The Pololu basic relay carrier modules allow simple control of a single-pole, double-throw (SPDT) switch from low-voltage, low-current control signals. This item includes the basic carrier PCB with a soldered-in 12 V relay, 5.0 mm terminal blocks for the switch connections, and straight 0.1" male header for the control connections. The included power relay is an Omron G5LE-14-DC12 and is rated for up to 10 A under most conditions.
I need a ‘potential free’ relay or otherwise open collector/optocoupler output. It will have 12V on the input side (when connected to a running 12V pump). It’s output is to be connected to a digital I/O input on a mini computer for alarming purposes. The digital IO operates on 3V3. Would Pololu Basic SPDT Relay Carrier with 12VDC Relay (Assembled) | POLOLU-2482 | Core Electronics Australia be a suitable solution to this?
This relay should work fine for this purpose. Depending on the mini computer you are connecting the 3.3V output to you may want to consider using a resistor to limit the current going into this device.
Hi Samuel.
This sort of thing never seems to have a gate current limiting resistor on board. A gate grounding resistor is always there but the all important current limiting resistor is not.
Do you know of any particular reason for this?
Also a couple of more small points regarding this particular module:
Why 2 indicator LEDs. They don’t show that the relay has actually operated anyway. They only show the relay has been requested to operate.
What is the Zener diode for.
Thanks for the included circuit schematic. Does not happen often.
Cheers Bob
I wouldn’t be able to tell you exactly why this specific relay doesn’t include a gate current-limiting resistor. From my understanding, it’s probably to reduce unwanted propagation delays in the gate signal so that applications with high switching speeds aren’t affected.
Aside from this, I would assume all the normal culprits of cost, size, and complexity apply to some extent.
The use of 2 indicator LEDs does seem strange to me as well, I guess so that they are visible from both sides of the relay? You can see how they are positioned in this product photo but your guess is as good as mine on this one.
The Zener diode’s purpose is lost on me as well, I would guess it is some kind of over-voltage protection for the LEDs but that’s not a guess I make with a high level of confidence.
Hi Samuel
The gate is a short to ground at switch on so I would have thought the possible inrush current damage to the driving device would have been more important than propagation delay. The Mosfet is only handling a few mA coil current and the relay probably takes 10mS to operate so a few µS is not going to be significant. Unfortunately this sort of damage can be cumulative and fail at some time in the future when there is a distinct possibility that the MCU or whatever is being used for something quite unrelated. Usually results in a hair pulling exercise.
I think your
could be the determining force.
Me either. If D3 is doing its job the reverse volts across the LEDs would only be 0.6V at worst. The zener is just stopping D3 doing its job until the reverse spike gets to 12V. All it is doing is allowing a 12V reverse pulse to develop across the coil instead of the whole pulse being suppressed… This I might add is added to the supply creating an effective 24V across the Mosfet for the pulse duration. That should not trouble the Mosfet unduly but I just wondered why it is there. D3 would prevent this without it.
Anyway these design guys are on a higher pay scale than me so who am I to wonder ???
Would just stop me using one without modification although having it already mounted might make some mods worthwhile.
Cheers Bob
Good note on the cumulative damage that could cause Bob.
This kind of schematic analysis is definitely beyond what I would feel confident weighing in on. This relay should be fine for @Niclas185318’s usecase despite the confusion caused by that diagram.
@Niclas185318 This product is not designed for what you want to do. It is designed to provide a high voltage high current connection to switch some piece of equipment on or off.
From what you have said you want to detect the pump running and provide a digital input to a computer. Something like the links below would do the job with the addition of a resistor in series on the input and one on the output. The values would depend on how you intend to detect the pump running and what the digital computer needs.
