rating 
> of the IPS is about twice the needed pulse current.  I would like a 
> higher voltage rating, but my initial experiments show clean traces 
with 
> .003 inch brass wire as the electrode with pulse rates of 30,000 
per 
> second.  Since the IPS handles all the proteection circuitry 
internally 
> ( the chips are meant for automotive uses ) the Garden of EDM 
circuit is 
> reduced to a complexity level that matches the RC EDM circuitry. 
> 
> My breadboarded circuit consists of  a 48 volt DC-DC converter ( 
too 
> close to the IPS limits, but it's what I've got at hand ) feeding 
a   
> ~10-20 uf polypropylene cap with a SCR triggering the pulse when 
the IPS 
> has switched off.  One half of the 556 controls the discharge rate 
and 
> the other half blocks the IPS while the gap is conductiong.  I'm 
using 
> the servo circuit from Ben Fleming's _The EDM How-To Book_  ( based 
on a 
> SN754410NE and LM-339 ) to drive an electrode of .003 brass wire 
held in 
> a modified pin vise over a bath of deionized water.  My initial 
runs 
> have burned clean .003 lines in 1 ounce copper glass epoxy board 
with 
> out burning the substrate at 60 inches per second.  Not too bad!
> 
> I'm thinking about an electrode  design using .003" ( or finer ) 
brass 
> wire feed through a plastic shield/electrolyte feed tube and the 
gap 
> controlled by a stepper driving a worm on a gear which would drive 
a 
> wire spool.  The gap could then be controlled by a simple step in 
step 
> out  sequence while the electrode chuck could be kept compact and 
light 
> in weight.
> 
> Finally a new area of inquiry.  Using microwaves to burn the copper 
( 
> and possibly the substrate for through holes ).  See the October 18 
2002 
> edition of Science magazine for an article on a microwave drill!
> 
> I'll try to put more time into the EDM project now that I can see 
the 

> light at the end of the tunnel on my Willys restoration.
> 
> -- 
> Curtis W Richards
> 
> --
> "Law and liberty cannot rationally become the objects of our love,
> unless they first become the objects of our knowledge." 
> --James Wilson
> 
> 
> 
> [Non-text portions of this message have been removed]

IPS 5751 is 50V/45A/R=0.019, better than 5451 (45V/35A/R=0.025) and cheaper.
Cristian

>I'm working on reconfiguring the Garden of EDM to the International
>Rectifiers  IPS5451 "intelligent power switch" instead of IGBTs or
>FETs.  The IPS5451 chips have a 50 volt and 45 amp pulse limit. and
>built in thermal and over current protection.

At 03:52 PM 9/28/2005, you wrote:
>--- In Homebrew_PCBs@yahoogroups.com, Curtis Richards <cwrxr@e...>
>wrote:
>   My initial runs
> > have burned clean .003 lines in 1 ounce copper glass epoxy board with
> > out burning the substrate at 60 inches per second.
>
>Curtis,
>
>That is very impressive. I am going to breadboard this on a pcb and try
>it this w/e.

May I have the schematics, please?
Cristian

--- cristian <cristianbip@...> wrote:

>May I have the schematics, please?
>Cristian


Cristian

I'm not sure whether you were referring to Carl's
schematics or mine.

I had already started a design slightly different to
Carl's using a pic as a controller rather than a 556.
The reason for this is that I can easily tweak the
timing by software and in circuit programming rather
than changing components, variable resistors etc.

Also I have been waiting some weeks for delivery of
Ben's book haere in UK thru Camden Steam Services, so
I currently have no details of the servo drive that
Carl use's ie Ben's design.

My own experiments had showed that circuit board
copper ablation was definitely possible, and Carl has
confirmed this. I personally think the success of this
project is in controlling the wire drive, for which I
have already constructed a miniature geared drive,
which will track the set peak discharge voltage.

You are welcome to my schematic in a few days on the
basis that it is purely experimental, but it would be
nice to collaborate with someone who also has a desire
to see trhis thing cutting track. I will attach the
pic asm code when i have got it working. Do you do
PIC's?

Both the IPS5451 and the IPS5751 are listed by Farnell
UK as high side drivers, and the 5751 seems quite
suitable. 

I really want this m/c to be sitting on the bench
turning out PCB's ready for roller tinning while I do
other useful things.

