Electronics Question: Substituting a Small Relay with a MOSFET

DigiKey has Switching Devices.
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Mouser has them also.
( sorted by manufacturer)

Good Luck.
Edwardo
Major Electronic Patent Holder

 

You may want to consider an opto-isolated switch. There is no electrical conection between the switch and the circuit that is being switched. If I understand your question. They come as small devices that look like ICs.

 

Rick,

Also,, Mouser has design experts,, online.
DigiKey, and the above, have Millions of parts.
Get busy.


Edwardo

 

It's a little unclear to me what you mean by "isolated" in this case. I.e., the blinker is distant/removed from the low fuel sensor switch or that the blinker circuit is electrically isolated (no current flow between the two) from the sensor circuit.

An opto-isolator is the most likely part to use if you want to maintain electrical isolation. However, the problem with an opto-isolator is that it requires power on the control side (just like a relay) and you also have power on the blinker side, so the two really aren't electrically isolated from each other after all unless you are willing to provide separate power supplies/batteries to each side and that doesn't seem likely on a motorcycle. If they're not truly electrically isolated, then the reason to use an opto-isolator is less. Also the driven (blinker) side of the opto-isolator looks just like a transistor, so if your transistor circuit didn't work, then you really haven't solved your root problem.

A mosfet can be used as a switch, it's just another type of transistor. A mosfet controls current based on an input voltage, while a bipolar transistor controls current based on an input current. However, when used as a switch, that difference between the two is not very important. Essentially whatever problem you have with the blinker side of your circuit is not going to get solved by changing to a mosfet, except accidentally (you do something different without realizing it).

So the question is why isn't your transistor circuit working, if it worked when breadboarded. One possibility is that the blinker battery supply wire has too high a voltage drop, your primary supposition. Is the high-current side (likely the blinker) closer to the power connection than the sensor side (likely the low current side)? Have you measured the current load of your blinker circuit then calculated the resistance of the length of wire and wire gauge from the battery to the blinker? Another possibility is that your sensor side control current is less than you think so the transistor is operating in its linear region (where it doesn't act like a switch), rather than its saturated region (this is where it does act like a switch). Have you measured the resistance and current of your low-fuel sensor side when triggered and not triggered? Etc.

If you want to draw up a circuit diagram and describe the characteristics of the key components (low fuel sensor, LED current load, etc) then I might see a few more possibilities.