Request Guidance: NiMH Low-Voltage Cut-Off

MikeML

Joined Oct 2, 2009
5,444
With respect to discharging a pack consisting of several series cells, if the criteria is to prevent the weakest cell from reversing polarity due to the load current, then the only game in town is to monitor the pack voltage, and stop discharging when the total pack voltage drops. Otherwise, you would need as many voltage monitors as there are cells in the pack.

Here is the discharge curve for 2000mAh NiMh cells, reproduced below.

In a four-cell pack, if you use a cutoff voltage of 0.95V per cell, that would mean a pack cutoff voltage of 3.8V. If the pack contained one weaker cell and three healthy cells that still had 1.1V per cell, the load cuttoff would occur just as the weak cell reaches 3.8-3*1.1 = 0.5V
 

Thread Starter

phototron

Joined Nov 22, 2013
22
Thanks again for the info MikeML. I want to make sure that the PFET you suggested will be able to handle the 2 watt load I intend to create. The spec sheet appears to suggest it can only handle 1.6 watts.
 

MikeML

Joined Oct 2, 2009
5,444
The power that the FET dissipates is a small fraction of the power being switched. Note the resistance from Drain to Source (Rds) of the FET when it is turned on. The power dissipated by the FET is Voltage from Drain to Source (Vds) times the Drain current (Id).
 

Thread Starter

phototron

Joined Nov 22, 2013
22
The power that the FET dissipates is a small fraction of the power being switched. Note the resistance from Drain to Source (Rds) of the FET when it is turned on. The power dissipated by the FET is Voltage from Drain to Source (Vds) times the Drain current (Id).
I have seen Rds and Vds and similar ratings in Spec sheets. Now I'm starting to understand what they mean. Thank you.

I am reading the semiconductor section on this site that describes using a JFET as a switch to get a better idea how the PFET functions, and how transistors can be used as switches in general. This will all be so very useful in so many projects!

Thank you!

Edit: Now I see which connections in the circuit are Drain, Source and Gate! Reading about MOSFETs on this site now.
 
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Thread Starter

phototron

Joined Nov 22, 2013
22
Ok. I'm ready to order the parts we've covered, plus the resistors to be used as heaters and the AA cell holders. However, I thought I'd check into the complexity of a temperature-controlled switch, just in case.

I checked this site and found this thread where they talk about using two transistors as a switch controlled by a thermistor powered by a 5v DC supply. One of the posters recommended using an LM393 in place of the two transistors.

I also found this page elsewhere that gives a rough overview of the operation of thermistors.

Now I'd like some opinions as to whether a temperature switch would be too complicated, or if it is something I might pursue with another thread. If it isn't more difficult than the circuit by Mike ML, I know it would increase the efficacy of the small battery I'll be using. Often the nights here only enter freezing temperatures for a couple of hours - it isn't until the middle of winter that we have longer periods of freezing.

Again, I appreciate all the guidance.
 

Thread Starter

phototron

Joined Nov 22, 2013
22
The power that the FET dissipates is a small fraction of the power being switched. Note the resistance from Drain to Source (Rds) of the FET when it is turned on. The power dissipated by the FET is Voltage from Drain to Source (Vds) times the Drain current (Id).
Hi again, Mike ML. Happy Thanksgiving. When you have time, could you please help me understand which components will carry the load in your circuit? I don't quite understand why the PFET doesn't have to have a 2W rating to switch a 2W load. In my head the load travels through the PFET. I am trying to understand, but a lot of this is new to me yet.
 
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