[Diy_efi] DIY-WBO2 circuit improvement suggestion

[bcroe at juno.com]

On Mon, 17 Jun 2002 11:23:55 -0700 Perry Harrington 
<pedward at apsoft.com> writes:
> The heater is a coil of resistive wire, correct?
> Well, let's assume it's a 15W wirewound resistor for argument.

OK, and lets say its resistance is 8 ohms.

> If you're pumping 10v at 1.5a into it, it's 15w.  If you pump
> 5v at 3a into it, it's still 15w.  

Better study up on Ohms law.  Volts = Current x Resistance.
If you control 2 of these, the other will be determined by the 
circuit.  

If you apply 10v to an 8 ohm resistor, the current will be 
I = V/R = 10/8 = 1.25 amp.  The power will be 
V x I = 10 x 1.25 = 12.5 watts.  Now if you lower 
V to 5 volts, current will be 5/8 = .625 amp.  Power will be
V x I = 5 x .625 = 3.125 watts.  Only 1/4 the power above.

Taking a step back, the current through the resistor will be 
proportional to the voltage.  That means when voltage 
decreases, current decreases.  Power into the resistor will 
be proportional to the square of the voltage.  So when you 
lose some voltage, you loose even more power.

> A wirewound resistor will follow power dissipation, not 
> current.  So why wouldn't the heater do the same?

See above

> Gar is always talking on about how commercial systems 
> work down to 6v. I would doubt, due to noise, that they 
> use a step-up switcher for powering it, so they HAVE to 
> pump more current at a lower voltage into it.

There are some tricks that can be played.  One is to maintain 
a certain sensor temperature.  If the engine is supplying heat, 
the control could reduce heater power.  This implies you know 
the sensor temperature.  The DIY-WB doesn't do anything this 
fancy.

> I think that a thermistor could be used for controlling the
> wattage into the heater, in a feedback loop, the same way that
> GM uses it for slow current rampup in their ECUs.  When more
> power is conducted, reduce the current, simple appropriate use
> of such a device.  This would limit power dissipation and not
> current only, like the design does now.  I'd have to sit down
> and do some research into exact implementation, but I understand
> the concepts, so I figure someone who is more familiar with the
> circuit would be better to implement the change.

I guess your goal is operation at lower battery voltage?  When 
you get it working, please share it with the list.  Bruce Roe

> --Perry
> 
> On Fri, Jun 14, 2002 at 06:16:51PM -0500, bcroe at juno.com wrote:

> > How do you plan to get the heater to work at lower voltage 
> > and higher current?
> > 
> > Bruce Roe

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