Wheatstone Bridge Help

[andybarrett1]

Hi all

Looking to build a Wheatstone bridge so as I cam simulate some suspect SG cells.

The cells are 700R @ 0.5mV / Volt (Excitation is 5v DC)

So I am guessing a Bridge with 4 x 700R resistors (1 in each corner) gives 0.00mv O/P

My question is how can I simulate the force throwing 1 resistor out of balance  using a pot maybe?

Thank you for any help / advice

BR
Andy

[Kleinstein]

To get a small adjustment, e.g. to set the slight imbalance, one could use a pot as a third branch and couple the slider through a relatively high resistor to the bridge. So something like a 10 K pot and a 350 K resistor to get a  +-10 mV/V adjustment range.

[andybarrett1]

/Hi Kleinstein

Explain more please

Sorry

[Dragon88]

Do you want to be able to make quick, specific adjustments? Then a small value trimmer in series or a very large value trimmer in parallel would do it.

Do you want to simulate a more analog effect? How about a power FET in contact with one of the resistors acting as a heater? The temperature change will change the value of the resistor based on its tempco. The heat will transfer to the entire bridge over time, but the resistor in contact with the heater will experience the most sudden change, upsetting the bridge. Flip the heater on and off to simulate disturbances.

[andybarrett1]

Hi

I just need to simulate the cells for the controller... Maybe a couple of calibrations etc.

Just to check for repeatability etc...

[uncle_bob]

Hi

The "normal way to do it":

Grab four 700 ohm resistors. It's probably worth it to get some fairly good ones.

Grab a 10 ohm pot and two 5 ohm resistors.

The pot goes in one leg, center tap is the output.

The two 5 ohm resistors go in the other leg where the pot would have been.

Net result is a bit of imbalance at the extremes. It shouldn't be enough to throw anything out of whack.

The pot will give you about a +/- 50 mV swing. If that's enough then you are done. If you need more, then it's a bigger pot and likely
smaller resistors. At some point the balance becomes an issue and you go to two pots on a single shaft. Just where the problem starts depends on what you are driving into and (as always) how picky you are.

Bob

[andybarrett1]

Thank you Bob

I have just got this...... The alerts not working 

Exactly what I was looking for .... Thank you

BR
Andy

 

[uncle_bob]

Thank you Bob

I have just got this...... The alerts not working 

Exactly what I was looking for .... Thank you

BR
Andy

Hi

No problem. The alert system seems to be a little flakey at times. (Unlike me, I'm flakey all the time ...)

Bob

[andybarrett1]