Jim Thomson wrote:
>> Some commercial amps are moving to Triacs and ramping the voltage up
>> instead of a "step start" and I understand they are working quite well.
>>
>> 73
>>
>> Roger (K8RI)
>>
>
> Yep. Emtron have been doing it for many years and it is superb.
>
> Alek,
> VK6APK
>
>
> ### ramping up over how long a period of time, 5 x secs, 10 x secs
> 1x sec, or what I suspect is from a zero cross point, up to the top
> of the cycle.. [IE: 4.17 msecs] ????
>
IIRC it's 5 seconds. I've not seen the circuit so I have no idea as to
how they trigger them. I'd guess, probably not at zero crossing as
that's how they vary the power (turn on time). Turning them off at a
zero crossing point is a different matter and it requires a pulse of
back current, or opposite voltage if they are not at the zero voltage
crossing point. A triac is nothing more than a pair of back to back
SCRs in one package. Once on they stay on, unless forced to turn off.
They are usually triggered by a diac in simple circuits. Normally they
turn on any where in the half cycle for each polarity and off at the
zero crossing although that is not necessary, but it certainly makes
life easier.
> ## will the triac handle the huge pulses every 8.3 msecs??
>
>
Yes, but they need to be sized appropriately. Also once the ramp up is
complete there is normally no need to keep the triac in the circuit, but
again I've not seen the circuit.
> ## My problem with the SSR's is... they need a BIG heatsink.
>
Not necessiarily. We used SCRs back to back in quarter megawatt
installation and those did use large heatsinks, but 4 or 5 KW , 220 VAC
switches are small with no heat sinks. A 20 or 30 A triac might take a
heat sink a couple inches on a side and maybe less for a 5 second ramp.
They generate very little heat when in the full on position. All but a
tiny percent of the heat is developed only during the actual switching.
I have a 1 KW solid state switching supply that uses a pair of heat
sinks that are about 2" X 3" or 4"
A 5 second ramp to 100% would require very little heatsink. Commercial
solid state switches are about the size of a normal open frame relay.
> They should also be supplemented with a breaker or fuses.
>
That's a given, but those should be present even with a mechanical relay.
> There is too much leakage current through them imo. If you
> are working on an amp that has them,un-plug from the
> 240v line 1st.
>
>
Again, that's a given with high voltage power supplies even with
mechanical switches and relays. I do not work on HV supplies with them
plugged in except when taking voltage measurements. Even then I hook up
everything with the supply unplugged and discharged. Even then you have
to be careful about unloaded filter caps building up a charge if they
haven't been off long enough to "settle" after discharging.
> ## IF you decide on using SSR's... get one that has a
> zero cross voltage turn on.... and a zero cross CURRENT
> turn OFF.
>
Zero current turn on doesn't work well for a ramp up unless you chop a
full cycle at a time which gives some timing issues as you typically
don't just add sequential cycles to get a smooth ramp up. By the time
you reach half power it's a half second on and half second off which can
really make the old transformers howl. It's fine for a on/off solid
state switch, but not a variable voltage. OTOH it takes other "noise
generating circuits" to force a non zero turn off as it takes a pulse
stronger than the forward voltage at that point in the cycle. I've
never seen a non zero turn off although I know it can be done. That's
where the problems start popping up with interaction of circuits.
Typically you pick where in each half of the cycle you want to turn on
and let it turn off naturally at the zero crossing point. At work we
used a pair of 1200 amp, 480 VAC SCRs to vary the power in some
installations. No problems with noise or hash. The slice of silicon
inside was about the size of a silver dollar only less than a mm thick.
Those were water cooled and about the size of a hockey puck.
As far as solid state switches, I'd purchase a commercial, ready made
product even if building a sensing trigger. Transistors take a much
larger heatsink than comparable Triacs when used as switches. We were
using them at work well over 20 years ago. Prices were comparable to
open frame relays of the same power handling capability, but I've not
looked at one since I went back to college in 87.
73
Roger (K8RI)
> Later... Jim VE7RF
>
>
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