Solid State Relay Questions

J

Jason

Can someone please explain the following to me (specifically regarding Crydom P/N CX240D5(R))?

1. Can you explain what the zero crossing circuit is used for?

2. What is meant by a "Random turn-on switching version" of this part?

I was planning on using this device to control a heating element (about 3 amps of current) using PWM to drive the input. The heating element is AC (110V). Is there another way of driving this type of load? I'm trying to keep everything that is DC operated at 5V. I've just looked into IGBTs, however, I have not read enough about them to know if they would work. Looks like it would have to be a DC voltage that was being switched in order to use one of those...

Thanks for the help.

Jason

T

Thomas Hergenhahn

> Can someone please explain the following to me (specifically regarding Crydom P/N CX240D5(R))?
>
> 1. Can you explain what the zero crossing circuit is used for?

The relay delays switching on until the line voltage (sinusoidal AC) goes through zero. The purpose is to minimize rush-in current. This is good for resistive loads but absolutely undesireable for inductive loads. So, it is okay and good for most heating applications. See also below.

> 2. What is meant by a "Random turn-on switching version" of this part?

I suppose this version just switches in the same instant it gets the command.

> I was planning on using this device to control >a heating element (about 3 amps of current) >using PWM to drive the input. The heating >element is AC (110V). Is there another way of >driving this type of load?

Yes, there are some other ways, in principle. Much depends on the thermal capacitance (hope this term is understood, I'm not a native speaker of english). Imagine your heater as a source of heat like a voltage source. Immagine whatever conducts the heat to where you need it as a resistor. Then the volume and mass of material which you want to heat up is "charged" with heat like a capacitor from a voltage source over a resistor. Hence there is a time constant. If this time constant is great with respect to the lines period (16.6 ms @ 60 cps), you can use SCR controllers, if not you need a switch that can turn off at any time. In any case the period of your PWM signal has to be a lot shorter than the time constant. If the time constant is greater than many periods (more than some ten seconds), you can youse the zero cross switch principle. The period off your PWM signal will be many line periods and when you switch on, the relay switches on at the next zero crossing, then delivering current for some line periods. When you switch off the control signal, current flows again until next zero crossing.

>I'm trying to keep everything that is DC >operated at 5V. I've just looked into IGBTs, >however, I have not read enough about them to >know if they would work. Looks like it would >have to be a DC voltage that was being switched >in order to use one of those...

I suppose the main problem you might encounter with IGBTs is that they do not provide any insulation between load and control circuit. You would have to realize this in a driver circuit. Usually, you need to supply the energy needed for control to the load side. The advantage of IGBTs is that you can turn them off at any time (opposed to SCRs which switch off at zero current).

HTH
Thomas

D

dan

> Can someone please explain the following to me (specifically regarding Crydom P/N CX240D5(R))?
>
> 1. Can you explain what the zero crossing circuit is used for?

The Zero crossing circuit delays the turn on of the relay until the AC voltage is at a zero crossing, i.e when the voltage across the switch is at zero volts. This is used to reduce surge current and EMI. The relay is probably a triac or two anti-parallel SCR's.

> 2. What is meant by a "Random turn-on switching version" of this part?

This just means that the relay turns on when ever the control signal is asserted. There is some delay from assertion of the control signal.

> I was planning on using this device to control a heating element (about 3 amps of current) using PWM to drive the input. The heating element is AC (110V). Is there another way of driving this type of load? I'm trying to keep everything that is DC operated at 5V. I've just looked into IGBTs, however, I have not read enough about them to know if they would work. Looks like it would have to be a DC voltage that was being switched in order to use one of those...

If you are using PWM, with this zero crossing type of part, the minimun on time would be 1/2 cycle or 8.33 msec.

Dan

M

Michael Griffin

On September 18, 2003 16:28, Jason wrote:
<clip>
> 1. Can you explain what the zero crossing circuit is used for?

To ensure the solid state relay turns on at the zero crossing of the AC cycle.
This reduces electrical noise.

> 2. What is meant by a "Random turn-on switching version" of this part?
<clip>

The solid state relay can be turned on at any point in the AC cycle.

