My first question: I plan on using a 12.6v c.t. 1A transformer with full wave rectification. This should net me 6.3v @ 2A right?
Next question: I am a little worried about the higher voltage this will produce. If this transformer provides 6.3v @ 2A, the smaller load I will be placing on it (~0.7A total) will cause the output to be higher than 6.3v. After rectification and at 2A, the output should be 7.92v (figuring rectifier 0.7v loss). But since I will only be drawing ~0.7A total, the output will be closer to 9v DC, if not higher.
I know I could regulate the supply and feed the rest of the circuit true 6v, but I thought that splitting the thermal load across the two cc transistors would be preferable (only have to heatsink the two transistors). If I regulate it first, the cc transistor disspation will be lower true, but will still need to be on a heatsink, as well as the regulator. Same thermal load, just split differently. What would you do?
The transformer will not change maximum output current unless it has two secondary windings connected parallel. With a center tapped transformer you can't do it. The secondary windings must be separate!
Use a 10VA transformer with either a single secondary winding of 9V/1.1A (or 2X9V, 2X555mA, e.g. Block UI39/8 209)
Use an LM317T adjustable voltage regulator and adjust it to the deisred voltage. It regulates pretty precisely independent of the load (minimum 10mA).
Nothing, if you use the suggested regulator. It works alright up to 1.5A.
Boncuk
Good project.
It seems like temperature sensing is the easiest way to do it.
How hot does the battery get before the charge is actually terminated?
Good project.
It seems like temperature sensing is the easiest way to do it.
How hot does the battery get before the charge is actually terminated?
surely it is more of a case of how hotter than the ambient does the battery have to get to be charged, I think I once saw a similar kit that used two sensors to acomplish this
That's probably a better approach, maybe a thermocouple could be used?
Temperature sensing works well and is easy, but I would rather have used a true delta voltage peak scheme. I tried experimenting by using a comparator. One input was hooked directly to the battery positive terminal, the other input was also hooked to the cell, but through an isolation diode that charges a capacitor. I figured the cap would "store" the highest voltage peak. However, I found that when the battery voltage started to drop at the end of the charge, the diode's reverse leakage current was very slowly discharging the cap so it wasn't holding a constant voltage. At the slow 0.3A charge rate I use, the delta peak happens slower than the cap discharges through the diode leakage so the comparator never shuts off the charge circuit.
Ni-MH cells are made in Japan for Energizer (maybe by Sanyo). They are all low self-discharge now. The AA cells are 2450mAh.
I get coupons online or from a local lumber store then I buy the cells when they are on sale at a food store.
The package for the newest Ni-MH AA cells says, "charge 150 times more and charge lasts 6 months longer than 2450mAh cells".
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