Eliminate the voltage regulator ?

[3v0]

EDIT: For clarification currently my design is using a LM317 in a constant current mode for charging the backup cells and a fixed voltage regulator for power the rest of the circuit. I am asking about getting rid of the fixed voltage regulator.

Given that a design has a constant current source for charging its backup NiMh at C/10. At this rate you can charge continuously.

The voltage generated by the LM317 across the battery terminals tracks the battery voltage.

Can we power the device from the battery terminals all the time?

My device functions with a power source between 2.5V and 3.6V. The 3 cell AAA battery likes to be charged to 4.1V. So I am thinking of finding a diode with a .5V Vf and forgetting about using a fixed voltage regulator.

I will need to increase the current output of the LM317 by the amount used by the circuit backed up.

 

[3v0]

I already have a proper low dropout regulator I am asking about getting rid of it. The TO-92 LDO is pointed to by the red arrow on the left. The TO-220 LM317 is on the right. Click on the image to enlarge.

**broken link removed**

 

[ronv]

I don't see why you couldn't do that but you need to be careful not to remove the battery while charging.

 

[3v0]

I don't see why you couldn't do that but you need to be careful not to remove the battery while charging.

I would not expect to happen with the 3 cell version am thinking off. The cells have solder tabs and the leads could be soldered.

 

[MikeMl]

I think you are asking about how the voltage regulation might be effected by introducing the dynamic impedance of the diode between the battery and the downstream circuit. To test this, I made a bunch of assumptions when creating this simulation:

1. The load draws a steady current of 20mA with intermittent spikes of 200mA (typical of digital circuits.)
2. The load bypass capacitor is 1uF.
3. The charger supplies 50mA to the battery.
4. The internal series resistance of the battery stack is 0.1Ω.

Using a 1N4001 shows that the diode subtracts a nominal 0.6V, but it also introduces a voltage ripple of 142mV due to the impedance of the diode. The 1uF bypass cap doesn't reduce the ripple. To get rid of the ripple, it would take a huge filter capacitor.

If the downsteam circuit contains anything that is effected by the ripple, then you should retain the LDO.

 

[3v0]

Thanks Mike I am chewing on it.

 

[Mr RB]

Why use a constant current system 3v0? It's lossy as the LM317 will drop about 3v from input to output, and it's also lossy as the full current will be expended all the time so you'll get some battery heating etc and probably reduced battery life.

Many commercial appliances use 3 cells for battery backup, many of the older style good quality VCRs did this with 3 or 4 NiCd cells. They used a regulated voltage (usually 5.0v) and a simple series resistor to keep the battery charged. The resistor sets the max current for a discharged battery and coupled with the regulated voltage it also sets a small continuous current into the full battery. Sometimes they had a series diode too to drop the regulated 5.0v to about 4.4v.

 

[3v0]

RB

The docs I have read indicate that the way to charge NiMh is with constant current. I am monitoring the battery voltage from the PIC. I can turn the charger and and off. And the docs say that at C/10 or less you can charge continuously without harm to the cells. But I would like to get the max life out of the cells so whatever is best in that regard is the ticket. I have attached the circuit.

FWIW I am backing up nodes on a radio linked monitoring and control network. Cost is an issue and I would like to use 3 cells rather then 4 on some nodes. The PIC and radio will work in the range of 2.5V to 3.6V, battery tops out at about 4.1V.

Your solution does sound less complex but I need to address the problem of the battery topping out at 4.1V and the radio at 3.6V. A simple diode drop should fix this problem or will we be back to the diode ripple problem mike talked about ?

I imagine the circuit would be something like this. I know the regulator will need to be higher then 4.1 due to voltage drop at the current limit resistor. Perhaps it would be better to bypass the resistor to power the micro when then power is on?

 

[Mr RB]

Thanks for providing the extra info 3v0. I think the needs of battery backup are different from what is normally known as "charging", especially something like cell phone charging.

Normally in battery backup you expect the battery to be trickle charged all the time,a nd be full all the time, with only the occasional power outage where the battery will then supply the power.

If that is the case for your app, I would use one of those tiny low Q-current low drop out 3.3v regulators and always run the device from the battery through the regulator. The regulated voltage will give you some safety and also ensure reliable transmit power and stable performance. Then as for keeping the battery trickle charged that depends on your input power source, mainly what voltage it is.

If your source is 5.0v then I would probably use one or two diodes, normal or schottky as needed to give you approx the right float voltage, then a resistor. Keeping a battery trickle charged from a regulated power source is very easy.

If these are going to be powered all the time I would never consider pumping C/10 into the battery constantly.