Modifications to improve 9V battery operation

[gyro]

I just posted the internal photos of my DSO138 enclosure and someone may have spotted and wondered about the diode soldered on the back of the PCB across C25...

I have made a few modifications to allow the board to work more efficiently with a 9V alkaline (or NiMh) cell battery pack.

There are three problems with getting the unit to work down to the fully discharged state of the batteries (0.8V/cell, 4.8V total). The first is the 5V supply, the 78L05 starts to drop out when the input voltage reaches about 7V. The second is that AV- also drops. The third is the voltage drop across D2.

The first problem is easy, I swapped the 78L05 (U5) for an LP2950-50 LDO regulator. This requires a bigger output capacitor for stability, so I changed C20 (100nF) to 100uF . It oscillated with 100n but is stable with 100uF.

The problem of AV- was a little more complicated. I was hoping that the F/W PWM would be able to regulate V- rather better, by monitoring V-MON, but this doesn't seem to work - maybe a possible F/W improvement?

There is no suitable LDO replacement for a 79L05. Instead I reduced the value of L2 from 1mH to 680uH (0.68mH). At this value Q1 is still happy (I know there have been problems with wrongly supplied 100uH in counterfeit kits) and V- is increased by a couple of volts.

At high input voltage there is a danger of V- exceeding the voltage rating of C25 (16V) and the input limit of the 79L05. As protection I have wired a 15V 400mW zener diode across C25 (cathode to GND). I know it is not ideal, but it doesn't start to conduct until the supply voltage reaches nearly 12V on my unit and even then it conducts very little current.

The voltage drop across D2 is a problem, it could be replaced with a schottky diode which would reduce it a little, but to achieve minimum possible input voltage I bypassed it. NOTE: This removes power supply reverse polarity protection!

With all of the above modifications, my DSO138 now operates correctly from 11V down to 5.4V, this equates to 0.9V per cell on the battery, which is pretty close to fully discharged. Input current is around 120mA so a set of AA batteries should give well over 12 hours of operation.

I'm only documenting what changes I have made on my unit for information. Many people will not care about battery operation, and I'm not suggesting that anyone else repeat my modifications.

[bobydunkel]

Tnx for Info...

Best regards,
boby