Resistance calculator

[Terry Harris]

It has been around for years, I never pushed or publicised it much. I think
most EEs would find it useful so I give it a mention here.

It is a Win32 program to show pairs of standard value resistors which give
a desired resistance for series or parallel combination or ratio for a
voltage divider.

Download here http://web.newsguy.com/pentangle/ResCAD20.zip

If you like ResCAD I also wrote a units conversion program which ended up
being rather more versatile. It comes with a bunch of conversions and
calculations (you want weird? photon wavelength to gallons of gasoline with
the same energy content weird enough?). Its real strength is the ability to
be configured to provide just the conversions and calculations you need
with a simple and small interface.

Download here http://web.newsguy.com/pentangle/uSheet_Beta_1.02_setup.exe


--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist

[Adam Field]

> It has been around for years, I never pushed or publicised it much. I think
> most EEs would find it useful so I give it a mention here.
>
> It is a Win32 program to show pairs of standard value resistors which give
> a desired resistance for series or parallel combination or ratio for a
> voltage divider.
>
> Download here http://web.newsguy.com/pentangle/ResCAD20.zip

It's a nice little utility. It runs on Wine 1.0.1 fine under Ubuntu if
anyone's wondering. The uSheet program functions with some visual
problems (labels don't show up well), which is too bad. Thanks.
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist

[Brendan Gillatt]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Terry Harris wrote:
> It has been around for years, I never pushed or publicised it much. I think
> most EEs would find it useful so I give it a mention here.
>
> It is a Win32 program to show pairs of standard value resistors which give
> a desired resistance for series or parallel combination or ratio for a
> voltage divider.
>
> Download here http://web.newsguy.com/pentangle/ResCAD20.zip

I have been searching for a little utility like this for far too long.
thank you very much!

- --
Brendan Gillatt | GPG Key: 0xBF6A0D94
brendan {a} brendangillatt (dot) co (dot) uk
http://www.brendangillatt.co.uk
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.7 (MingW32)

iD8DBQFKPkX2rlc7Stqch40RAnv9AJ4tHB4bT+QnzWQ/O+CkZZuIGEsV8QCgo8P2
uTN0yz1XXY3Wo94/sPAOTE0=
=dXky
-----END PGP SIGNATURE-----
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist

[Russell McMahon]

> Terry Harris wrote: ...
>> It is a Win32 program to show pairs of standard value resistors which
>> give
>> a desired resistance for series or parallel combination or ratio for a
>> voltage divider.
>>
>> Download here http://web.newsguy.com/pentangle/ResCAD20.zip

==> VERY USEFUL PARALLEL RESISTOR EQUATION
What resistor do I need to make X from Y (X < Y)

________

Such utilities are useful.
It's also useful to learn another form of a very well known formula.

*** Resistance of two parallel resistors:

It is generally well known that for RC = RA + RB in parallel then

         RC = (RA x RB) / ( RA + RB)

A simple rearrangemnent of this formula is less generally committed to
memory.

*** Resistor required to reduce a resistor to a required (lower) value.

Target R = (Target x existing) / (Target - existing)

        RB = (RA x RC) / (RC - RA)

This simple formula is immensely useful when prototyping

eg 87k is required.
Assume only E12 values at 1% are available.

RA = 100K
RC=  87K

RB = (100 x 87) / (100 - 87) = 8700/13 = 669k
680k will probably suffice.
[ 680 x 100/780 = 87.179 ~ 2% high][within error margin of resistor
tolerance]

Amazingly useful equation despite simplicity. Can often be worked out
mentally (slower in older brains :-) ).

           Russell McMahon

--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist

[Terry Harris]

On Mon, 22 Jun 2009 22:23:58 +1200, Russell McMahon wrote:

>Target R = (Target x existing) / (Target - existing)
>
>        RB = (RA x RC) / (RC - RA)
>
>This simple formula is immensely useful when prototyping
>
>eg 87k is required.
>Assume only E12 values at 1% are available.
>
>RA = 100K
>RC=  87K
>
>RB = (100 x 87) / (100 - 87) = 8700/13 = 669k
>680k will probably suffice.

But how or why did you choose 100k for RA?

It happens to be right for 87k and E12s but isn't for say 83k and E12s or
87k and E24s.

