Re: Jerry/Solid state switches

Oh - just power resistors - Why not heat sink the resistors to cool them down.
It is done all of the time.  Sometimes with a fan sometimes with fins.....

A 100 watt light bulb gets hot - and it gives some of the energy off in light.
Across your relay contacts - is there arcing ? - might need a snubbing circuit
there.  A single 100w maybe heat sink to the metal case.  That would sink a lot of heat.

Martin

Ignoramus15775 wrote:

Update... Adding a 250 Ohm resistor fully solved the restart problem.

However, the resistor gets quite warm after a few restarts, which can
probably be fine, but I do not like it. I think that I need a lot less
ohms, rather than a heftier reststor. 

The power generated in the resistor is volts^2/ohms, or about 200 watt
for a 250 ohm resistor.

I have several resistor labeled "0.2 kOhm, 100W, maybe I will use one
of them one instead. Using two would mean 400 ohms, or about 0.6 amps,
or about 144 watts, or 70 watts per resistor. Two could handle 250V on
a continuous basis, but they are quite big (5 inches or so). 

i

On Sun, 21 Aug 2005 04:57:00 GMT, Ignoramus15775 <ignoramus15775@xxxxxxxxxxxxxxxxxxxx> wrote:

Thanks to Jerry for making me think about these issues. Here are some
results.

I accidentally found a 90A solid state relay this morning. It turns
out that I took it out of a very big UPS a few months ago and it was
in my pile of electronic thingamabobs. Now that I knew more about
electronic devices, I recognized it for what it is. It has a zero
crossing feature, I think both for turning on as well as off.

It is Crydom H12D4890. I have been playing with it today.

I also found 360 mF worth of 240 VAC capacitors that I also used today. 

I tried using the relay for controlling capacitors. I rewired my
second 7.5 HP phase converter as follows.

I used two light switches, one single pole (I call it primary) and one
double pole (I call it secondary). Got this DP switch for free at a
garage sale today, in a big pile of "please take this free stuff away"
stuff. As a side note, also got a 1/2 HP TEFC motor that I will use
for a home made grinder, for $2.

Schematic: 

The primary switch conducts electricity from POWER output of the
contactor to SIGNAL input.

The secondary, two pole switch, does two things: 

1) passes power from power input to power output of the contactor.
That allows the primary switch to conduct electricity to the signal
input of the contactor. That turns the contactor on and keeps it on as
long as the primary switch is on. (but would not turn it on by itself
without the secondary switch)

2) Passes power to a 21V DC power supply (salvaged from a POS $18 "24V
cordless drill" from ebay). That power supply is connected to inputs
of the Crydom solid state relay and engages the relay and capacitors.

DC, applied to the crydom, would turn capacitors on, as long as
secondary switch is engaged. When I disengage the secondary (two pole)
switch, power to the 21 VDC power supply goes out, 21 VDC is not
supplied to the relay, and the relay turns capacitors off. The RPC
continues to run (see 1) with much less noise and vibration than it
did with caps left in.

Here's the problem.

The solid state relay acts quite strangely. Sometimes, upon restart,
it flat out refuses to engage, so the caps are not in the
circuit. Shorting caps prior to restart seems to rectify the problem. 

I suspect that there is some issue with remaining charge in the caps
and zero crossing logic. I am thinking to just add a relatively low
ohm resistor between caps. I can get away with it, because the
starting circuit is only engaged for a few seconds and the resistor
would not overheat. I welcome any thoughts on this....

i






--
Martin Eastburn
@ home at Lions' Lair with our computer lionslair at consolidated dot net
NRA LOH, NRA Life
NRA Second Amendment Task Force Charter Founder

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