# How do I find the switching voltages from a relay's data sheet?

I'm doing a school project and the course so far has taught us nothing but resistors really. Some project to be made. So I'm trying to figure something out myself.

I'm using a photocell to control a relay to control a motor (DPDT relay, so two-direction DC motor control based on light). My question is:

How do I know what relay to use? I have all other stats on things, but the relays are confusing me. Specifically I have the datasheet for JW2SN-DC12V and I don't really understand it; how do I find the switching voltage (point at which it changes from one position to the other)?

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• 8 years ago

"switching voltage" has a different meaning than the one you are using. It usually means the voltage that can be switched by the contacts with no damage. Which, for DC and for the "standard" relay, is 30 volts and 5 amps.

You seem to be asking for the coil ratings. Further down on the datasheet, apparently this relay can be ordered with various types of coils. The DC12V part on the part number indicates that the coil requires 12 volts DC to operate. And at 12 volts nominal, the pickup voltage (voltage required to close the contacts) is 8.4 volts and the dropout voltage is 1.2 volts, ie, the voltage that will cause the contacts to open once closed.

But the way it is made to operate is, apply a nominal 12 volts (which could be as low as 10 volts) to the coil and the contacts close. Remove the 12 volts, and they open.

If you are using this to reverse the direction of a motor, you have to verify that the relay is break-before-make, otherwise you will have a momentary short while the relay switches which is disastrous.

• John
Lv 5
8 years ago

That relay is a 12v relay. Generally, relays operate with the nominal voltage +/- some percentage and you sometimes can find it in the datasheet. If you look on pg3, you see figure 2. Operate/release time and that shows a +/-20% voltage of operation and gives you a curve. A word of caution though, the hold on a relay is often lower than what is shown, these are the spec limits.

How do you pick a relay? You need to know the operating current and voltage of the contacts, how many contacts you need, and then the control voltage of the relay and the resistance of the coil. The last two are used to create the circuit to control the relay and drive it on and off. You will need something besides the photocell to drive the relay, it can't handle the current even if it's resistance was low enough, so some transistors would be useful.

I just did a search for

photocell relay controller schematic

in google and the very first item shows a schematic that does a single photocell on off control (street lamp control).

• tanny
Lv 4
3 years ago

there's a drawing that exhibits the connections. You grant a hundred and twenty VAC 60 Hz to pins 7 and eight. connect a thermocouple, the E52-THE@ Thermistor (replaceable element) or a platinum resistance thermometer to terminals a million (advantageous) and a pair of (adverse). for output: If the burden circuit is a heating administration device, be optimistic to connect the burden to terminals 4 and 5. If the burden circuit is a cooling administration device, be optimistic to connect the burden to terminals 4 and six. They advise utilising an exterior relay to amplify the electrical powered existence of inner relays while driving a extensive means load. it is taken into consideration necessary while the output relay is switched regularly (e.g., with proportional administration). what's the voltage of your heater? If it is going to function on a hundred and twenty VAC, below 3 amps (360 watts) then you could use the relay touch outputs. no rely if it is greater beneficial than 3 amps, you desire an exterior relay. no rely if it is a distinctive voltage, you ought to use the contacts, as long by way of fact the present is below 3 amps. looks to me that the indoors relay is incredibly undesirable, you would be able to desire to continuously use an exterior relay. .