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Tube coil related questions--"Super Staccato" Controller
Running dialog with Staccato Controller, and possible "Super Staccato"
where multiple consective positive alternations are sent to VTTC
off time is adjustable as before.
1. You are right, 12V should have been used as input voltage instead of 15V
(fingers slipped :^) )
2. Correct again, when pin 6 voltage (Vthresh [R3/R4]) drops below ~ 5.15V
with AC riding on DC level will cause U1a to trigger and pin 5 output goes
3. You should be able to adjust attenuator values R1/R2, R3/R4 to work with
transformer you've ordered. Be sure to bypass the hel.. out of the circuit. A
three terminal regulator will significantly stabilize circuit.
4. R10/R11/C10 setup a trigger level of 6 volts, C is a high frequency bypass
which triggers U1b with negative transition of U1a (i.e. when U1a goes LOW).
Network acts as a differentiator.
5. I'm thinking about your comment last night concerning multiple cycle on
times and adjustable off time. There may be
an eloquently simple way to do this, may require
me to pull my breadboard back out. :^) I sugggest this controller be
"Super Staccato" Controller, in that ON time is multiple consecutive
alternations, with adjustable OFF time (PRF rate). I'm thinking setting at
4 alternations ON time, and adjustable OFF time as presently designed.
6. Quick modelling reality check suggests that U1a turns on about 250usec
to actual utility '0' crossing, which dramatically limits SCR power
conduction losses only. This equates to approximately 1.5W per A of passed
current. Even a small SCR should be capable of controlling a many kW VTTC
unit, provided properly heatsinked, timing is synchronized to AC line, and SCR
can tolerate an occasional "BURP" when a tube flashes over.
Dave Sharpe, TCBOR
Sent by: David Sharpe <sccr4us-at-erols-dot-com>
09/05/2000 09:42 PM AST
Subject: Re: Tube coil related questions
Thanks for the help. I have a better idea of how this works now, and
thanks for including the equations relating frequency to the 556
monostable RC values, that is useful to know.
A few more points I want to clarify though: I don't know why you used
15 V in the trigger reference calculation, since the R3 /R4 divider
only gets 12 volts as shown, and thus the 75K would divide out ~5.15
V. From your timing diagram, I see that when the AC in to the schmidt
trigger is at about 0 volts, triggering occurs. This would mean that
if pin 6 voltage falls BELOW the 5.15 V set by R3 and R4, triggering
occurs? or do I have the phasing backwards, and more than 5.15 volts
turns it on?
In either case, you seem to have the trigger of U1b turning ON the
monostable oscillator when it goes higher than 6.0 V (R10 - R11 10k
divider + out from pin 5)
What adjustments would I need to make if I replaced the 25.2 VCT
transformer with a 14.5 VAC center-tapped? I have a 5000 microfarad
filter cap and I don't plan on using a regulator so I should see a
fairly stable 10 volts DC (sqrt 2 * 7.25 V) out to the circuit. From
what I gather, the only potential sticking points would be the value
of the R1 R2 divider and then the values of the trigger biases for
pins 6 and 8, if the trigger is a fixed voltage (like 5 volts) for the
556 chip that is independent of Vcc.
Thanks for the pointer towards radioshack-dot-com. I already purchased a
couple 600 VRRM, 25 amp SCRs from Digi-Key at a similarly exorbitant
price :^). If they blow, I'll have to get the 800 or 1200 volt version
>R1 and R2 (AC attenuator) and R3 and R4 (Schmidt level adjustment).
The circuit (R1/R2) basically atttenuates the >AC
>input and guarantees that the peak
>AC input levels do not exceed the rail voltages (latchup and magic
smoke release from 556 assured). R3/R4 sets the >DC
>trigger reference at ~6.43V (15V*75000/(100000+75000). With 12.6V in
through R1/R2 attenuator allow input AC of >2.93V.
Check Radio Shack online (www.radioshack-dot-com) and look under
semiconductors, power devices, SCR's. They have a 800VRRM,
25A device (2N6509) from
Motorola (now ON Semiconductor), that is HORRIBLE expensive, as in
$1.89....each :^). I'm using them in my high power
push-pull VTTC, and with these
ratings should be capable of controlling AT LEAST a 10kW machine. It
is a TO220 case, and I'm mounting on a small heat
sink just to be safe and stack the odds in my favor.