I need help with Relay Specs?

Question

Hi,

I am a physics undergrad working in a lab at UCLA, and we are currently trying to figure out how to automate our current setup.

I am quite new at practical electronics. I have handled a few things in class before but most of that was theoretical, and Relays were never really discussed.

I am currently looking at some solid state relays on Newark, but i don't really know what i am looking for

the schematic i drew up uses two relays. Our LabView system can put out 10VDC at 2mA, so i am using a relay to draw current from the wall @ a small DC voltage, and then using that voltage to operate another relay that will draw a 120V/60Hz current out of the wall to power some solenoid valves.

The 2nd Relay is already taken care of (turns on a 3V 10mA), but we now need the smaller one to activate it, and to be honest i am not sure what the specs i am looking at mean. what do Load Voltage Max, Load Current, Isolation Voltage, and Forward Current mean?

I figure that the Load current is how much it draws out of the power supply and that the load voltage max is how much voltage is required to switch it, but what is the forward current and the isolation voltage?

Answer 1

To summarize and check I understand it, you have a logic board which gives 10V 2ma output, and you want to drive a solenoid valve that has needs 3V 10 ma (minimum) to operate it?



You don;t really need a another "low power" relay to interface the two, and since relays are mechanical devices they are orders of magnitude less reliable than electronic components.



A better solution would be a simple buffer amplifier that will make your 2ma output control a bigger current for driving higher powered devices. Most likely the logic board supplier will also sell a compatible board, if you want to buy one (it will probably have multiple driver circuits to handle several devices, e.g. 4 or 8). If you want to make you own, a standard 1-transistor relay driver circuit will work fine. Google for the circuit, or look in any textbook on practical circuit design.



If you only need to handle 10V and less than 100 mA, the choice of transistor is completely non-critical, any "general purpose medium power transistor" should work fine without any need to bother with heatsinks, etc

Answer 2

Load Voltage and Load current refer to the maximums that the relay can switch and carry, i.e. how much of a load it can withstand.



Forward current *probably* means the current that the relay will draw from the switching signal (reverse current would be infinitesimal since we're talking about a diode-type device here). I don't see what else it could mean.



Isolation voltage is the amount of voltage difference there can be between the load circuit and the switching circuit or between load circuit and ground before the insulation will break down and allow current to flow where it shouldn't.



Reverse or breakdown voltage is how much wrong-polarity voltage the switching circuit can handle without breaking.

Answer 3

Note that relays have two separate circuits, the load circuit, usually at high voltages and currents. And the drive circuit, at much lower voltage and current. The two are isolated from each other, in a conventional relay by a magnetic field, and in SSRs by an optical isolator.



"load" specs all apply to the contacts, not the drive circuitry.



isolation voltage is the max voltage you can have between the drive circuitry and the load circuitry.

forward current is a new one to me, need the context.

load current is the max current on the load circuit. I don't understand your reference to the power supply.



.

Answer 4

The one you need seems to be able to handle both AC and DC. The one you have is DC only. Check the power you are going to use it (5 V, 12V, 24 v , etc.) Check the amps you plan to draw If what you have can handle the amps at the specific Voltage, you are OK.