Roger



	
	
		
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--- In Homebrew_PCBs@yahoogroups.com, roger lucas <ralucas4277@y...> 
wrote:
>>snip<<

 
> My own experiments had showed that circuit board
> copper ablation was definitely possible, and Carl has
> confirmed this. I personally think the success of this
> project is in controlling the wire drive, for which I
> have already constructed a miniature geared drive,
> which will track the set peak discharge voltage.
> 
> You are welcome to my schematic in a few days on the
> basis that it is purely experimental, but it would be
> nice to collaborate with someone who also has a desire
> to see trhis thing cutting track. I will attach the
> pic asm code when i have got it working. Do you do
> PIC's?
> 
> Both the IPS5451 and the IPS5751 are listed by Farnell
> UK as high side drivers, and the 5751 seems quite
> suitable. 
> 
> I really want this m/c to be sitting on the bench
> turning out PCB's ready for roller tinning while I do
> other useful things.
> 
> Roger
>

Roger,

I'm a 'Johnny come lately' both on this board and on this thread so 
I've probably missed something vital but something puzzles me about 
this process:

Presumably the 3 thou brass wire is one electrode of the discharge, 
and the copper board target forms the other. This being the case, as 
the cut progresses to the point where one section of copper is 
totally isolated the cut will cease. The implication being that the 
motion driving software (presumably G code?) must have knowledge of 
this limitation and forward plan accordingly. I assume then that this 
rules out off the shelf packages intended for isolation milling like 
PlatinCNC ( http://www.platincnc.com/ )

I'm currently using PlatinCNC on my Bridgeport Interact 1 CNC mill 
but really only for hole drilling as it's quicker to etch the boards 
than mill them as I have the luxuary of a set of Mega tanks set up.

The concept of EDM'ing a board is attractive - I have an EDM 
diesinking machine in my workshop so am familiar with the process. A 
big contributor to the speed of the process is the operating fluid 
used, as each micro discharge vaporises the fluid carrying away the 
metal fragments into the bulk of the liquid, and as the vapour  
condenses the rush back of liquid into the void ensures fresh 
dielectric. I'm using "Ionoplus" from Oel-Held ( http://www.oel-
held.de/ )which unlike paraffin based conventional ones isn't 
inflamable and doesn't stink - it also has the side benefit of being 
a lurid fluorescent green which entertains visitors <G>

AWEM

>You are welcome to my schematic in a few days on the
>basis that it is purely experimental, but it would be
>nice to collaborate with someone who also has a desire
>to see trhis thing cutting track. I will attach the
>pic asm code when i have got it working. Do you do
>PIC's?

I own a CNC milling machine from www.bungard.de and I'm interested to replace
milling with EDM.
Also I've just bought a high power UV led and now I'm looking for a good 
collimator, to "CNC draw'
on a sensitized PCB, as an alternative.
Mainly I'm an electronic engineer. I've manage to PIC some projects, but 
I'm far from a PIC guru.
Also I own a full electronic lab plus a mill and a lathe.

Farnell is here too (Romania).
What do you think, the polypropylene cap # 400-2143  22u/100Vdc is good ?
They don't list high current discharge caps.
All the best to you there,
Cristian

--- Andrew Mawson <andrew@...> wrote:



I'm a 'Johnny come lately' both on this board and on
this thread so 
I've probably missed something vital but something
puzzles me about 
this process:

Presumably the 3 thou brass wire is one electrode of
the discharge, 
and the copper board target forms the other. This
being the case, as 
the cut progresses to the point where one section of
copper is 
totally isolated the cut will cease. The implication
being that the 
motion driving software (presumably G code?) must have
knowledge of 
this limitation and forward plan accordingly. I assume
then that this 
rules out off the shelf packages intended for
isolation milling like 
PlatinCNC ( http://www.platincnc.com/ )

I'm currently using PlatinCNC on my Bridgeport
Interact 1 CNC mill 
but really only for hole drilling as it's quicker to
etch the boards 
than mill them as I have the luxuary of a set of Mega
tanks set up.

