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Solid State Relays

B

breister

Well Known Member
Can someone out there help me make sense out of the dizzying array of solid state relays?

I run a 12v (ok 14v) system and want to learn how to decide what is an appropriate solid state relay that will accept as input a few milliamps and output a couple of amps continuously (when receiving the milliamp input signal). Searching on "solid state relay" produces literally thousands of examples, but not knowing how to interpret the specs I can't seem to tell if they are suitable for continuous duty (some clearly say they aren't) etc.

I'm also interested in replacing aging mechanical relays for trim and flaps which are NOT continuous duty but may have more total amperage draw.

Thanks!
 
Dean_aeroleds said:
This one might do the trick... you will have to condition the control signal to keep up under 10VDC, but that can be done easily with a resistor and zener diode:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=CC1124-ND
Click to expand...

Ok, so what is the trick to understanding the specs? If I read that right, it can't be used to power anything requiring more than 10v. Reducing the control voltage would be ok, but my output voltage has to be 12-14v DC.

This one looks closer, and looks like it will accept any input voltage between 5-30. But I still don't understand the nomenclature.
 
The 10 amp one....

breister said:
Ok, so what is the trick to understanding the specs? If I read that right, it can't be used to power anything requiring more than 10v. Reducing the control voltage would be ok, but my output voltage has to be 12-14v DC.

This one looks closer, and looks like it will accept any input voltage between 5-30. But I still don't understand the nomenclature.
Click to expand...

...you link to has a voltage drop of 1.0 volts at full load. Note the requirement for a heat sink, that 1 volt drop at 10 amps turns into 12 watts of heat inside the unit.

Sort of defeats the use of a solid state relay vs. a good quality mechanical relay that has essentially zero voltage drop.
 
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breister said:
Ok, so what is the trick to understanding the specs? If I read that right, it can't be used to power anything requiring more than 10v. Reducing the control voltage would be ok, but my output voltage has to be 12-14v DC.

This one looks closer, and looks like it will accept any input voltage between 5-30. But I still don't understand the nomenclature.
Click to expand...

No, the part I linked to can switch voltages up to 100V. Only the control voltage is limited to below 10V. A resistor and zener diode is cheap, and easy to implement. The circuit looks like this:

Battery Voltage
Panel Mounted Switch
10Kohm resistor
Input to the solid state relay control attaches here
10V zener diode
Ground

Also, the on resistance is 0.04 ohms, which means it won't burn a lot of power when turned on...
 
Dean_aeroleds said:
No, the part I linked to can switch voltages up to 100V. Only the control voltage is limited to below 10V. A resistor and zener diode is cheap, and easy to implement. The circuit looks like this:

Battery Voltage
Panel Mounted Switch
10Kohm resistor
Input to the solid state relay control attaches here
10V zener diode
Ground

Also, the on resistance is 0.04 ohms, which means it won't burn a lot of power when turned on...
Click to expand...

Actually, make that a 1K resistor. It needs more current to drive the input than a 10K resistor will allow.
 
Even lower resistance?

Dean_aeroleds said:
Actually, make that a 1K resistor. It needs more current to drive the input than a 10K resistor will allow.
Click to expand...

Description
Control Voltage Range 3.0-10.0 VDC
Must Turn On Voltage 3.0 VDC
Must Turn Off Voltage 1 .0 VDC
Typycal Input Current @ 5 VDC 15 mAdc {spelling error from data sheet}
Nominal Input Impedance 300 Ohm
Maximum Turn-On Time [msec] 1.0
Maximum Turn-Off Time [μsec] 300

I think a 470 ohm would be better....:)

The DigiKey range you linked to is much better than the other link, with the Digikey 10 amp device being 1.8 watts at max current.
 
Dean_aeroleds said:
No, the part I linked to can switch voltages up to 100V. Only the control voltage is limited to below 10V. A resistor and zener diode is cheap, and easy to implement. The circuit looks like this:

Battery Voltage
Panel Mounted Switch
10Kohm resistor
Input to the solid state relay control attaches here
10V zener diode
Ground

Also, the on resistance is 0.04 ohms, which means it won't burn a lot of power when turned on...
Click to expand...

Well, that just points out that I don't understand the nomenclature on the spec.

I understand the resistor - don't want to feed 14.5v into a 10v max circuit - but what is the purpose of the diode in the circuit?
 
breister said:
I understand the resistor - don't want to feed 14.5v into a 10v max circuit - but what is the purpose of the diode in the circuit?
Click to expand...

Anytime that a relay coil is driven by a circuit that is not specifically designed to drive a relay, you should use a quenching/suppression diode connected in parallel with the relay coil. You may think the diode serves no purpose because the voltage applied to the relay cannot pass through the diode. This is true when the relay is energized. The diode comes into play when the power source is removed from the relay coil. When power is applied to the relay coil, a magnetic field is created and energy is stored in the coil. When power is removed, the magnetic field collapses causing a reverse voltage to be generated (it's called inductive kickback or back EMF). The back EMF can easily reach high voltages. The diode will absorb the reverse voltage spike. This voltage, if not absorbed by the diode, will cause premature failure of switch contacts in the relay. Its convenient to purchase these modern relays with the diodes built in. Its also important to remember that when using a diode supression in the circuit, then the +- voltage polarity now becomes important. Without this diode, usually it is not important.
 
