30+ Amp Transistor? Need to build a high current PWM controller

[BrandonKelm]

So, I saw one of John's posts about using a TIP120 to PWM control a car headlight.  http://www.efx-tek.com/php/smf/index.php?topic=1874.msg11087#msg11087 That wasn't the original post, but it has the important bits in it.  Unfortunately, the TIP120 is only good up to 5A.

I'm converting my mechanical, water pump driven cooling fan on my Jeep Stroker project to an electric fan.  It frees up a little power, cools better and provides better throttle response.  Others have claimed a 5% increase in economy.  I'm not doing it for economy, this motor was built for monster torque, not sipping fuel.  Anyhow..

The motor is already equipped with a coolant temperature sender.  It's range is ~380k ohms on the cold side (-40 F) to 370 ohms on the hot side (+248 F).  It uses a 5v signal going in, then the PCM interprets the voltage on the output to adjust air/fuel mixture, etc. 

I suspect I could tap into that to have the Prop1 read the output voltage and I could code accordingly?  That is step one.  Step two is controlling the fan.  One of the most popular electric fans are, oddly enough, sourced from mid 90's Taurus's.  They move a TON of air, between 3000-4000CFM seems to be the consensus.  Unfortunately, they also pull quite a bit of current.  ~120A inrush on startup, ~32A FLA on the high speed wire.  The Taurus fan has two windings in it, one for high speed, the other for low.  I don't know the low speed, doesn't really matter, it would never be used, hence why I want a PWM controller.    I suppose I could use the low speed wire for startup, IE P0 controls a standard relay.  When the controller turns the fan on, it first hits P0 to spin the fan up, then P1 (which would be the PWM channel) takes over and P0 shuts off.

Back to the topic at hand..  Any recommendations for a high current transistor to handle PWM control of a 12vdc fan?

[JonnyMac]

Check the Crydom web site for DC "hockey puck" type SSRs -- you'll find lots of choices in the "hockey puck" style.

[BrandonKelm]

Wow, I can't believe I didn't think of that.  Actually, I didn't think they would even work for a PWM setup, but now that I think about it, I can't see why they wouldn't.  I use a bunch of Crydom's Standard Series output modules and Opto22 G1 stuff at the haunt, hung off of my controllers for 120vac light control.  This was prior to starting to use DMX.

Thanks for the insight.  I'm have a bunch of puck SSR's at the haunt, but I doubt I have one for 12vdc.  I'll have to take a look when I get back into town.

As far as using the Prop1 goes, can I set it up to read a voltage from my temp sender (5v input), or will I have to install a second sender and use it as a variable resistor?  I'm hoping to not have to use a second sensor.  The wiring harness that has the temp sender wire in it is right where I want to install the controller, so it would just be a matter of some heat shrink and a soldering iron to tap that wire.

[JonnyMac]

It's not going to be easy to read 5v from the sender without an external ADC.  Here's an example from Scott Edwards, the guy who started the column I took over.

* http://www.parallax.com/Portals/0/Downloads/docs/cols/nv/vol1/col/nv4.pdf

The LTC1298 may not be easy to find; I'd suggest the MCP3202 -- though the code will require some mods.

[JonnyMac]

Project #2 in this document uses the ADC0831 which is pretty easy to come by.

* http://www.parallax.com/Portals/0/Downloads/appnt/stamps/bs1Appnotes.pdf

[BrandonKelm]

Thanks again John.

I may just T-off on the thermostat housing and install another temp sender.  The sender should then be able to be directly connected to the Prop1, it's just a variable resistor.  This will save me some time assembling a board and using some (at my currently level) complex code.

Regarding the SSR, I've done a little digging into it and it seems that some SSR's may not switch fast enough to be "PWM compatible", if you will.  I looked around a little and came across this; http://search.digikey.com/us/en/products/2N5302G/2N5302GOS-ND/918233 (Datasheet: http://www.onsemi.com/pub_link/Collateral/2N5302-D.PDF )  Seems like it would fit the bill.  I'm also going with a different fan, a little less flow but it only pulls 15A at full tilt.  Thoughts?

[JonnyMac]

It's easy to construct a transistor driver and it's always best to spec a component at 200% of your target need.

Now... I mist the PWM part of the 1st post.  You won't be able to use the PWM command in the Prop-1 language.  In the AppNotes document

http://www.parallax.com/Portals/0/Downloads/appnt/stamps/bs1Appnotes.pdf

...have a look at project 21 called "Fun With Trains."  It shows how to do low-frequency PWM in code for running trains; this would be appropriate for running your fan.