LDO Voltage Regulator Behaviour

Thread Starter

qwertyuiop23

Joined Aug 21, 2013
14
Hi there,

I have a straing gauge circuit containing an INA126 amplifier. The design is to use 5V however, I only have a 12V power supply available (from the wall), and was going to use an LDO to control the voltage to 5V. In theory I though this would work, in reality the voltage that the LDO outputs changes with the input voltage.

At Vin equal to 5V it outputs 5V and the linearly decreases as the input voltage increases. I have checked the current draw and it is at 5mA well below what the LDO can draw.

However, the output and ground terminals of the LDO are connected to the E+ and E- terminals of the strain bridge. If I unsolder the strain bridge the voltage goes back to outputting at the correct voltage. This leads me to believe that due to the voltage divider created by the full strain bridge the internal voltage divider inside the LDO has had its values change and therefore is no longer accurate.

I have not attempted to fix this problem yet but I believe I can buffer the output from the LDO using a simple voltage follower op amp, but I would rather not do that if there is another way to fix this.

I have attached a picture of the circuit. (Apologies about the paint drawing no Altium here to draw a proper one)

Regards,
Lance Henderson
Mechatronics Engineer
 

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ScottWang

Joined Aug 23, 2012
7,397
If the pins connection is correct, then 7805 could be damaged, usually the Vin-Vout>=2.5V, unless have the internal shorted or wiring shorted.
 

Alec_t

Joined Sep 17, 2013
14,280
The MC7805CT (or LM7805CT) is not a LDO (low drop-out) regulator; it needs at least 7V input to regulate properly.
What is the bridge resistance and required current? The regulator short circuit current is only 230mA.
 

Thread Starter

qwertyuiop23

Joined Aug 21, 2013
14
The chip needs capacitors, too.
Are they not only input/output bypass capacitors? They simply help with the stability and transient response of the controller, nothing to do with the output voltage. I could be wrong though.

The MC7805CT (or LM7805CT) is not a LDO (low drop-out) regulator; it needs at least 7V input to regulate properly.
What is the bridge resistance and required current? The regulator short circuit current is only 230mA.
Yea I am powering with 12V so it shouldn't have that problem. Unsure of the resistance of the strain gauge. Not high I would suspect. I would have thought that it could supply 2Amp even with a small resistance

EDIT:: The bridge resistance is 1.005kOhm so nowhere near enough to draw the current max of the circuit. I canonly thnik that the fact the internal resistor is now in parallel it struggles to meet the demand

Looking at what you've indicated is your pin out and that given in the datasheet, you haven't connected it properly.
I know, I can assure you that it is correct on the board.
 
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