Solid State Relay vs Mechanical Relay?

Hi

I've currently completed a project which I'm quite happy with but am trying to improve.
The mechanical relay on my PCB triggers a volt-free (or near volt-free) contact on another device, which at the moment works fine, but I would like to change as the coil from the conventional relay is causing interference with one of the other components on the board (magnetometer).

I've been looking at SSRs or Solid State Relays and I'm trying to understand whether or not they would be suitable, for as far as I can see the "load" side of an SSD requires a through-current, which I wouldn't want.

Would I be right in saying that these types of relays do not provide simple "volt-free" contacts and would require a current to be passing on the load terminals?

Any help would be great!

Please explain “volt-free contact”

If there is no voltage on the contacts, then what voltage is it controlling?

As mentioned above, it is called "volt-free", but in reality they have a micro voltage monitoring for closure.

Practically every SSR I have come across require a "minimum load current", which varies from relay to relay, but generally around 100mA.

micro voltage from where? for closure of what?

A regular-old electro-mechanical relay is often the most “foolproof” solution because it’s simply an electrically operated switch. Regular relays also tend to be “electrically rugged” (not important for your low-power application).

Maybe you can move the relay or “filter” the power to reduce/eliminate the interference?

Depending on what you’re switching an [u]analog switch[/u] may be the answer. They are used for switching low-power signals (audio signals for example). But, they are not electrically isolated so you’d have to add an opto-isolator if you need isolation.

Or if you just need a high/low switch you might be able to get-away with just a transistor or just an opto-isolator.

warning, X/Y problem !

instead of asking us the fitness of a device for your circuit.

define the circuit and we will offer suitable device.

what voltage ? AC or DC ? and how much power are you switching ?
is this isolated from the Arduino power ?

As to the answer to your question about SSR units, most are AC only and yes, they need to draw a load in order to work.

they are just a complied circuit of things for a specifc task in an nice package.
you can dupiclate the circuit by just ordering the parts and putting it together yourself

if you are just switching a DC load, there might be much easier ways.

According to the OP he isn't switching any voltage, or micro volts. So there is no current, voltage or power to be switched. I'm still perplexed on how this works.

adwsystems:
According to the OP he isn’t switching any voltage, or micro volts. So there is no current, voltage or power to be switched. I’m still perplexed on how this works.

If there is indeed no voltage, then eventually the relay contacts will develop enough resistance to not pass ANY current. A vacuum relay such as a reed relay will work.

Paul

:o Right, I can see I have caused a lot of confusion about what I am switching.

I am using my circuit to control a FAAC gate control board, which when you short two terminals together on the control board it opens my gates at home.

Up until now I have been using my circuit which detects vehicles and triggers the mechanical relay to short the two terminals on the gate control board which, in turn, opens my gates.

The manufacturer of the gate control board has listed the terminals as “volt-free” contacts. What exactly this means I have no idea, but I guess there must be some sort of trickle current between the terminals, to monitor a short.

All I wanted to know, without getting into specifics and quadratics is if there is an alternative I could use, instead of a clunky mechanical relay, to short those two contacts together.

OOOOF :o

rebble:
:o Right, I can see I have caused a lot of confusion about what I am switching.

I am using my circuit to control a FAAC gate control board, which when you short two terminals together on the control board it opens my gates at home.

Up until now I have been using my circuit which detects vehicles and triggers the mechanical relay to short the two terminals on the gate control board which, in turn, opens my gates.

The manufacturer of the gate control board has listed the terminals as "volt-free" contacts. What exactly this means I have no idea, but I guess there must be some sort of trickle current between the terminals, to monitor a short.

All I wanted to know, without getting into specifics and quadratics is if there is an alternative I could use, instead of a clunky mechanical relay, to short those two contacts together.

OOOOF :o

Get your digital meter out and find if the connection is AC or DC. If DC, determine which wire is + and which is -.

Then see if the - wire is really connected to the circuit board ground.

Paul

Paul_KD7HB:
Get your digital meter out and find if the connection is AC or DC. If DC, determine which wire is + and which is -.

Then see if the - wire is really connected to the circuit board ground.

Paul

Right, done that. Between the "Input" terminals (GND & Input) it's giving 22V DC.
Doesn't sound very "volt-free" to me.

rebble:
Right, done that. Between the "Input" terminals (GND & Input) it's giving 22V DC.
Doesn't sound very "volt-free" to me.

Ok, now if you can find a low value resistor, say 5 to 20 ohms. Put that in place of the relay contacts and see if the project thinks that is the relay contacts. If so, them a MOSFET will be a great replacement for the relay.

Paul

...so long as the Arduino power supply can share a ground with the thing being controlled.

Paul_KD7HB:
Ok, now if you can find a low value resistor, say 5 to 20 ohms. Put that in place of the relay contacts and see if the project thinks that is the relay contacts. If so, them a MOSFET will be a great replacement for the relay.

Paul

Right, I think I understand now. So basically the “contacts” on these SSRs have a substantial resistance, depending on their load.

I will give this a go, see if it works. And yes, the Arduino can share GND with the board

Put your multimeter on current and see what current it takes.

If < a mA or so an optocoupler could be a good solution.

I’d be distrustful of common earths in such an environment…

But a simple relay would work well

Allan

what relay are you using ? a reed ?
with almost no power, the contacts only need to be minimal so one of the 10 amp Songle relay boards would be way too much and as stated other devices pick up the coils magnetic field.

a mechanical relay connects contacts to the original signal passes.
once you go into silicon, you can just sense the input, then create an output of a suitable value.
an optoisolator

if your problem is that your magnetometer senses a very strong magnetic field, you might be able to use a much weaker field and move it some distance from the magnetometer.

A 5V Songle relay as found on most cheap boards has a coil that is listed for 89 mA
a 5v, non-latching reed will use 5 to 10mA. and contacts are rated for 500mA

without links to parts, it would appear that you are looking at high current devices and not signal level devices.
a 5V signal relay will also be in the 10mA-30mA range for a coil and if you go for a latching, then the pulse may be higher, closer to 30mA-50mA, but would be a pulse, not continuous.

a simple reed relay is low enough power that you can power it from a pin of the Arduino board without all the extra circuitry.

if you look at the circuit for an a DC SSR it is not hard to make from parts in your bins. the pre-packaged units are also often very high current rated and would be overkill for a voltage-free signal.