Output Current Limits

[JamesD]

Hello,

What are the constant current limits for the S8 outputs? I'm installing an S8 onto FS racecar with a GSXR600 motor and would like to speed control (PWM) my small fuel pump/radiator fan and as such was wondering if these could be directly driven (sinked) off the ECU's high current outputs. We've measured current on both and steady state each consumes less than 10-12A; obviously the in-rush current at start is higher (not sure of that number). In a direct connection we would soft start the motors to avoid high in-rush currents.

This is more of a speculative question as we will most likely run through HELLA automotive SSRs to ensure that the ECU circuitry is isolated from the main chassis wiring and devices, but if the ECU's allowable current is high enough running directly through it is attractive due to simplicity and space constraints.

Thanks!
Jim

[pat]

Jim,

The output transistors in the S8 are rated at 10A for the Injectors and 20A for the ignition. If you wanted to drive a fuel pump or a rad fan then it would be best to use an ignition output, and to add a Schottky diode for flyback suppression - the ignition outputs will protect themselves against excessive flyback but they will effectively be working as heaters whilst doing so. By limiting the flyback voltage to less than half a volt above the supply voltage, the disspation in the S8 will be drastically reduced.

Hope this helps,

Pat.

[JamesD]

Thanks for the quick response Pat.

Adding a diode is simple enough to limit flyback voltage and save a little load on the output channels. Now knowing that the ignition outputs do have protection against excessive flyback, do they also have protection against over current as say might happen if a coil internally shorts?

[pat]

Jim,

The ignition output IGBTs have got flyback clamping but they don't have over-current shutdown; if you are concerned that they may be subjected to a short circuit then it would make sense to put some sort of protection in series with them. Either a conventional or a silicon fuse may prove useful if you suspect that you may end up with a shorted coil.

Hope this helps,

Pat.

[JamesD]

Pat, thanks for the details!

Best!
Jim

[T04GTR]

how does one size the diode for the application? does it have to be rated to the same current as the load? im talking for fly-back protection for solid state relays, and my pdu.
cheers
ive killed 2 ssr's trying to pwm them to do a fan.... i have read they need a "diode" if connected to an inductive load. but what size?

and to protect he ecu im assuming they need to be in "series", nit parallel like for a ssr driving a motor? can someone clarify this?

[stevieturbo]

how does one size the diode for the application? does it have to be rated to the same current as the load? im talking for fly-back protection for solid state relays, and my pdu.
cheers
ive killed 2 ssr's trying to pwm them to do a fan.... i have read they need a "diode" if connected to an inductive load. but what size?

and to protect he ecu im assuming they need to be in "series", nit parallel like for a ssr driving a motor? can someone clarify this?

Out of interest....how did you kill them ?

I killed one myself, but pretty sure it was a wiring short the way I soldered them onto the diode for convenience then heat shrinked them.....and the heat was a little too much for the wiring sheath and as I crossed the wires over each other, pretty sure they shorted at some point. lol

Mine went up in lots of smoke, raging as it was a proper Crydom one, not even a cheapy ! lol

Given the actual load through the flyback diode would be veyr small...I dont think it would need a huge diode ? I think mine was around a 5A piece.

Mine worked fine for initial testing, went for a drive and again all seemed well. Got home, turned the car off but the pumps stayed running, although definitely not at full power
It was during probing to find out WTF was going on the SSR went up in a cloud of smoke.

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DSC_0443 (Large).JPG (410.34 KiB) Viewed 30235 times

[T04GTR]

i killed one, a 40 dollar china one, from jaycar. testing the fan pwm. it took about 1 min to fail. i tested it at 1000hz, not much happened under 70% duty. put it down to 500hz and within about 10 secs i heard the fan motor belt squeal as it went closed circuit and gave it full power.

i bought another one (different brand), and it was turning on straight away, apon testing i noticed that the pwm output (fan3) was showing 100ohms to ground with the output switched off, and removed from the cal....

so i moved it to slave1 output 20. (slave 1 ignition4 according to the pinout sheet i got) re tested and same thing. with a 10 amp diode going from the load+ to ground.
(i had bat to the relay, from the relay thru the motor to ground.
signal was a 500ohm resister from the bat + to sig +. then sig- out to the ecu.

i used the 500 ohm resister because i was scared some bad shit was being sent thu the signal wire into the ecu. somehow.. not sure how i killed an output. but version 1 was with no resister. so im guessing it was getting flyback thu the mosfet when it was "on"
and i had a 1k resister across the signal posts.

i had a 3rd relay at work, so i wired it up a little differently, bat + to motor, from motor to relay then to ground. sig was wired the same as the earlier ones. this time i had it on the hoist, and just touched the pwm sig to the relay to test different frequency. 500hz seems to work well, but it gets to hot to touch after about 5 secs.. ive given up on the ssr idea. i need to bite the bullet and get a proper fan module. but i cant find any info on the pwm frequency sent to the oem modules.

im sure the guys have that info. as its an 600w audi module i think il get.

[TimH]

Did you heat-sink the SSR? at 60A, a 100A SSR will need a 1C/W heatseak to keep it cool and for it not to die.

I would have no hesitation in using a decent 100A SSR (e.g. Crydom) properly heatsunk, with catch diodes wired as per manufacturers recommendations. And if you can use the Syvecs software to limit the initial PWM so it soft starts (surely possible?) then even better.

[stevieturbo]

Anything I've read about the cheap ones, is dont go over 100Hz.

That's why I bought the Crydom, it stated it can run to 1000Hz although I think I tried it at 800.

The Chinese SSR's I bought were more like £10, so no idea how they'll work.

But before I drove mine, I was ran the pumps all way down to around 45% up to 100% and it seemed to do it no problem, SSR never even got hot.

I'm sure the failure was my own fault with the diode/heatshrink/wiring

I did not use any resistors etc, it all worked fine without them, and turned off ok during testing. SSR's seem to want the current to flow in one direction only too....ie the supply/load are fixed terminals, whereas with an old relay you can use either terminal.