INA 125 Instrument Amplifier
for rocket motor test stand

This little amplifier, or lack thereof, has stood in my way of having a digital test stand for a long time.  I had studied Richard Nakka's excellent tutorial on building a load cell, and even purchased some strain gages to make a crude variation on his theme.  Purchased a Dataq A/D starter kit, hooked it up, and got no reading.  None whatsoever.  Someone offered a good deal on a commercially-made load cell and I got one.  It gave no reading either.  Stuart Leslie informed me that an amplifier was needed, and sent me info on making it.  I wasn't ready, having no recent experience in electronics, and I was soon distracted with other things...  

Enter Steve Ghioto.  The kind of friend every rocket geek should have.  He knows electronics.  He knows rockets.  And he lives within driving distance.  He built me a cute little test stand using the 20kg load cell from Aerocon, and another larger stand using the 500lb cell.  And he built the amplifier for them.  All I had to do was get a cheap data acquisition unit, make motors, and start testing.  

So life is good.  I am getting test data of unprecedented resolution and accuracy.  There is a danger that I will be doing real science with this equipment.  Suddenly, I have lost my excuses for sloppy work.  Darn!

But something still bothers me.  I don't know the amp.  It is a black box, literally and figuratively.   Perhaps this is a character flaw - I am intolerant of ignorance, especially my own.  I feel a need to master this amplifier.

Besides, I would like to be able to build such setups from scratch.  The load cell/test stand is mechanical, and now that I have Steve's example I can look at it and figure it out.  For variety I can look at several others on the Aerocon site, which offers a tutorial on load cells, and a pictorial essay illustrating many different styles of test stands.  

Now the A/D converter is another black box, but enough is too much.  I'll burn that bridge when I come to it.  Anyway, the basic Dataq unit is cheap and easy.

After staring for many minutes at Steve's amplifier to see how it was constructed, seeing schematics in Terry McCreary's book and on Mike Bennett's and Scott Fintel's websites, and reading Stuart's instructions and a zillion e-mails on the rocketry lists, I get the notion that I might be able to build the amp myself.

On a dare, I ordered some INA125P chips from Digi-Key and the ball started to roll..

Board 1 small photo
Board 1

A minimalist approach, with a tiny circuitboard and just enough stuff to make it work.  
And it works just fine!
Board 2, small photo
Board 2

A slightly larger board with some extra room.
Easier to work on, more precisely adjustable, and it even has a power-on LED.  

This is the one I recommend.

The Box

Whichever board is made, it needs to go in a box of some sort.  

Here's mine - I'm following Steve's model.

Four Components
Full Setup

The amp doesn't do much by itself. Check out the other stuff.

Thrust Curve Sample
Static Tests
using this equipment
Spreadsheets & videos

Aerocon Systems Instrument Amp
If you would rather not build this amp, note that Aerocon Systems is selling an instrument amp that is similar to this one and probably better.

If I had known that earlier, I might not have gone to all this trouble!

Back to Recrystallized Rocketry
Jimmy Yawn
rev. 12/7/04

Amp Docs - a few accessory files to assist in building the INA 125/122 amp

JY 1/31/16