--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist

[Gerhard Fiedler]

Russell McMahon wrote:

>> Terry Harris wrote: ...
>>> It is a Win32 program to show pairs of standard value resistors which
>>> give
>>> a desired resistance for series or parallel combination or ratio for a
>>> voltage divider.
>>>
>>> Download here http://web.newsguy.com/pentangle/ResCAD20.zip
>
> ==> VERY USEFUL PARALLEL RESISTOR EQUATION
> What resistor do I need to make X from Y (X < Y)
>
> ________
>
> Such utilities are useful.
> It's also useful to learn another form of a very well known formula.
>
> *** Resistance of two parallel resistors:
>
> It is generally well known that for RC = RA + RB in parallel then
>
>          RC = (RA x RB) / ( RA + RB)

All of this is much easier to remember in the original form (the above
is a derived form). Just as you add the resistances for series, you add
the conductance (1/resistance) for parallel.

  1/Rc = 1/Ra + 1/Rb

> A simple rearrangemnent of this formula is less generally committed to
> memory.

Due to (probably genetically) bad memory for irrelevant (IMO :) details,
I hate anything that relies on memorization of irrelevant (IMO :)
details :)

> *** Resistor required to reduce a resistor to a required (lower) value.
>
> Target R = (Target x existing) / (Target - existing)
>
>         RB = (RA x RC) / (RC - RA)

Considering the above, this becomes almost obvious:

  1/Rb = 1/Rc - 1/Ra

Gerhard
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist

[Steve Maroney]

Hello gang,

I'm new to microcontrollers and new to electronic circuit design. I've
read up on how a transistor works and I think I get the theory but they
don't function as expected when tested.

I have a PIC that outputs to an LCD which works great.  I also need to
share 4/8 of the LCD data lines as an input to another microcontroller.
I need a method of keeping data not destined for the 2nd microcontroller
away from it while addressing the LCD.  I thought I could put some
2n2222's in line with my 4 shared data lines.  When I need send data to
the other microcontroller, I would bring the base of on my 4 transistors
high, send the data, and bring it back low.

When testing my transistors with 5v applied to the base, I get the same
voltage from the collector and emitter. The collector and emitter are
not tied in with anything so I would expected nothing from the collector
or the emitter.  

I also applied 5v to the collector and an LED w/resistor to the emitter.
When I apply 5v to the base the LED turns on, if remove 5v from the
collector, the LED stays on because of the 5v applied to the base

Should it work this way ?

Sorry for the long post, thanks.

Best Regards,
Steve Maroney

Business Computer Support, LLC
Mobile Phone:[phone]
Office Phone: [phone]
Fax: [phone]



-----Original Message-----
From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf
Of Terry Harris
Sent: Monday, June 22, 2009 6:32 AM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Resistance calculator

On Mon, 22 Jun 2009 22:23:58 +1200, Russell McMahon wrote:

>Target R = (Target x existing) / (Target - existing)
>
>        RB = (RA x RC) / (RC - RA)
>
>This simple formula is immensely useful when prototyping
>
>eg 87k is required.
>Assume only E12 values at 1% are available.
>
>RA = 100K
>RC=  87K
>
>RB = (100 x 87) / (100 - 87) = 8700/13 = 669k
>680k will probably suffice.

But how or why did you choose 100k for RA?

It happens to be right for 87k and E12s but isn't for say 83k and E12s
or
87k and E24s.

--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist




--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist

On Wed, 1 Jul 2009 23:56:02 -0500
"Steve Maroney" <steve@...> wrote:

> I need a method of keeping data not destined for the 2nd microcontroller
> away from it while addressing the LCD.  I thought I could put some
> 2n2222's in line with my 4 shared data lines.  When I need send data to
> the other microcontroller, I would bring the base of on my 4 transistors
> high, send the data, and bring it back low.

I suggest you check the CD4066 quad analog switch. Seems a lot simpler
than using discrete transistors. No resistors necessary either.

John
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist

[Tamas Rudnai]

Hi Steve,

Somehow I have the feeling that you forgot the base resistor? Depending your
transistor and your circuit you have to choose an appropriate value to get
your transistor work as expected.

