Hi All,

I was wondering, for switching a fuel pump such as:
Kavan fuel pump

Would a relay or a transistor be best? what is the advantage of relays? is it something to do with being able to handle AC?

Cheers
Shaun

 

How do you plan to switch the relay? What will your on-cycle be?
How is AC involved? It appears to be a DC pump.

John

 

Relays are not ESD sensitive, and can take a bit more electrical abuse. They do wear out over time though.

Transistors have a much higher MTBF (mean time between failure) rate, which means they don't have contacts that will wear out.

Over the long haul, transistors are better, but you have to be very sure you have hooked them up correctly and have protections for special problems in place. For example, a transistor driving a motor or a coil needs a surge supression diode, relays could care less.

 

How do you plan to switch the relay? What will your on-cycle be?
How is AC involved? It appears to be a DC pump.

John

I would use a PIC18F4431 MCU... I am thinking something like a 10k resistor to ground and a 10k to the base of something like a BC183 which would switch the coil of the relay. It will likely only be on for a few seconds every now and then.

It is a DC pump.. I was just wondering if relays were used more in AC circuits...

Cheers
Shaun

 

Relays are not ESD sensitive, and can take a bit more electrical abuse. They do wear out over time though.

Transistors have a much higher MTBF (mean time between failure) rate, which means they don't have contacts that will wear out.

Over the long haul, transistors are better, but you have to be very sure you have hooked them up correctly and have protections for special problems in place. For example, a transistor driving a motor or a coil needs a surge supression diode, relays could care less.

Ok then, so relays are less sensitive, but wear out quicker. Going by my previous post, would you recommend a transistor then? I have a FRS1 relay that was hanging about that I thought I'd use.

Cheers
Shaun

 

From what you describe, I would stick with solid state devices ("transistors"). You will likely need an interface between the PIC and the power transistor (mosfet?) to drive its gate. Although, that is not certain. I have run logic level mosfets with low gate capacitance directly from a PIC. Keep an eye on the total current and current per pin specs of the PIC. John

 

From what you describe, I would stick with solid state devices ("transistors"). You will likely need an interface between the PIC and the power transistor (mosfet?) to drive its gate. Although, that is not certain. I have run logic level mosfets with low gate capacitance directly from a PIC. Keep an eye on the total current and current per pin specs of the PIC. John

ok cheers

I have heard of someone switching a mosfet directly as they switch on voltage rather than current... but I may still put that BC183 is anyway... it can take 100mA I think so 4 times that of PIC.

Thanks for your replies.
Cheers
Shaun

 

It is true they "run" on voltage, but you still have the gate capacitance to charge, which you want to happen rapidly. In your situation, switching is intermittant and infrequent and the load current is not much. An IRLZ24 might work. I am using that mosfet directly off a CMOS logic chip. John

 

It is true they "run" on voltage, but you still have the gate capacitance to charge, which you want to happen rapidly. In your situation, switching is intermittant and infrequent and the load current is not much. An IRLZ24 might work. I am using that mosfet directly off a CMOS logic chip. John

Thanks for the suggestion... I'll give it a try

Cheers
Shaun