Battery discharging circuit not working

[Harneet Arora]

I have a circuit that aims to discharge a battery and hence monitors it's voltage.
The circuit either drains 4 uA current through a 680k resistor or 20mA current which is driven by the DAC. The switching between these two scenarios is done using a 74hc125 ( have tried N FET as well)

The circuit diagram is attached.

Problem: When the switch corresponding to 680k is enabled it gets grounded but when switch corresponding to 10 ohm resistor is enabled it doesn't gets grounded.
I'm unable to spot the problem with the circuit . Why is this so? Is there a problem with any component or circuit?

Please Help!!

 

[Old Steve]

If the 10 ohm resistor isn't getting grounded at all, doesn't it have to be a problem related to the dsPIC? Are you sure the dsPIC output pin is going low?

 

[Harneet Arora]

If the 10 ohm resistor isn't getting grounded at all, doesn't it have to be a problem related to the dsPIC? Are you sure the dsPIC output pin is going low?

Thanks much for your reply.

Actually the pins on the dspic are showing accurate values corresponding to the written high or low values. So, I think there some problem with the hardware not with the software.

 

[Old Steve]

Thanks much for your reply.

Actually the pins on the dspic are showing accurate values corresponding to the written high or low values. So, I think there some problem with the hardware not with the software.

Well, if the dsPIC pin is going low high, but the bottom of the resistor isn't being grounded, there's only one thing in between. Hard to read, but looks like 74HC something. Is it a 74HC05 open-drain inverter, in which case I guess the dsPIC pin must go high to ground the resistor. The 74HC05 can only sink 4mA though. Blown?

I was looking at this datasheet, http://www.ti.com/product/sn74hc05?CMP=AFC-conv_SF_SEP which says 4mA, but just saw another that says 25mA max:- http://www.nxp.com/documents/data_sheet/74HC05.pdf

Which chip is it? And is it definitely a 74HC05? Very hard to read the writing on the schematic.

Edit: Somehow, I missed the chip type in your original post - a 74HC125 line driver. Non-inverting, so I was right originally. The dsPIC output must go low to ground the resistor. Either way, again, different chips, different specs. One datasheet says 6mA, another says 35mA.
Same story though. If the dsPIC pin is going low but the 10 ohm resistor isn't being grounded, it must be that chip.
I trust you're using the OE to allow it to float high as a tri-state output, then enabling it with OE to drive it low?

 

[Harneet Arora]

Well, if the dsPIC pin is going low high, but the bottom of the resistor isn't being grounded, there's only one thing in between. Hard to read, but looks like 74HC something. Is it a 74HC05 open-drain inverter, in which case I guess the dsPIC pin must go high to ground the resistor. The 74HC05 can only sink 4mA though. Blown?

I was looking at this datasheet, http://www.ti.com/product/sn74hc05?CMP=AFC-conv_SF_SEP which says 4mA, but just saw another that says 25mA max:- http://www.nxp.com/documents/data_sheet/74HC05.pdf

Which chip is it? And is it definitely a 74HC05? Very hard to read the writing on the schematic.

Sorry about the unclear handwriting. It is 74hc125. A tri-state buffer. I have tried replacing it with a N FET but still the results are the same.

 

[Old Steve]

Sorry about the unclear handwriting. It is 74hc125. A tri-state buffer. I have tried replacing it with a N FET but still the results are the same.

Right, now we're on the same page. You said the resistor is not being grounded, but it must be I guess. You've measured from the bottom of the resistor to ground?
What you really meant, I suppose, is that the other end of the resistor isn't connecting to the battery through the Darlington pair?

 

[Old Steve]

Sorry I was so slow to pick up on the chip name and the fact that you'd also tried an NFET last night. It was very late and I was exhausted after a hard day writing two uC programs. Have you solved this yet?