K8064 DC controlled dimmer.... Triac failure

Hi to all

Can anyone help with a problem with this dimmer?

I have 3 of these kits each with a load of 8 50W ( total 400W ) halogen bulbs and it works great. BUT every time a bulb fails the triac blows, usually MT1-MT2 short, this is getting a bit tedious changing the triac every time a bulb goes.

I have replaced the orignal fuse with a 3amp fast blow but this has made no difference. Has anyone had the same problem? and if so have you found a solution?

Any help would be appreciated


It is quite normal that the triac blows. When a bulb fails, an enormous current flows, and fuses are way to slow to respond. However, it is not normal that the bulbs fail often. Either you use a low quality bulb, or you use them for long periods at very low intensity, which causes the halogen cyclus to stop and causes the filament to degrade.

I’ve not encountered this problem yet, but I always solder a VDR next to Triacs, between A1-A2.
Therefore, any voltage overshoot is short-circuited by the VDR.

Hope this will help.

This will not help in this case. Triac failure is caused by current overshoot, not voltage overshoot.

I have the schematic diagram in front of me.

Let me know, but I believe I’m not wrong if I say that the power section is made up of 3 elements when the circuit is closed with mains :

  • The Triac,
  • The 1.5 mH inductance,
  • The external load (e.g. incandescent bulb(s), or other resistive load).

In normal operation, the Triac controls the output voltage (and current too, according to Ohm’s law).
If there is no load connected to the output, any attempt to drive the Triac from 0% to 100% will not ruin anything.

But, if the Triac is delivering, let’s say 75%, to an existing load, and if you open the circuit (as a blowing bulb does), what happens ?

What appears between the terminals of the inductance ? (Not really a resistive load).

Don’t you believe that a VDR, able to short-circuit 6 500 A during 25 µs (0.70 EUR VAT included), won’t protect the Triac from blowing, if it’s mounted between A1/A2 ? (According to Ohm’s law : I = U/R between A1/A2).

Anyway, as it’s usual to protect the switching circuit of a relay in a DC domain by a diode, it’s a recommended practise, in the AC domain, to protect a triac with an appropriate VDR, either if it just switches a load with zero-crossing detector, or if it acts as light dimmer.

Since 1976, when I started to engineer light dimmers for amateur theatre lightning, driving up to 3 500 W each, I never lost a Triac when a spotlight bulb broke down on duty.

Trying this solution by soldering a VDR on the PCB kit will not affect, in any way, the behaviour of the original circuit.

Philippe J. BOUCHON
Audio Engineering Society, Inc.
French section Elected Officer

Usually, there is no problem when the bulb blows while operating. The triac sometimes fails when the bulb is off and turned on. A part of the filament shorts, and this combined with the low turn-on resistance of the bulb cause the triac to fail due to overcurrent.