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Multiplexing Fet / Opto help please.

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quant

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untitled-jpg.35646
 

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Electronworks

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What is wrong with driving the columns of your matrix with a high side P_FET and driving the rows with a low side N_FET? You will need some level translation circuitry to high side drive your PFET (and possibly another opto coupler). It would be an EMC nightmare, but structurally it looks quite straight forward.

I also notice you have no back emf diode across your solenoid.
 

quant

New Member
What is wrong with driving the columns of your matrix with a high side P_FET and driving the rows with a low side N_FET? You will need some level translation circuitry to high side drive your PFET (and possibly another opto coupler). It would be an EMC nightmare, but structurally it looks quite straight forward.

I also notice you have no back emf diode across your solenoid.

EMF Diode is connected on pads 11 & 12?

Please explain your concept a little more clearly with detail and schem. for this noob to understand, maybe an example?

thanks
 

Electronworks

New Member
Please see attached.

To switch on L6, drive M8 and M4. To drive L2, drive M7 and M5.

Please note, there is no connection between the horizontal lines and the vertical ones.

Does this OK, or have I misunderstood you?:confused:
 

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mneary

New Member
You need a back emf diode on each solenoid, but equally important you need a forward diode too.
To switch on L6, drive M8 and M4.
The sneak path through L3 will energize M5, which will drive L2 and L5 in series, plus L1 and L4 in series.
To drive L2, drive M7 and M5.
..The path through L5 will energize M4, and drive L3 and L6 in series plus L3 and L4 in series.
 

Electronworks

New Member
Why do you need a forward diode too? The PFETs are only ever driving positive and the NFETs are only ever sinking current?

The sneak path through L3 will energize M5, which will drive L2 and L5 in series, plus L1 and L4 in series.

What is a sneak path and how will it energise a MOSFET that is turned off?

(To keep the circuit simple, I have disregarded the driving circuitry around the MOSFETs.)

Please explain

Thanks
 

mneary

New Member
Black arrows show where you intend the current to flow.
Red arrows show where it also flows.
 

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Electronworks

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I see your point - so you will need a diode in series with each coil and one in parallel pointing in the opposite direction to snub the back emf.

Thanks for the clarification. I was using the same circuit as I have used hundreds of times with LEDs... but then an LED by its nature only conducts one way..

Thanks
 

quant

New Member
Gentlemen,
Thank you so much for all the help and advice provided and Happppy Thanksgiving too...
I'll comb through all the information and being a NewB I'm sure I'll need further assistance.

thanks
 

quant

New Member
Can anyone to please show a sample circuit with the Diodes placed and their part numbers and types given as well as the types and numbers for the Fets?

Thanks very much

p.s
Please note that in my own circuits I have already included a Diode(on Pads 11 & 12), does that stay or it now gets eliminated?
 
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quant

New Member
I have the following available, is any of them any good?

1- BC547BG Amplifier Transistors NPN Silicon

2- 2SC945P NPN Silicon Transistor(AF amplifier and low speed switching)

3- BC109A - npn sisicon transistor

4- 2N3904 NPN General Purpose Amplifier

p.s.
obviously all NPN! Can I use my Arduino Mega to substitude the PMos until Sun. when I can get anything required?
 
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quant

New Member
Notes for future Noobs:

NMOS and PMOS
In NMOS transistors, the silicon channel between the source and drain is of p-type silicon. When a positive voltage is placed on the gate electrode, it repulses the holes in the p-type material forming a conducting (pseudo n-type) channel and turning the transistor on. A negative voltage turns the transistor off. With a PMOS transistor, the opposite occurs. A positive voltage on the gate turns the transistor off, and a negative voltage turns it on. NMOS transistors switch faster than PMOS.
 
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mneary

New Member
Sorry for the fuzzy image, didn't want to draw it from scratch. (When you draw, please make sure it's big enough, and if possible please get rid of the background lines.

I don't remember what the current and voltage are, so I won't comment on appropriate transistors and diodes yet. Here's the corrected schematic.
 

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Electronworks

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If I were you, when wiring up this circuit, replace the coils with 1k resistors. Switch on a high side PFET and a low side NFET and you should see a voltage across ONE resistor only. Go around the circuit with a voltmeter and check the voltage across all other resistors and they should be 0V.

What you are trying to do is present a voltage to ONE column only and a short to ground (via the NFET) in ONE row only. The coil that gets energised is at the junction of that row and column. All other points in the circuit will either have 0V on them, or a row voltage (and no column voltage) or a column voltage (and no row voltage). Your circuit did not do this
 
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