AdrianWint:
Are you sure they are BTA08-600B ?
The BTB08-600B is a non isolated tab 8A 600V device, the BTA08-600B is its insulated cousin. The trailing 'B' tells you about the gate sensitivity.
Like you, I found out the hardway, years ago, about the different varieties of TRIACs, ironically also fixing NJD controllers (NJD = New Junction Discotheques, - those controllers where made in Sandiacre, Nottingham... about 5 miles from where I type this)
Hello Adrian, yes definitely marked BTA08-600B. The centre pin connects to the heat sink tab. Perhaps they are wrongly marked or a sub-standard faulty batch? I have used N.J.D. stuff for years, although its design is a bit antiquated nowadays. But I like retro!
Z.
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