The concept of EDM'ing a board is attractive - I have
an EDM 
diesinking machine in my workshop so am familiar with
the process. A 
big contributor to the speed of the process is the
operating fluid 
used, as each micro discharge vaporises the fluid
carrying away the 
metal fragments into the bulk of the liquid, and as
the vapour  
condenses the rush back of liquid into the void
ensures fresh 
dielectric. I'm using "Ionoplus" from Oel-Held (
http://www.oel-
held.de/ )which unlike paraffin based conventional
ones isn't 
inflamable and doesn't stink - it also has the side
benefit of being 
a lurid fluorescent green which entertains visitors
<G>

AWEM



Andrew,

The pcb must be edm'ed in a raster fashion, ie
consecutive passes with the electrode so that the
electrode always approaches fresh copper, analagous to
the old dot matrix printers.

However, I have no knowledge of an off the shelf
package to convert pcb layout to raster scan format.
At the moment I am converting to bitmap and cobbling
from there. The resulting file needs to be edited for
spark on/off to pass over copper that has to remain.
Maybe someone can help here, by suggesting a better
process!

For further enlightenment see the following messages
on the Homebrew_PCB forum,

9593, 9594, 9602, 9604, 9614, 9805, 10006, 10024,
10096, 

I am interested in the dielectric fluid you mention,
and will try to obtain some for trials.

Roger



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--- cristian <cristianbip@...> wrote:


---------------------------------

>You are welcome to my schematic in a few days on the
>basis that it is purely experimental, but it would be
>nice to collaborate with someone who also has a
desire
>to see trhis thing cutting track. I will attach the
>pic asm code when i have got it working. Do you do
>PIC's?

I own a CNC milling machine from www.bungard.de and
I'm interested to replace
milling with EDM.
Also I've just bought a high power UV led and now I'm
looking for a good 
collimator, to "CNC draw'
on a sensitized PCB, as an alternative.
Mainly I'm an electronic engineer. I've manage to PIC
some projects, but 
I'm far from a PIC guru.
Also I own a full electronic lab plus a mill and a
lathe.

Farnell is here too (Romania).
What do you think, the polypropylene cap # 400-2143 
22u/100Vdc is good ?
They don't list high current discharge caps.
All the best to you there,
Cristian


Cristian,

I have used polyprop caps, don't know what their ESR
is, but they have worked ok for trials. I may just try
the ones you list.
The SCR I have selected is TYN812 (Farnell 682-524),
on the basis that it should cope with the voltage and
current for trials. May need a higher power item as
speed, ie average current increases.

Roger




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--- In Homebrew_PCBs@yahoogroups.com, roger lucas <ralucas4277@y...> 
wrote:

>>SNIP<<

> Andrew,
> 
> The pcb must be edm'ed in a raster fashion, ie
> consecutive passes with the electrode so that the
> electrode always approaches fresh copper, analagous to
> the old dot matrix printers.
> 
> However, I have no knowledge of an off the shelf
> package to convert pcb layout to raster scan format.
> At the moment I am converting to bitmap and cobbling
> from there. The resulting file needs to be edited for
> spark on/off to pass over copper that has to remain.
> Maybe someone can help here, by suggesting a better
> process!
> 
> For further enlightenment see the following messages
> on the Homebrew_PCB forum,
> 
> 9593, 9594, 9602, 9604, 9614, 9805, 10006, 10024,
> 10096, 
> 
> I am interested in the dielectric fluid you mention,
> and will try to obtain some for trials.
> 
> Roger
> 
>

Roger,

If you were to make your scanning system move both the EDM mechanism 
and an opto head looking at a conventional translucent PCB image, the 
digital signal from the opto sensor could turn off/on the spark 
source and there would be no need for custom software ! The original 
Muirhead wet fax machines used a similar set up with a light sensor 
looking at the source document, it's output sent down 'phone lines to 
a receiver where 'almost'point contact between a fixed horizontal 
blade, and a rotating helical blade (like a cylinder lawn mower) 
passed electricity though a paper soaked in an iodine compound to 
make the mark.

I'm afraid  the dielectric was rather expensive - particularly as I 
needed 19 gallons (!!!) In fact I seem to remember paying almost as 
much for the fluid as I did for the (none working at the time) EDM 
machine <G>

Andrew,


Yes, I've just been quoted \ufffd2-08/litre by the UK
distributor, MIN QTY 55 litres, Gulp, but that is
delivered!!


I was using a document scanner on my desk when your
post arrived, and it set me wondering whether this
might be a possible interface to the EDM PCB mill as
you described. Must investigate further.

