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simple design that doesnt work.

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alphadog

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Hello,
I'm using a simple design thats supposed to drive a lacthing single coil relay.
My question is, why is M2 (N-channel mosfet) in cut-off?

relay-jpg.26203


Thanks.
 

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I don't think it likes the way M3 and M4 are oriented.
 
Could you explain please?
I'm really down 'cause i dont succeed in designing a driver circuit for single coil latching relay.
My intention in that design is that a ~0.2A would flow through the relay, and i dont understand why it doesnt like that.
 
Look carefully at this -

**broken link removed**
 
Thank you very much for the good drafting!
As you can see in my design, i followed your drawing.
M3 gate is LOW, therefore its channel conducts current.
M4 gate is HIGH but its channel still conducts current,
Why is that?
 
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Continue:
The problem is that M4 Drain is not LOW as your sketching, but that is what i dont understand.
Shouldnt M4 Drain be LOW?
 
Look at the datasheet for your P-channel mosfets. There is a diode between drain and source.
Since your P-channel Mosfets are connected upside-down then their diodes are always conducting.

Your schematic should [b[always[/b] have part numbers for the Mosfets so we can see if they can turn on properly when their gate voltage is only 5V.
Most Mosfets need a gate voltage of 10V to properly turn on.
 
Thanks for your help.
I tried to used Mosfest with catalog number in pspice, but every mosfet i used (except MBreakN/P), the simulation gave an error that the Mosfet is not defined.

You know how to solve it?
 
You are simulating random circuits.
You need to learn about electronics to properly design a circuit that works.

You have your P-channel Mosfets connected upside-down so of course they will not work properly.
 
Now your P-channel Mosfets are connected properly but without a part number for the Mosfets then we don't know if they can turn on with only 5V on the gate.
 
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I think i know now how to handle the pspice model.
I looked at a few datasheets of mosfets but i haven't seen Lambada and Kp parameters, only rds_on.
In order to enter Lambada and Kp into Pspice model, should i calculate:
Linear zone: Kp = 0.5 / (rds * [VGS - VT - VDS])
saturation zone: Lambada = 1 / (rds * IDS)
?

Thanks!
 
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Quit looking at pspice and start looking at spec sheet, as clever duck suggested. Look at the curve for source-drain current vs. Vgs. At 5V, this may not be enough to deliver the 200ma through the 25Ω load.

Also, you may have blown the drivers since you had them upside-down.

...assuming you actually built this. Are you just doing simulations?
 
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The curves for Mosfets on their datasheets are for "typical" ones. Their written spec's show that there is a very wide range from the curves.
If the curves show a Mosfet conducting 40A when its gate voltage is 4V but its written guaranteed spec says it needs a gate voltage of 10V then I give it 10V, not 4V.
 
Quit looking at pspice and start looking at spec sheet, as clever duck suggested. Look at the curve for source-drain current vs. Vgs. At 5V, this may not be enough to deliver the 200ma through the 25Ω load.

Also, you may have blown the drivers since you had them upside-down.

...assuming you actually built this. Are you just doing simulations?

I'm am going to build this.
I looked at the graphes and it really helped me.
So, Thanks!!

I still want to simulate it on Pspice,
So how can i do this if i dont know the Kp and Lambda of the mosfet? (all i know directly from the spec is its VTO).
 
All transistors have dozens of parameters that aren't listed on spec sheets because they aren't typically important.

Plug your part number into one of these search engines -
**broken link removed**
 
I think the Mosfets will have "shoot-through" current when one is turning on when the other is turning off. They short-circuit the supply.
 
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