LM723 Voltage Regulator, Regulation Question

[Stanfi]

I have a circuit design, where I am using a LM723 Voltage regulator to give me 12VDC. If I understand correctly the LM723 can supply 150ma without an external transistor. However, I have a load connected to it that is varriable, and the current draw will range from 40 to 50ma. I notice that when I reach 50ma. That I will see appromimately .3V drop in my 12V signal. Is this normal? or do I have a problem?

Thanks

 

[bearing01]

The amount of current the regulator will deliver (power it will source / dissipate) depends on its temperature. I am not familiar with the LM723 but I have found with the LM317 (rated for 1A) that when it sources around 200mA it gets pretty hot and then the 12V out will slow drop. The thermal shutdown inside the regulator kicks in to turn the device off.

If you blow on the device (to remove heat) and the voltage picks back up that is a sign you need a heatsink and maybe even a fan to get full current rating.

 

[Madcow]

sounds a little high.
what is the input voltage.
What is the value of Rsc.
If Rsc is 10 ohms like the app.note has
you maybe in current limit at 50mA.
Try lowering the current sense resister.

 

[Stanfi]

The input voltage is 15 Volts. Rsc is 11 ohms.

 

[felixc]

Madcow is probably right. The LM723 has a spec for the minimum voltage drop to maintain between input and output, to which you add the voltage loss of the current sense. You may be getting too close to this limit. To verify the theory Move up the input voltage to 16 volts, does it make a difference?

 

[sreid]

I've just got to ask; why not just use a 7812 regulator?

 

[felixc]

Or maybe one with a better LDO spec, like an LM1086-adj.

 

[Stanfi]

The 12V I am using to excite a load cell (Wheatstone bridge). By using a potentiometer with one of my external resistors, it enables me to "tweak" the voltage to exactly 12V.. I reviewed some DC power supply designs, many used the 723, so I assumed it was a good choice for my application.

 

[OperaHouse]

That's an antique! I would have thought they would be hard to find. Brings back memories of supplies I was designing in the late 70's. You can even make them into a switcher. Still, a victim of the need for lower parts count. A 7805 can make a nice adjustable 12V regulator. I think your problem is the small voltage difference between input and output. That would be a problem with many series regulators that aren't specifically designed for this situation.

 

[Stanfi]

Actually the LM723 is readily available from digikey.

One of the power supplies that I reviewed was the Power-One supplies that digikey sells

 

[cbarn24050]

Hi, with a small load current and small variation you could probably just use a resistor with a zener.

 

[felixc]

The output of your wheatstone bridge is ratiometric to the power supply voltage. Since you already have the precision circuitry to read your load cell, can you use it to also read the excitation voltage at the load cell? this is called the four-wire measurement technique. This would compensate for any variation of the supply.