Roger


--- Andrew Mawson <andrew@...> wrote:


---------------------------------
--- In Homebrew_PCBs@yahoogroups.com, roger lucas
<ralucas4277@y...> 
wrote:

>>SNIP<<

> Andrew,
> 
> The pcb must be edm'ed in a raster fashion, ie
> consecutive passes with the electrode so that the
> electrode always approaches fresh copper, analagous
to
> the old dot matrix printers.
> 
> However, I have no knowledge of an off the shelf
> package to convert pcb layout to raster scan format.
> At the moment I am converting to bitmap and cobbling
> from there. The resulting file needs to be edited
for
> spark on/off to pass over copper that has to remain.
> Maybe someone can help here, by suggesting a better
> process!
> 
> For further enlightenment see the following messages
> on the Homebrew_PCB forum,
> 
> 9593, 9594, 9602, 9604, 9614, 9805, 10006, 10024,
> 10096, 
> 
> I am interested in the dielectric fluid you mention,
> and will try to obtain some for trials.
> 
> Roger
> 
>

Roger,

If you were to make your scanning system move both the
EDM mechanism 
and an opto head looking at a conventional translucent
PCB image, the 
digital signal from the opto sensor could turn off/on
the spark 
source and there would be no need for custom software
! The original 
Muirhead wet fax machines used a similar set up with a
light sensor 
looking at the source document, it's output sent down
'phone lines to 
a receiver where 'almost'point contact between a fixed
horizontal 
blade, and a rotating helical blade (like a cylinder
lawn mower) 
passed electricity though a paper soaked in an iodine
compound to 
make the mark.

I'm afraid  the dielectric was rather expensive -
particularly as I 
needed 19 gallons (!!!) In fact I seem to remember
paying almost as 
much for the fluid as I did for the (none working at
the time) EDM 
machine <G>




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--- In Homebrew_PCBs@yahoogroups.com, roger lucas <ralucas4277@y...> 
wrote:
> Andrew,
> 
> 
> Yes, I've just been quoted £2-08/litre by the UK
> distributor, MIN QTY 55 litres, Gulp, but that is
> delivered!!
> 
> 
> I was using a document scanner on my desk when your
> post arrived, and it set me wondering whether this
> might be a possible interface to the EDM PCB mill as
> you described. Must investigate further.
> 
> Roger
>

Yep - I had to buy two of those drums - nearly £270 including the 
VAT ! But at least the workshop doesn't stink of paraffin and so far 
hasn't burnt down <G>


AWEM

-> 
> BTW, can I ask what spec of SCR you used?
> 
> TIA
> 
> Roger

Hi Roger, Cristian, Andrew, et al,

I used junk box parts that I had around!  The SCR was a NTE5369 (
125A, 1200V )which is a stud mount type and was already on a heatsink.
 One half of the 556 timer was set to trigger the pulse at 12,500 Hz.
 It triggers the IPS to charge the polypropylene discharge cap while
the SCR is off.  When the IPS is turned off the SCR is triggered and
the pulse travels through .01 ohm current sense resistor.  The second
half of the 556 monitors the current through the electrode and
inhibits the IPS until current through the resistor drops to below the
SCR holding value.

The "electrode" is a piece of .003 inch brass wire in a pin vise with
a threaded shank which is mated to a threaded sleeve driven by a junk
box stepper.  The gap is controlled by the servo circuit in Ben
Flemmings book.

I used the IPS5451 since I had some on hand and since the pulse
current is in the 10 - 15 amp range ( I tell you more when I get a
chance to put it on the scope .  The IPS chip should be good for pulse
rates up to about 15,000.

RC type EDM uses the gap breakdown voltage to control pulse rate by
switching caps into and out of the discharge circuit.  I use the high
speed SCR to increase the pulse rate and thus can use the servo
control to keep the gap breakdown voltage at a selected voltage for
each run.  This allows differant gap volteges to be set and speed
tests to br run.

If anyone were going to experiment with really high pulse speed I'd
recommend using two SCRs controlled by optoisolated signals from a
PIC.  The IPS is the limiting factor in my tests as it takes ~65 usec
to turn off.  Using two SCRs would allow faster switching, but with
attendant commutation problems.  A PIC should have enough speed to
test for commutation of the SCRs while monitoring gap breakdown
voltage and current.  Using SCRs simplifies the support circuitry
whereas IGBTs or FETs add condiderable cost.  If you can settle for 30
to 60 inch per scond travel the IPS solution seems to work OK, but
remember with allowances for acceleration at the traverse endpoints
it'll take 20-25 minutes to cover a 8 x 11 board.

