Relay vs Triac Long-term Reliability / Safety?

Discussion in 'General Electronics Chat' started by flat-16, Jan 14, 2006.

  1. flat-16

    Thread Starter New Member

    Jan 14, 2006
    Hello to all,

    I have some musical instruments that need to go on the road to gigs, therefore long-term reliability is an issue.

    The units all have an internal mains transformer that emits serious EMI, and is also mechanically noisy.

    The solution is to move the tfmr to a separate case; easy enough, but there are 2 slight complications:

    A thermal fuse needs to control the tfmr primary, thus shutting down all power in the event of it blowing.

    There is only a tiny area on the instrument to fit the jack to couple the tfmr to the regulation board, thus limiting options in terms of connectors due to 7 pins being needed and a fair current capability.

    A 7-pin connector has been found, but alas it's only rated for 50v, albeit up to 6A. The only connectors I can find that have a mains-rating and fit size-wise are mil-spec, and at £16 each this isn't funny as I'll need 8 of each...

    The solution I have rigged up works well: a BTA06-600T triac, with the gate in series with the thermal fuse. 5 pins on the connector are used for the secondaries, and 2 for the Gate loop. The Gate is controlled by the Triac A2 AC pin, with a 41K resistor in series to limit the current for safety reasons (I live in a 230-240v area, and this means the Gate has around 5.6ma for its trigger).

    Having consulted several audio engineers, they are highly dubious of Triacs and have told me to rig up a mechanical relay... I have the parts to do this, and can rig up a simple unregulated supply, and put the thermal fuse in series with the DC control signal to the relay; however, this will be a fair amount of work, and the current idea works ok...

    So, my questions are: do you think the Triac idea is safe? (Everything is insulated in heatshrink, even though the Gate voltage exceeds the connector's rating...)

    Will a Triac be more reliable long-term than a conventional electro-mechanical relay? I would have thought the Triac to be far more reliable, particularly as it's working well below its max ratings (max demand is 200ma, and Triac's rated for 6A), but certain engineers tell me otherwise... Triacs don't seem to be held in high regard...maybe this is a dogma in audio engineers....(RFI is not an issue btw)

    Will this system work day-in, day-out on the road?

    I should also add that putting 4m of cable along the Gate control signal (2m there, 2m back) can cause false triggering in sensitive Triacs, so that's also an issue...although I'd be more worried about false switching off.

    I'd like to take this opportunity to say Thanks in advance, and convey gratitude to the organisers of the main site; I spend a lot of time studying the reference pages, and recommend the site to counterparts regularly.
  2. beenthere

    Retired Moderator

    Apr 20, 2004

    I'd go for the relay. You may have space problems in the case, but you can mount a fairly small relay that has adequate contacts for the mains side - your current there is going to be lower than on the secondary output.

    Triacs have some bad habits. I've seen some that essentailly shut down in quadrants 2 and 4, which will shut off half your power. They also tend to fail in a manner that eats the gate drive circuitry, whch is just poor behavior. They don't like to get hot. Realys will work up to the point where the insulation burns off the coil.
  3. flat-16

    Thread Starter New Member

    Jan 14, 2006
    That seems to be the consensus amongst the audio engineers I consulted... But then I thought, "these things are used in microwave ovens and lighting, surely, with no moving parts, they're more reliable than a relay?"

    Thanks for your reply. The case against Triacs is mounting...does anyone like them?