DIY-EGOR ideas (was: No Subject)

Bernd Felsche bernie at innovative.iinet.net.au
Wed Nov 22 15:37:31 GMT 2000


Jörgen Karlsson tapped away at the keyboard with:

> I have been thinking...

That's a good start. :-)

> I think that the obvious choice is to use a Atmel microcontroller,
> these are as inexpensive as the cheaper PICs and have the
> performance of the most expensive ones. They are made to be

My choice as well. You may have seen my proposed circuit at
http://members.iinet.net.au/~metapro/bernie/tech/EFI/vapour/DDL.html

> programmed with a parallell cable, the only electronic parts is
> three resistors and they are optional. Finnished circuit boards
> are available from dontronics.com, check the simmm100. When you
> are at Dons site check out the demo version of Bascom AVR, pretty
> large code can be built with the demo version, up to 2KB code can
> be used with the demo. That should be enough...

> Since this is a diy project I think that one of the most importand
> aspects is to keep the initial cost down, in production that is
> not a problem. By using an atmel processor we _can_ get started
> with an AVR, an old printer cable and a broken power supply for a
> computer... Lets say $10. Or we can do it simple, we get a

Actually; I'd recommend a starter kit of some sort to begin.  The
STK200 allows you to test your ideas with simulated analogue stuff
connected via ribbon cable on a "bread-board". (Switches and LEDS
are on the STK200 for a number of IO pins)

> complete simm100 simstick from Dontronics, if you are in
> scandinavia get it from the manufacturer directly, that is
> www.lawicel.se. I think that the fully assembled board are around
> $35, the circuit board it self are around $10. I think that it is
> beneficial to the project if no programmer has to be bought or
> built, being able to get an assembled computer for $35 is good
> too.

Good stuff for experimenting but watch the temperature rating of the
components if you want to go "live" with it.

> We cannot pwm anything across the pump cell, it has to be a
> constant flow there, I think that the way to go is to filter the
> pwm output to get a stable current. If the sensor ground is put at

I haven't been able to find a reference about not being able to PWM
the pump current. Are you able to tell us where you've found that
information? What sort of load (RLC) does the cell present?

>From my understanding of how it works, (which could be very wrong)
only the average current is significant as long as the peak current
remains within the safe limits for the cell.

If you wanted to, you could take the PWM signal and (RC) filter it
before using it to drive the bases of plain, old NPN/PNP transistors
within their linear range, in the bridge circuit instead of the
FETs. A bipolar cap on the output could be used to filter any
significant AC component. The power disspation is going to be far
higher than raw PWM, but you're only driving a few milliamps.

> 2.5v above the circuits ground the processor can drive the pump
> cell and filter circuit directly.

Depends if the cell is floating or tied to ground somehow. If it's
floating, then a bridge circuit can be used to reverse current flow.
It's quite common in DC motor drives.

What sort of filtering is necessary?

> We know that Bruce and the guys used up towards 1.1A heater
> current, lets assume that is max. Let someone else do the damage
> ;) First of all we filter the pwm output from the mcu and then we
> use the filtered voltage to drive the power transistor.

> We also need to measure the current across the sensor, we only put
> a series resistor and measure the voltage drop. By using a

That's why there's a current-sensing resistor in the ground-lead of
the H-bridge in the circuit described earlier.

> 'current mirror' we can put the voltage potential of the resistor
> almost where we want it, if we connect one of the pins to ground
> we can use a simple op amp circuit to get the voltage drop over
> the resistor. We can probably omit the amplifier completely,  but
> I am not sure. I think that we need to use the drive transistor as
> one of the transistors in the current mirror, we might have some
> problems with the headroom available if we don't.

> Or we can use a more complex op amp circuit on the unmirrored
> heater drive to get a ground referenced voltage across the
> resistor. This kind of circuits are available in the LM124 data
> sheet from national.

> We also need the voltage across the heater element it self, if we
> have the current and the voltage across the heater we know the
> resistance, that we can translate to temperature. I think that
> aiming for a set resistance here is the way to control the heater
> correctly, If the resistance goes up we must lower the heater

Why not measure the voltage drop "directly" using an ADC channel in
the AVR? As in my discussion circuit where there's a current sensing
resistor, and a voltage-divider to scale the nominal voltage to
below AREF. That reduces the number of external components.

> current and the other way around. I think that we can be pretty
> lay back while doing this, I think that we will use one voltage
> across the sensor for initial fast heating, then at some
> resistance we drop the voltage to prevent cracking, the correct
> values can probably be found in the base shematic. After that we
> let the raising resistance of the sensor take care of the
> adjustment, we just put two more regulation points, one that
> lowers the voltage a bit when the sensor is close to maximum temp
> and one that almost completely shuts of the current when it gets
> even hotter.

Sounds very complicated. If you're controlling the current through
the heater, then you're controlling its power output. 

See the circuit at the URL above. It covers most of the aspects
you've addressed; differently, of course - and there's nothing wrong
with that as we're working pretty much in the dark largely due to
lack of manufacturer's specifications.

-- 
 /"\  Bernd Felsche - Innovative Reckoning, Perth, Western Australia
 \ /      ASCII ribbon campaign | I'm a .signature virus!       |
  X       against HTML mail     | Copy me into your ~/.signature|
 / \      and postings          | to help me spread!            |
----------------------------------------------------------------------------
To unsubscribe from diy_efi, send "unsubscribe diy_efi" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org




More information about the Diy_efi mailing list