Amplifier triac protection circuit problem

[N_Cook]

This is probably the circuit, split primary is used , ZD is a 5V zener
checked ok
, V are 1N4007 checked ok, 2.2uF,100V polyester for high C unpolarised, ,
pnp BC214 emitter to top , 94R is 2x 47R in series
view/message source, may have equi-spaced font to read the ASCII image


240V

| mains primary
|C
|C------- 120V -----------------------------------------|
|C |
| |
| 47R |
| ___ |
| -|___|-| |
| /´ | |
| ,´ | |
| ,´ |< BC214 |
| / -| |
| / / |\ |
100R,25W| ,´ / ------- |
| ,´ / | | |
|----- / |-------| | V 2.2uF , 100V |
.-. _|_/ --- --10uF | - || |
| | V_A 10uF--- --- V ---||---| |
| | | | ___ | - | || .|. |
'-' |--------- -|___|- ZD | V | | |
|----| | - | | 94R,10W |
| 2K7 | | '-' |
|TIC236D | | | |
| |-----|---|--- |
0V | |
| |
|------------------|
120V

An exiting commercial 600W audio amp, runs perfectly happily if this triac
stuff is bypassed.
I'm assuming the triac circuit protects the amp from over current but I find
it confusing. Plenty of gain on the pnp, all Rs check out ok and triac
checks ok tried out in a dimmer circuit.
Running up the transformer , without any load, with this cct via a variac
there comes an ac voltage where large peaks are produced at the primary and
overvoltage at the secondary and nasty saturation type noise from the
windings. Checked waveform on a scope via isolation transformer and
monitored over 35R resistor placed in series with the primary. Peaks are one
a cycle (about 3mS of 20mS UK mains) but not at peaks , troughs or inflexion
point of the mains and amplitude of the peaks maybe as much as twice the
mains peak to peak .
Swapping the TIC to a lower rating TIC226D the amp (no load) works fine, no
nasty mechanical noises.
Swapping the triac to a 25A, 600V the same effect is observed. I'm assuming
that something is causing the triac to cut , once a cycle, when a high
voltage is across the transformer and the inductance of the primary is
generating these peaks, but what is the likely culprit?

 

[Phil Allison]

" N.Kook"

This is probably the circuit,

( snip ASCII art mess)

An exiting commercial 600W audio amp, runs perfectly happily if this triac
stuff is bypassed.
I'm assuming the triac circuit protects the amp from over current ...

** Nope - it's a "soft start" circuit.

It's job is to reduce or eliminate inrush surge current.

Running up the transformer , without any load, with this cct via a variac
...

** That is not how it is ever meant to be operated.

The circuit expects to be switched onto the AC supply and the tranny needs
to have a rectifer and filter bank load on the secondary. Without any load,
the primary generates large back emf spikes that play havoc with the triac
drive.


...... Phil

 

[N_Cook]

" N.Kook"


( snip ASCII art mess)


** Nope - it's a "soft start" circuit.

It's job is to reduce or eliminate inrush surge current.




** That is not how it is ever meant to be operated.

The circuit expects to be switched onto the AC supply and the tranny needs
to have a rectifer and filter bank load on the secondary. Without any load,
the primary generates large back emf spikes that play havoc with the triac
drive.


..... Phil

Thanks for that, it explains why there is no reset type function When first
observed the problem ,the transformer was overheating and I'd disconnected
the load on the secondary but this "buck boost" type effect was operating
with R and C load as well as in full use.

Looks as though it was me being a plonker. As the 2 10uF were high ESR I
desoldered them, to replace with new, but because new and old were short
pinned radial format, I managed to solder in the old ones again. Will redo
later today

 

[N_Cook]

for the archives this is a corrected (but not necessarily correct) version
of the soft start circuit, now makes more sense

240V
| mains primary
|C
|C------- 120V ac---------------------------------------|
|C |
| |
| 47R |
| ___ |
| -|___|-| |
| /´ | |
| ,´ | |
| ,´ |< BC214 |
| / -| |
| / / |\ |
100R,25W| ,´ / ------- |
| ,´ / | | |
|----- / / | V 2.2uF , 250V |
.-. _|_/ / | - || ___ |
| | V_A / V ----||------|___|------|
| | | / - | || 94R,10W
'-' | | ZD | V
|----| | | -
| | | |
|TIC236D |----------|-----|
| |
| |
| |-------|
| --- --220uF
| 220uF --- ---
| | ___ |
|------------|___|-
| 2K7
0 V