Author Topic: Burrell Single Crank Compound Traction Engines - 4" Scale  (Read 57201 times)

Offline Alex

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #255 on: December 13, 2020, 02:11:24 PM »
Glad to see the update; you do wonderful work. Keep going! JohnS.

Offline Jo

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #256 on: December 13, 2020, 02:16:09 PM »
Thanks for the update Andrew, I am going to miss seeing your progress at Stratfield Saye  :(

Jo
Enjoyment is more important than achievement.

Offline Admiral_dk

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #257 on: December 13, 2020, 05:42:40 PM »
Really great result with the latest batch of parts  :ThumbsUp:

Quote
By large I mean half a farad or so.

 :o  - oh man - that is almost a battery and not a capacitor ...!... does it make sense at all to use an OpAmp ?

Offline Don1966

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #258 on: December 13, 2020, 07:04:09 PM »
Awesome rsesults Andrew........ :Love:




 :cheers:
Don

Offline Kim

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #259 on: December 14, 2020, 05:43:50 AM »
Great to see an update, Andrew!  Nice work all around.

Boy, seeing those wheels up on your portable bench shure gives me a size reference!  They're huge!

Kim

Offline jadge

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #260 on: December 17, 2020, 08:18:47 PM »
Boy, seeing those wheels up on your portable bench shure gives me a size reference!  They're huge!

I was told they'd be surprisingly heavy, and it's true they are much heavier than the wheels with metal strakes. Just as well I have concrete floors in the bungalow.

Andrew

Offline jadge

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #261 on: December 17, 2020, 08:43:54 PM »
... does it make sense at all to use an OpAmp ?

The sensor has three pins, a reference electrode (RE), working electrode (WE) and a counter electrode (CE). Internally the RE is between the CE and WE. When the target gas meets the sensing element an oxidising or reducing reaction occurs that generates a current into, or out of, the WE. To keep the sensor in balance the voltage on the RE needs to be kept the same as that on the WE. To do this once the reaction occurs the voltage on the CE needs to change in order to force another reaction that generates a current (through the large capacitor and parallel resistor) to equal that generated by the WE. At least I think that's how it works.  :) The amplifier that does this is called a potentiostat. The voltage difference between RE and WE needs to be small, ideally microvolts. So the potentiostat needs very high DC gain, but doesn't need a high bnadwisth as the sensor takes many seconds to react to target gases. So I've got an opamp with capacitive feedback, making the DC gain essentially the same as the open loop gain of the opamp.

Andrew

Offline Admiral_dk

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #262 on: December 17, 2020, 10:32:58 PM »
Quote
To keep the sensor in balance the voltage on the RE needs to be kept the same as that on the WE.

Ok that sentence alone kind of explains why one really has to consider an OpAmp - no matter if it's an off the shelf item or one made from discrete parts. This doesn't help your first mentioned concern, and I wouldn't have been surprised if my first solution (if I had that assignment) at least needed tweaking if not a complete redesign .... Modern simulators are great, but they do not always solve all the problems - so you usually need to build a prototype (but I'm sure I don't need to tell you that)  ;)

Offline jadge

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #263 on: December 20, 2020, 11:03:30 AM »
Simulators are a mixed blessing. I've used ones ranging from free to the Mentor Graphics offerings. They're useful for trying out ideas and playing with component values. I don't quite agree with the Bob Pease attitude, but I always prototype analogue circuits. I normally build the circuits "dead bug" style on single sided blank PCB material. I used to prototype buck converters as they never did quite what they said on the tin. But the newer ICs are much better. I still buy dev kits where needed - just ordered dev kits for the selected ADC and DAC on the gas sensor project. They should allow me to check a few things on the analogue side and then let the software writer get a head start.

Andrew