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any idea getting the gate to 0v?

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well, hi, i builded this alkaline battery charger with constant current and pulse charging but have a little problem getting the gate voltage down to zero. i used a pnp to discharge the gate when the 555 is of state. the thing is that it goes down to 0.6v which is the saturation voltage of the pnp.
Any way to get it down to 0 without using a relay? the problem is that as the gates remains at 0.6v, theres still some current flowing so i cant get belown 2mA

**broken link removed**

the lm317 is in fact a lm385 but i coulnd find the part in the simulator :(

actual circuit in the breadboard
**broken link removed**

ps: it works at 0.35Hz / 25%
 
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Q2 can not pull down below 0.6 volts. You are lucky to get that low.
If you used a 2n2222 with it's E on ground and C connected to the gate, you will get about 0.3 volts. This inverts the logic! Also you will need a base resistor.
 
I now see you have a base resistor.
You will get the same thing if you use a diode for Q2. (and 1k changes to 0 ohms)
 
Do I have this right? C4 charges up by 100k, Discharges with 330k.
....Invert logic.....
Make top R=220k bottom R=100k, remove 1N4148.
Now charge is 320k and discharge 100k.
Change Q1 to 2N2222A with E at gnd.

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Next idea is to do the above but;
remove Q1. Add diode from 555 pin 3 to U2 pin 2. Add 10k resistor from U2-2 to 1 ohm resistor. This way the 555 will pull up U2-2 to shut off the FET.
 
there, done it and works a lot better, also reversed the led soe ill torn on only when off (25%) so it stays as before.
now the off voltage goes down to 0, but the mosfet still lets some current flow when setting the dividers to 0v.
seems the only next step posible would be to find a voltage comparator with lower low-level voltage as it's reading arround 16mV (which im gratefull as the datasheet says 250mV tipical)

sumarisin i cant get below 1,22mA....ill see how it goes on a pcb, as moving the wires and the resistances sometimes it goes up to 1,70mA and sometimes down to 0,86mA
thanks for the help ron :p

ps: could a logic level mosfet help? i saw Dave from EEVBlog do this with a logic level one, but as i cant get those here i used one with low impedance i had floating arround....bah the only model left i had xD
 
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Try a 10KΩ resistor from the gate to ground.

Boncuk
 
Try a 10KΩ resistor from the gate to ground.

Boncuk

it just takes it to 0v, tryed different values and they only limit the maximum current i can get from the drain.
it doesn't even divide the gate voltage because as theres a 1MOhm resistor before the gate, it just makes the voltage comparator give more voltages/current, as if it worked like a pwm. it just limits the maximum gate voltage.
 
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The problem is there is current in the FOSFET when the gate=0V! WHY?
Replace the MOSFET. Bad part.
Use a smaller FET. The IRFZ24N is a 17A part. I think you are charging at about 1A. Try a 2 to 4A part.
 
the problem is when it's in the ON part. even if i set the 393 to 0v, there still is some voltage. it must be some leakeage from somewere.
i'll to as ron says, try with a lower capasity mosfet.

ps: lol i got it wrong, the mosfet isnt a IRFZ24N, it's a IRFZ 44 N, which is a 42A mosfet.
and yes the maximum current is 1.26A which is the minimum voltage the 385 can give.
 
Here's a way to get the gate voltage slightly below the FET source voltage when Q2 conducts. That should turn the FET off completely :-

Insert a high current handling (>1A) forward-biassed diode in series with the 1 ohm FET source resistor, between the resistor and ground, with cathode to ground.
Connect the 'Adj' terminal of the LM317 and the bottoms of the two pots to the diode anode instead of to ground, so that the reference voltage for the comparator is raised by one diode drop (~ .7V).
 
ill try it, i dont understand how that works but will see if it does the job, and btw i said in the first post that the 317 isnt a 317 but a lm385 but as i didn't have it in the simulator used a 317 instead
 
It works because the FET source voltage is then higher than ~0.7V, whereas Q2 emitter is at 0V and when Q2 switches on its collector (hence the FET gate) is at just the Q2 saturation voltage (<0.2V).
Whether you're using a 317 or a 385 makes no difference to the method I suggested in post #12.
 
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