Solid state AUX relay

[Bitsyncmaster]

Well I decided to solder on the stud terminals. I wanted to measure if I need more copper. For this test board I just laid out 1 oz. copper on one side.

Testing with 50.0 amps.

MOSFET = 0.000202 ohms
Top of terminal to terminal = 0.000564 ohms
To first points where copper meets terminal = 0.000486 ohms.

So the copper will be 2 oz. on the production boards. Still deciding if I will lay more copper on other side. It would need some via's through the MOSFET pads.

[Helirich]

Dave I have a question that only you can answer. I have e-fans on a custom truck that fail quite often. They are Tyco brand. They look to be well made in Portugal. A friend of mine says the reason they are failing is the way in which I have them wired. The truck has a main power relay. When I turn that 500 amp relay on, it powers the main bus. From there, the power goes on to the fan relays. I have thermostat switches that trigger the fans on. I have it wired this way so when the main power is off, I know there is no drain on the battery.

So if I?m driving and the fans come on and then I stop. If I shut the motor down, the fans are still running because the water is still hot. If I turn the main power off, everything stops of course. My friend says you can shut the small relays off by the main power. He says I need to shut the relay ?triggers? off first, then I can shut the main power off. Would you agree?

[Bitsyncmaster]

Some cars the fans do continue to run after engine shut down. I would guess it is to cool the radiator. You would not want to keep the fans running if your sensors are not mounted at the radiator. There is no coolant flow after shut down.

Your relays should have a control pin and a ground. You should control the relay via one of those pins. Those are low current circuits so a switch or sensor can control it.

Dave I have a question that only you can answer. I have e-fans on a custom truck that fail quite often. They are Tyco brand. They look to be well made in Portugal. A friend of mine says the reason they are failing is the way in which I have them wired. The truck has a main power relay. When I turn that 500 amp relay on, it powers the main bus. From there, the power goes on to the fan relays. I have thermostat switches that trigger the fans on. I have it wired this way so when the main power is off, I know there is no drain on the battery.

So if I?m driving and the fans come on and then I stop. If I shut the motor down, the fans are still running because the water is still hot. If I turn the main power off, everything stops of course. My friend says you can shut the small relays off by the main power. He says I need to shut the relay ?triggers? off first, then I can shut the main power off. Would you agree?

[Helirich]

Some cars the fans do continue to run after engine shut down. I would guess it is to cool the radiator. You would not want to keep the fans running if your sensors are not mounted at the radiator. There is no coolant flow after shut down.

Your relays should have a control pin and a ground. You should control the relay via one of those pins. Those are low current circuits so a switch or sensor can control it.

What my friend says is you can?t cut the power to the trigger at the same time as the circuit. He called it ?flyback?. When I searched, it said most relays have protection from flyback.

[Bitsyncmaster]

What my friend says is you can?t cut the power to the trigger at the same time as the circuit. He called it ?flyback?. When I searched, it said most relays have protection from flyback.

When you switch off and inductive load, you do get a voltage spike. The relay is inductive (coil of wire) but it's spike is marginal. Yes some relays do put a diode inside them but they are very rare. The coil voltage spike has nothing to do with load voltage spike which is very large for electric fans.

[Helirich]

When you switch off and inductive load, you do get a voltage spike. The relay is inductive (coil of wire) but it's spike is marginal. Yes some relays do put a diode inside them but they are very rare. The coil voltage spike has nothing to do with load voltage spike which is very large for electric fans.

Thankyou

[jackb]

[Bitsyncmaster]

With the new MOSFET the board is very narrow. So I laid out a new 3D case and printed 4 of them at one time. The four takes 43.58 mL of resin and took 3 hours and 10 minuets to print. Resin printing has another requirement of washing in 99% IPA so that also adds to my cost.

[Bitsyncmaster]

I got the new circuit boards in and assembled 10 of them. The average on resistance is 0.000418 ohms. A little higher than I expected using 2 oz. copper on both sides of the board. I think my calculations did not account for the solder joint resistance. My old boards that used 4 MOSFETs was 0.00050 ohms so new units are a little better with less cost. The control current with 13.5 volts is 1.56 ma. which is much better then the old board which is 3.50 ma.

I did pot 4 of the new boards into my 3D printed cases. While doing that potting I was thinking that case could be designed with some clips to hold the boards in. However the resin I use is not very flexible so it would need another different resin.

Our AUX relays never fail so don't expect many owners will need these. My relays do produce much less heat (AUX relays are always on) and are silent when switching. I may run across different users that are in need of high current DC relays.

Anyway, I can sell the populated PCBs if someone wants to design there own case. You do need to replace your "push on" terminals on the power and load harness, to ring terminals. The control and ground terminals are the same as stock.

I have not calculated the price yet. The populated board should be less than $100 each.