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MOSFET circuit question

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earckens

Member
Hi, this diagram is for a detection circuit that does the following:
1. at the input a current transformer detects for the presence of 20kHz PWM signals. When these are present then Q1 is switched on
2. when detection is present the LED D5 should light up
3. when detection is present the relay output should be connected to +V through the P-channel Q4, and hence be activated.

However, as I read it, when detection is present then the output of Q2 goes high (no conduction because collector Q1 is low and thus Gate-Q2 is low); therefor Gate-Q4 is high and since this is a P-channel: no conduction drain-source, and the relay is off. Contrary to what it should be.
Is this reasoning wrong?
 

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dougy83

Well-Known Member
Points 1 & 2 are correct. Point 3 is not correct. When there is a suitable AC signal from the transformer, Q1 will be pulsing, Q2 will be off, Q3 will be on and Q4 will be off. The relay pin will be pulled low through the 47R resistor.
 

earckens

Member
Ah ok, I was wondering what the purpose is of the 47R resistor.

So if the relay pin is pulled low when detection is present there is no activation of the relay; when detection is gone, the Q4 conducts and the relay pin is connected to Vcc: relay activated.

But I want the relay to be off when no detection, and activated when detection is present: replace the p-channel with an n-channel and place the relay connection on the source instead of the drain?
 

dougy83

Well-Known Member
Can you just connect the relay between V+ and the RELAY pin? Then, when the pin is low, the relay is activated.
 

earckens

Member
Sure, that can be done; but when relay pin is low, Q4 is also switched off? Where does the relay current drain when that pin is low?
 

dougy83

Well-Known Member
The current to drive the relay would go through Q3 via the 47 ohm resistor. Are you worried that there won't be enough drive current?
 

earckens

Member
Indeed: draining the relay current upsets the LED functioning, and what purpose serves Q4 in that scenario?
Also, Q4 is a p-channel, should the load not be between Q4 and ground?
 

AnalogKid

Well-Known Member
Most Helpful Member
Q1 is pulsing, but its collector is held at a constant low by C1. If you connect the Q4 gate directly to the Q1 <edit> collector, Q4 will turn on the relay when the signal is present. The LED can be connected in parallel with the relay output, eliminating Q2 and Q3. Add a protection diode across the relay output to GND.

R2-C1 has a corner freq of 34 Hz, plenty low enough to filter a 20 kHz signal. The 4.7 ms time constant sets the delay between when the signal disappears and when the relay releases.

ak
 
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Tony Stewart

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Most Helpful Member
Change Q3 to NPN but reverse so base senses Rs current drop at 0.7V to regulate constant current for variation in LED voltage. with collector biasing MOSFET Vgs. Normally this is done with Rs near 1 Ohm for 700 mA...not 47 Ohms
 

earckens

Member
Q1 is pulsing, but its collector is held at a constant low by C1. If you connect the Q4 gate directly to the Q1 emitter, Q4 will turn on the relay when the signal is present. The LED can be connected in parallel with the relay output, eliminating Q2 and Q3. Add a protection diode across the relay output to GND.

R2-C1 has a corner freq of 34 Hz, plenty low enough to filter a 20 kHz signal. The 4.7 ms time constant sets the delay between when the signal disappears and when the relay releases.

ak
If you connect the Q4 gate directly to the Q1 emitter,... ..you mean Q1 collector?

Thanks for your calculation and feedback.
 

Tony Stewart

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Most Helpful Member
Mr Earckens, what is your LED current? and Vf? It seems like an over complicated way for current sensing.
 

earckens

Member
Change Q3 to NPN but reverse so base senses Rs current drop at 0.7V to regulate constant current for variation in LED voltage. with collector biasing MOSFET Vgs. Normally this is done with Rs near 1 Ohm for 700 mA...not 47 Ohms
Change Q3 to NPN but reverse ...... what do you mean? ....so base senses Rs... R6? ....to regulate constant current for variation in LED voltage. with collector biasing MOSFET Vgs ....what do you mean?
 

Tony Stewart

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Most Helpful Member
Change Q3 to NPN but reverse ...... what do you mean? ....so base senses Rs... R6? ....to regulate constant current for variation in LED voltage. with collector biasing MOSFET Vgs ....what do you mean?
ignore above , I see you are trying sense current not drive LEDs

but where are your specs?
 

Tony Stewart

Well-Known Member
Most Helpful Member
I see it is a model train noise current sensor..

This makes more sense now .

The schematic is correct. Visualize each drain or collector as an inverted output and follow the logic, detect noise input =1 and thus low side drive to relay=0
 

earckens

Member
I see it is a model train noise current sensor..

This makes more sense now .

The schematic is correct. Visualize each drain or collector as an inverted output and follow the logic, detect noise input =1 and thus low side drive to relay=0
Now I am confused. The purpose of this schematic is to do the following: when a detection occurs, ie. when Q1 is switched "ON", then the LED should light up AND the relay should be activated.
We are talking a p-channel for Q4 hence, upon detection this MOSFET is OFF, hence no relay activation (p-channel means the relay is connected between drain of Q4 and ground.

I want the relay to be ON when detection occurs, so I follow AnalogKid logic: connect gate of Q4 to the collector of Q1.

Is this wrong anywhere?
 

Tony Stewart

Well-Known Member
Most Helpful Member
when noise occurs Q1 in is 1 Q1c is low (0) the Q2D out is 1 then Q3-d out is 0 and LEd is On and. same ime Q3 out =0 drives Q4d =1 so if relay is between RELAY and 0V(GND) it is ON at the same time, but if between RELAY and +V it is inverted , assuming correct coil polarity in each case. so the schematic is correct.

If put in parallel with another sensor driver, the LED is ON externally driven.

However what is MISSING, is a spike clamp using
reverse diode across driver to,+V for a low side coil drive. BD6 is only for series or Totem Pole connections, parallel is better I think. as in wired OR =0 or wired NOR


If you read me carefully, I use inverting logic quickly to understand for common source amp switch like a logic inverter
 
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Tony Stewart

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Most Helpful Member
p.s. this design is suboptimal but works, if you follow my link for concerns.... better ways but not my project
 
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