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Amplifier triac protection circuit problem

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?

Reply to
N_Cook
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" 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

Reply to
Phil Allison

triac

variac

load,

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

Reply to
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
Reply to
N_Cook

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