Solid State Relays

[breister]

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]

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/Dk...name=CC1124-ND

[RV8RIVETER]

There is also this one for about the same price and you don't have to drop the control volts.
http://search.digikey.com/scripts/Dk...name=CC1128-ND

If you click on the "datasheets" link you can see all the relays in the family they produce.

[breister]

Originally Posted by Dean_aeroleds

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/Dk...name=CC1124-ND

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.

[az_gila]

Originally Posted by breister

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.

...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.

[Dean_aeroleds]

Originally Posted by breister

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.

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]

Originally Posted by Dean_aeroleds

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...

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

[az_gila]

Originally Posted by Dean_aeroleds

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

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.

[breister]

Originally Posted by Dean_aeroleds

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...

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?

[Kahuna]

Originally Posted by breister

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?

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.