I'll try to convert my "napkin scribbles" into a circuit diagram this
evening.

Curt

--- curt_rxr <cwrxr@...> wrote:


>it'll take 20-25 minutes to cover a 8 x 11 board.

>I'll try to convert my "napkin scribbles" into a
>circuit diagram this
>evening.

>Curt


Curt,

Many thanks for the info.

Your schematic will be great, and allow me to finish
the PIC controller without having to go over all the
ground you have already covered.

At the moment speed is not a major criteria, although
no doubt it will become so as the system gets fired
up. Initially I will be happy to have a system which
will produce 5 x 3 pcb's ready for roller tinning
unattended. 

I think the next hurdle will be to convert a bitmap to
an xy raster for edm'ing. I might go for a second PIC
for this, ie download .bmp file to a PIC running
compact flash memory, and use the PIC to interpret and
drive the xy axes and spark on/off, maybe even set
voltage from it by driving Ben's Z servo, (when I get
delivery of the book).

This is certainly turning into a very interesting
project.

Roger




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On Thu, 29 Sep 2005 17:37:32 +0200, Andrew Mawson <andrew@...>  
wrote:

>
> Yep - I had to buy two of those drums - nearly \ufffd270 including the
> VAT ! But at least the workshop doesn't stink of paraffin and so far
> hasn't burnt down <G>
> AWEM


Would not water be sufficient for PCBs?

A ion exchanger would not be expensive. Rainwater would be even cheaper.

How do you maintain the expensive EDM fluid in working order?

ST

--- In Homebrew_PCBs@yahoogroups.com, "Stefan Trethan" 
<stefan_trethan@g...> wrote:
> On Thu, 29 Sep 2005 17:37:32 +0200, Andrew Mawson <andrew@m...>  
> wrote:
> 
> >
> > Yep - I had to buy two of those drums - nearly £270 including
the
> > VAT ! But at least the workshop doesn't stink of paraffin and so
far
> > hasn't burnt down <G>
> > AWEM
> 
> 
> Would not water be sufficient for PCBs?
> 
> A ion exchanger would not be expensive. Rainwater would be even 
cheaper.
> 
> How do you maintain the expensive EDM fluid in working order?
> 
> ST

Stefan,

This is a 'Die Sinker' edm m/c - the fluid is constantly pumped
through 
two big cylindrical filters the size of lorry (truck?) air filters.

The true edm process (as opposed to spark ablation)controls the 
discharge very carefully to create the explosive expansion and
collapse 
to keep the discharge channel free of debris. This expansion and 
collapse is very much affected by the dielectric properties. I'm not 
saying deionised or distilled water won't work in your application as 
the conditions are very different. The depth of fluid is quite 
important as if too shallow the vaporised bits leave the surface and 
give a fog in the room. If deep enough they re-condense within the 
body of the fluid.

AWEM

--- In Homebrew_PCBs@yahoogroups.com, "curt_rxr" <cwrxr@e...> wrote:
> 
> If anyone were going to experiment with really high pulse speed I'd
> recommend using two SCRs controlled by optoisolated signals from a
> PIC.  The IPS is the limiting factor in my tests as it takes ~65 
usec
> to turn off.  Using two SCRs would allow faster switching, but with
> attendant commutation problems.  A PIC should have enough speed to
> test for commutation of the SCRs while monitoring gap breakdown
> voltage and current.  Using SCRs simplifies the support circuitry
> whereas IGBTs or FETs add condiderable cost.  If you can settle for 
30
> to 60 inch per scond travel the IPS solution seems to work OK, but
> remember with allowances for acceleration at the traverse endpoints
> it'll take 20-25 minutes to cover a 8 x 11 board.
> 
> I'll try to convert my "napkin scribbles" into a circuit diagram 
this
> evening.
> 
> Curt


Curt,

I assume it would be neccessary to use a raw rectified feed thru the 
first scr to the discharge cap, so that the voltage drops below the 
scr holding voltage every cycle so that when the cap has reached the 
charged state, the scr will switch off, or can you get scr's which 
can be triggered off as well as on?


As you may guess, I haven't had much to do with scr's.

Roger