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Michael Griffin
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M

matt hyatt

I am working witha eng group right now who is doing just that ,using a SSR to PWM a heater element at 120vac, use a FET switch to turn the neg. pin on and off and leave the pos. pin tied to 5 volts. this way if the relay input fails, it will not remain on. Don't worry about the zero crossing and random turn-on voltage.

Just drive the gate of a N-Chnl FET (with the source tied to ground, could be the drain, its late so I am not sure right now) with a pulsed 5 volt signal, at full heat (100% on) you will have 5 volts on the gate, at 50% power to the heater, you may have an avgerage of 2-3 volts on the gate (using a voltmeter).

Works great and is very reliable.

matt hyatt
technical consultants

F

Fred Townsend

Jason, you seem worried about using a SSR. A zero crossing SSR is probably an idea solution to your problem. A random turn on switch can be very problematic. Here is why.

When a zero crossing switch turns on / off, the current and voltage increases / decreases sinusoidally. Random switch current and voltage increases / decreases as a combination of sinusoidally and exponentially current and voltage. Worse case is at 90 and 270 degree where the current and voltage changes only exponentially. Exponential turn on is a problem to capacitive loads. Exponential turn off is a problem to inductive loads. All exponential changes generate more EMI, particularly when used with PWM, than sinusoid changes.

Fred Townsend

Z

Zvi

Hello Jason,

1. Zero crossover for an AC switching device means that the the load will be switched when the AC voltage "crosses over" or is at zero. This is useful to avoid problems when an inductive load is connected or interrupted.

2. Random switching means the device is NOT zero crossover. These may cost a little less.

For your 3 ampere resistive load, either one will do the job.

Note that the spec sheet for your CX240D5 shows that its rating will drop to 3 amps at 60 degrees Celsius, so be sure it's well ventilated.

Good luck,
Zvi

B

Bob Welker

The zero crossing part will turn on the output at the point (after the input has been turned on) the AC waveform crosses through 0 volts, whereas a random turn-on part will turn on the output nearly immediately after the input goes high.

The pro of using a zero-crossing component is EMI generation is decreased, since, when the output turns on, very little current is capable of flowing into the load. The con is that, since there is a variable time lag between when the input signal is asserted and the output turns on (up to 1/2 an AC cycle - 8.3 milliseconds) this consideration needs to be taken into account if driving it with PWM.

Random turn-on means that the output switches almost immediately after the input goes high (look at the spec sheet to find out what this minimum time lag is), and thus current will start flowing into the load at any phase of the power waveform (hence, the 'random' moniker).

A random turn-on component is probably a better choice when driving with a PWM signal.

A

Anonymous

Hello

0 crossing means that the ssr will turn on when the AC load sine wave is at 0 volts. Random will turn on with no relationship to the output. Power dissapation in the SSR is reduced if no current has to be switched.

The input voltage to activate the SSR is 3-15 VDC so your 5 VDC control voltage would be OK.

SSR's are used alot for heating applications because of the fact you can use a PWM control which would destroy a contactor or relay over time.

It is agood idea to use a safety relay in series with the load because SSR's can fail on.

Good Luck.

P

PT

Talk to a large heating element supplier. They sell PWM power control modules for every twpe of restive load imaginable. Single phase,3 phase, all common power voltages, all KW outputs.Pulse in,0-5vin, 4-20 ma in, etc.

J

Jason

(Wow, I'm glad that I decided to check back on this web site. I was concerned that after about a week, I was not able to find my post...)

Thank you for all the inputs. It sounds like the thing to use in this case is the SSR. As described by one of the replys, I think the time constant of the heating element would be fairly long. This leads me to believe that I can safely use a zero-crossing SSR.

I was going to use the PWM output of a microcontroller, however, now, I think that the problem would be better solved by just using an I/O pin, and switching between on/off (more like an oven heating element controller works). I think that this would reduce the complexity of the design considerably.

I think that I would be okay with the current rating of the SSR due to the fact that the SSR will actually be located away from the heat source, so that shouldn't be a factor (indoor use only!).

I would however, like to use the PWM for this project, so I may give it a try. I assume that the update PWM signal would have to be fairly long, in order to account for the long time constant of the heating element.

Thank you once again for all the replys.
Jason