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breister said:
Well, that just points out that I don't understand the nomenclature on the spec.

I understand the resistor - don't want to feed 14.5v into a 10v max circuit - but what is the purpose of the diode in the circuit?
Click to expand...

The 10V zener diode is there to ensure that the input voltage to the solid state relay control line never exceeds its rated voltage of 10V. Typically, the input current will drop enough voltage accross the current limiting resistor to ensure this, but a voltage spike or other transient could come along that would cause the voltage to go above 10V if the zener diode wasn't in place.

There is no coil in a solid state relay, so there won't be any inductive kick-back to worry about. The zener is only required to clamp the voltage to stay below the max allowed.

Dean
 
Thanks - it's like an ignition coil and the diode prevents "flashing" back down the channel. Makes perfect sense.
 
breister said:
Thanks - it's like an ignition coil and the diode prevents "flashing" back down the channel. Makes perfect sense.
Click to expand...

No, it is nothing like an ignition coil. There is no inductive fly back to worry about.

The zener diode is simply required to limit the voltage that can be applied to the control input to of the SSR (Solid State Relay).

Dean
 
Sorry, my first response was to Kahuna's first post. Both answers make sense in the intended situations.

One of the diagrams also listed having a LOAD on the relay which was inductive, and also included a diode in the diagram. Is a Facet pump an inductive load?
 
Hmmm, in my more critical application (automatic fuel transfer to a header tank) I'm more concerned about failure than using an extra amp or two. Also, the solid-state fuel sensors I plan to use only provide a very slight "control current" (12v but really low amps).

However, I really like the "latching" relays - hadn't heard of those before. Introduces some ideas...

My original plan called for a single optical sensor in the tank to provide positive signal when the tank was "less than full," and another optical sensor to provide positive signal so long as there was "wing fuel remaining." Each sensor would require it's own relay; power to one relay and servo would only be available if the other relay was energized. Thus, when fuel level falls below the "full" level (not actually full, don't want to pump fuel overboard) and provided there is still fuel in the transfer pump source line I would want the transfer (Facet) pump to turn on. The pump doesn't pull very much amperage, so "wasted current" would be small. The nice thing would be that the entire circuit would be solid state and therefore unlikely to "wear out" (although I fully intend an "override" mode to force the pump on). The only downside I can see to this would be that the pump would tend to cycle on and off very rapidly until the wing tank is dry, which might affect the life of the pump.

Another approach would be to have two sensors in the tank and a latching relay - but that would depend on activation current required by the latching servo. Since the optical sensors will only provide a very small current, I don't know if this would work.
 
Caution, you cannot use AC rated solid state relays in DC applications.

I've designed my HRII electrical system to use a combination of electromechanical relays (cheap, good for up to 40A loads and easy to source if/when a failure occurs) and solid-state relays of my own design.

The SSR that I've designed are used for flap and trim systems (aka relay deck) and are good for up to 5 amps continuous loads.

Big advantage over electromechanical relays is that there is no possibility of stuck contacts and it's *much* easier to perform speed control, which I have built into the devices.

The SSR can also drive a pair of normal loads up to 5A. Above that, it gets a lot more expensive, so the electromechanical units are a better bet.

I haven't decided whether or not to market these devices through Vx Aviation. It all depends on the business case, which takes more time to write than designing the products!

Vern
 
What are you trying to do? Maybe you don't need a SSR but simply a MOSFET from the return of the device you are powering to ground. You may need some voltage dropping resistors to drive the gate of the device, but the whole thing can be cheap and with very low power dissipation.
 
vlittle said:
Caution, you cannot use AC rated solid state relays in DC applications.

I've designed my HRII electrical system to use a combination of electromechanical relays (cheap, good for up to 40A loads and easy to source if/when a failure occurs) and solid-state relays of my own design.

The SSR that I've designed are used for flap and trim systems (aka relay deck) and are good for up to 5 amps continuous loads.

Big advantage over electromechanical relays is that there is no possibility of stuck contacts and it's *much* easier to perform speed control, which I have built into the devices.

The SSR can also drive a pair of normal loads up to 5A. Above that, it gets a lot more expensive, so the electromechanical units are a better bet.

I haven't decided whether or not to market these devices through Vx Aviation. It all depends on the business case, which takes more time to write than designing the products!

Vern
Click to expand...

Thanks Vern,

Once upon a time PP Avionics sold an automated fuel transfer system, but their web site is no more. Their solution was dependent upon a feature built in to Facet pumps that apparently is not reliable any more, so in later days they simply sold the light sensors and a solution where the transfer pump switches flashed at you once the fuel was exhausted.

I felt their solution was unsatisfactory as it still required you to turn the pumps on and monitor the quantity in the header tank. In my mind fuel transfer should be totally automatic and reliable (barring pump failure, which is rare). Thus, the system should know when the header tank is not full and then turn the pump on if and only if the supply tank is not empty, turning the pump off again when the header tank approaches full.

Realistically, I think a turbine system or a system with a supercharger or turbocharger could accomplish all of this very simply with air pressure rather than pumps, and simple floats and needle valves. However, I don't have any of the above.

You might find a ready audience if you would simply package the right combination of your simple devices with the right light sensors for such a solution. Several homebuilt models require fuel transfer.

In my case I also want reliable relays for a pitch trim system I'm installing - since it is seldom active the inefficiency should be irrelevant.
 
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