Jerry/Solid state switches

I

I sent you a picture of a solid state relay that I will mail to you if you want it to "mess around with". I am overloaded with good stuff that I'll probably never get to use anyway.

Jerry

Thanks... what's zero crossing?

i

i The point in time where a harmonic wave (can you say "sine wave") intersects the ordinate, or "x" axis. Voltage is zero at that time thus it is desireable for switching to occur at that time rather than earlier or later. There can be no transient voltage at the time of zero crossing; no chance of transient voltage damage to the switch.

Bob Swinney

Thanks Bob...

i

I always thought the main reason for the zero switching was to reduce generation of electrical/RF noise caused by switching. Back in my traffic signal tech days, I used thousands of those things.

Vaughn

At zero crossing the current flow is zero. That means the load current comes up as the voltage phase comes back up, which means you can switch larger currents with smaller (cheaper) devices, make less (or zero) switching noise for the same reason, and interface with lower voltage drivers such as low current computer stuff, since the gate signal can be smaller.

No transients, no RF. It's that simple. However in the case at hand we are more interested in zero current flow at switch time.

And less energy to be delivered to an antenna - e.g. wire attached. Also - the current is reversing at this instant as well. (If an a.c. model like most are today.)

Martin

That was part of it, but the main reason was to minimize turnon current to incandescent loads by not switching them on in mid-cycle. Even then, loadswitches that lasted used triacs of considerably higher ratings than seemed necessary to the designers.

I didn't design loadswitches, but I did design controllers. I'll admit to having been the original designer of the California 170. Ducking incoming but I think some of those are still in service 30 years later.

I designed 'em, but only a traffic engineer could program 'em to work well. Most fully-actuated intersections are not programmed well at all.

Harvey Goldenberg, the traffic engineer for the State of New York, was genius good at it. (New York adopted the 170 along with CA). I saw him tune an 8-phase dual-ring intersection of two arterials so it ran like magic during rush hour. It's an art as much as a science.

Non-ZeroCross switchs can be use to do PWM at waht ever you AC frecquency is... In effect that makes them a 'light dimmer'.

Most all of these devices are made from either back to back silicon controled rectifiers or triacs. Both of these devices 'turn on' when given a control current and will stay 'on' with out the control current till the voltage they are passing goes to zero... If you synchronize a small pulse of control current with the 'zero crossing' then you can pick any place on the sine wave to turn on, letting it turn off when it crosses zero. so in effect you are using the sine wave period as a PWM time base....

Simple examples... turn on at zero cross (well just a bit after) and it's full power, Turn on at mid sine wave and it's half power.. turn on 1/4 of the way through and its 70% power... (cos(45))

Yes the spike from the turn on any where but zero can cause plenty of RF but ya just 'choke' the hell out of it.... this IS basically what those 'light dimers in a switch' do.. ever wondered why you hear the ligh bulb 'sing' some times, that's the practicaly straight up energy pulse hitting it.

This is also what the cheap speed controllers for DC motors do, using just one SCR and is why there are 90VDC motors out there...

I've built a bidirectional variable speed DC motor controller using a triac.. you choose which half of the sine wave for direction and then do the PWM thing for speed... with a sutibally rated triac you could even use reverse voltage as a brake... (I had a 1 HP Bodine gear motor moving a 1 ton piece of equipment and NEEDED the reverse power to stop the damn thing... BTW this is abusive to a PM DC motor and after a year or so ya tend to demag them, but work just fine on a field coil motor.

The real cool thing on a SCR/Triac based controller is that with a PM DC motor you can sense the generated voltage when the SCR/Triac is off... it IS directly proportional to the speed the motor is turning (it's a generator) and adjust the PWM to keep you speed set.....

--.- Dave

All true -- but zero-crossing loadswitches will not turn on until the next zero crossing even if a drive signal is applied mid-cycle.

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

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

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.

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

Thanks, I would like to keep things simple.

Well, the switch is, as I was wisely instructed, a solid state switch, no arcing. It's nice after all the bugs are worked out.

i