Tamas



On Thu, Jul 2, 2009 at 5:56 AM, Steve Maroney <steve@...>wrote:

> Hello gang,
>
> I'm new to microcontrollers and new to electronic circuit design. I've
> read up on how a transistor works and I think I get the theory but they
> don't function as expected when tested.
>
> I have a PIC that outputs to an LCD which works great.  I also need to
> share 4/8 of the LCD data lines as an input to another microcontroller.
> I need a method of keeping data not destined for the 2nd microcontroller
> away from it while addressing the LCD.  I thought I could put some
> 2n2222's in line with my 4 shared data lines.  When I need send data to
> the other microcontroller, I would bring the base of on my 4 transistors
> high, send the data, and bring it back low.
>
> When testing my transistors with 5v applied to the base, I get the same
> voltage from the collector and emitter. The collector and emitter are
> not tied in with anything so I would expected nothing from the collector
> or the emitter.
>
> I also applied 5v to the collector and an LED w/resistor to the emitter.
> When I apply 5v to the base the LED turns on, if remove 5v from the
> collector, the LED stays on because of the 5v applied to the base
>
> Should it work this way ?
>
> Sorry for the long post, thanks.
>
> Best Regards,
> Steve Maroney
>
> Business Computer Support, LLC
> Mobile Phone:[phone]
> Office Phone: [phone]
> Fax: [phone]
>
>
>
> -----Original Message-----
> From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf
> Of Terry Harris
> Sent: Monday, June 22, 2009 6:32 AM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] Resistance calculator
>
> On Mon, 22 Jun 2009 22:23:58 +1200, Russell McMahon wrote:
>
> >Target R = (Target x existing) / (Target - existing)
> >
> >        RB = (RA x RC) / (RC - RA)
> >
> >This simple formula is immensely useful when prototyping
> >
> >eg 87k is required.
> >Assume only E12 values at 1% are available.
> >
> >RA = 100K
> >RC=  87K
> >
> >RB = (100 x 87) / (100 - 87) = 8700/13 = 669k
> >680k will probably suffice.
>
> But how or why did you choose 100k for RA?
>
> It happens to be right for 87k and E12s but isn't for say 83k and E12s
> or
> 87k and E24s.
>
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
>
>
>
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>

--
http://www.mcuhobby.com
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist

[Jinx-4]

> I need a method of keeping data not destined for the 2nd
> microcontroller away from it while addressing the LCD

Steve, how does the 2nd micro know which data is meant for it ?

If you use switches like the 4066, or a bus IC, to disconnect the
2nd micro from the data, it will still have data on its pins, although
presumably that would be fixed with pullups or pulldowns (eg 0000
or 1111), as you shouldn't and wouldn't leave them floating

One other alternative would be to have the data go to both, and the
2nd micro monitors the LCD's EN

--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist

[CDB-3]

possibly a lateral option - change the way you address the LCD to use
less lines by getting th epic to send data to something like a 74HC595
serial to parallel latch. The LCD will connect to the latch and you'll
only need 2 or 3 pins for the 74HC595.

Colin
--
cdb, colin@... on 2/07/2009
 
Web presence: www.btech-online.co.uk  
 
Hosted by:  www.1and1.co.uk/?k_id=7988359
 

 
 
 
 

--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist

[Olin Lathrop]

Steve Maroney wrote:
> I'm new to microcontrollers and new to electronic circuit design. I've
> read up on how a transistor works and I think I get the theory but
> they don't function as expected when tested.

What is your training in and what level have you achieved (high school
student, college senior majoring in basket weaving, 20 years of professional
experience installing air conditioners a McMurdo Station, something else
perhaps)?  What is your background in electronics?

Where are you located?

What level of budget do you have for this activity?

> I have a PIC that outputs to an LCD which works great.  I also need to
> share 4/8 of the LCD data lines as an input to another
> microcontroller. I need a method of keeping data not destined for the
> 2nd microcontroller away from it while addressing the LCD.

Why can't you use separate communication lines?  A single UART line for each
direction pops to mind.

> I thought
> I could put some 2n2222's in line with my 4 shared data lines.

"In line" doesn't describe anything that makes sense here.  Show your
schematic.

> When testing my transistors with 5v applied to the base,

Oops.  If the emitters were tied to ground then the transistors are now
toast.

> I get the
> same voltage from the collector and emitter. The collector and
> emitter are not tied in with anything so I would expected nothing
> from the collector or the emitter.
>
> I also applied 5v to the collector and an LED w/resistor to the
> emitter. When I apply 5v to the base the LED turns on, if remove 5v
> from the collector, the LED stays on because of the 5v applied to the
> base
>
> Should it work this way ?

I have no idea what you're doing.  Show the schematic.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
[phone].  Gold level PIC consultants since 2000.
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist