At 06:22 PM 12/30/2011, you wrote:
>Hi,
>
>I'm building some simple pic-based thermostats, which will have to
>keep some technical rooms above freezing.
>Controller part is fine, and one version of power part is fine, too.
>It's based on MOC3062 optocoupler and BT138 triac. Connections are
>directly as per MOC's datasheet, with resistors of 360 ohms.
>I used ordinary 1/4W resistors, without any problem, that device is
>working fine in test mode, switching a 500w heater.
>(230 VAC 50Hz here)
>
>For another application, halogen lamp is used as a heater, mainly
>because they can be obtained really really cheap, and will direct the
>heat, so I can mount it out of way. But those burn out very quickly
>when switching on-off too often.
>So, I decided to use one-way thyristors instead of triac, so i could
>turn on half power initially, then power another one in opposite
>direction after a delay. Looking thru available info, it seemed like
>same circuit should be fine, in this case, two of them. But the gate
>resistor is letting out magic smoke, it receives much more power!
>Thyristors used are BT151, from datasheet their gate currents are even
>smaller than for BT138.
>
>I have forgotten many things about these, so probably I'm missing
>something. Probably MOC3062 can not detect zero crossings, and is
>feeding gate current differently? Probably thyristor requires
>different circuit? Any ideas?

The SCR will block in the reverse direction, but the triac-output
optocoupler will turn on. The SCR will not trigger, but the gate
will break down with reverse voltage (well in excess of the abs max,
which is usually just a few volts), so gate current will flow-
peak of maybe 0.5A (on the negative half-cycle- think of the load
in series with the 360 ohm resistor in series with the optocoupler
output in series with a zener diode representing the SCR gate--
all across the 230V for the negative half cycles).

The result is too much heat in the SCR gate (maybe > 1W)
and WAY too much heat in the series resistor (>50 watts).

To fix it, just put something like a 1A 1kV diode in series with
the opto output- that should work. In fact, you can retain the
triac and add the diode to get half cycles- short the diode to
get full power.

(I suppose you know that drawing much current with just half
cycles is frowned upon- it creates harmonics and can overheat
transformers-- since you've only got a few A it shouldn't be a
problem). Also, thyristors (including triacs) tend to fail
'on' so please take that into account in your safety
considerations.


--sp







>I really really do not want to just use bigger wattage resistors,
>since whole device will be mounted in not-so-big plastic case and
>unnecessary heat is unwelcome.
>
>--
>KPL
>--
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