[Diy_efi] DIY-WBO2 circuit improvement suggestion

[Perry Harrington]

The heater is a coil of resistive wire, correct?

Well, let's assume it's a 15W wirewound resistor for argument.

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

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.

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.

--Perry

On Fri, Jun 14, 2002 at 06:16:51PM -0500, bcroe at juno.com wrote:
> Perry, 
> 
> The problem is getting nearly 1.5 amps to the heater at 
> 10 volts, with current limiting.  When you have that, it is  
> easy to steal .025 amp at 8 volts for the amplifiers.  A 
> 0.12 amp charge pump won't do it.
> 
> How do you plan to get the heater to work at lower voltage 
> and higher current?
> 
> Bruce Roe
> 
> On Thu, 13 Jun 2002 17:30:45 -0700 Perry Harrington 
> <pedward at apsoft.com> writes:
> 
> > With a little work the heater circuit could be redesigned 
> > to work at lower voltages.
> > Since what the real issue is total power dissipation in 
> > the heater, a higher current
> > at lower voltage would probably acheive the same results.
> > 
> > It seems the analog portion is the problem WRT dropouts 
> > and low voltage.
> > 
> > For this I propose a simple solution: A capacitor charge 
> > pump.
> > 
> > Maxim makes a couple of suitable devices which they sell 
> > for generating 12v programming voltages for flash.  The 
> > parts are MAX662A, MAX734, and MAX732.  They generate
> > 30ma, 120ma, and 200ma respectively.
> > 
> > If you redesign the +-4v regulator circuit to use the 662, it 
> > would permit operation off a supply down to 4.75v.  I'd 
> > recommend putting a low dropout +5v regulator in front of 
> > the pump and just regulate the input, then you could 
> > regulate the 12v
> > output to 8v with a low dropout (LM2936) regulator.
> > 
> > The 662 part should be suitable, as the power supply 
> > uses the 324 in voltage follower mode for the gnd tap.  
> > Since the output of this device can only sink 25ma or so, 
> > the 30ma supply should work.
> > 
> > --Perry
> 
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-- 
Perry Harrington			Data Acquisition & Instrumentation, Inc	
perry at dainst dot com					 http://www.dainst.com/		 

Those who would give up essential liberty to purchase a little temporary safety
deserve neither liberty or safety. Nor, are they likely to end up with either.
                             -- Benjamin Franklin

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