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P-Channel Mosfet Application

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Gary_SMi

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Hello, I have 5 x 8A mosfets that need to drive individual loads. They need to be triggered in 2 ways, 1.) all 5 triggered together from 1 microcontroller output or 2.) They need to be triggered individually from 24Vac plc outputs. Have tried as attached but produces undesirable results and the fets dont trigger correctly. The 24V section on the drawing is duplicated throughout the other channels. Can anyone possibly assist where I am going wrong with this? Thanks.

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FvM

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You are talking about PMOSFET and have PMOS symbols in the schematic, but are connecting it like NMOS (positive Vgs and Vds).

I assume that you need opto isolation for the 24 V AC inputs. But the common LED cathode ground and connection to 5 VDC control source is cancelling the isolation. Only an AC circuit should be connected to each LED. The DC control path should go the OC output side.
 

Gary_SMi

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Hi FvM, Yes P channel, connected as attached? Will rework the isolation as per your suggestion and retest. As for the 5V trigger illustrated by S1 and the diodes, maybe it will be better to place this then on the OC output side with transistors rather?

 

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Now the PMOSFETs are connected correctly.

I believe you'll use transistors, respectively a single transistors and decoupling diodes for the 5V trigger.
 

Vbase

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Hello, I have 5 x 8A mosfets that need to drive individual loads. They need to be triggered in 2 ways, 1.) all 5 triggered together from 1 microcontroller output or 2.) They need to be triggered individually from 24Vac plc outputs. Have tried as attached but produces undesirable results and the fets dont trigger correctly. The 24V section on the drawing is duplicated throughout the other channels. Can anyone possibly assist where I am going wrong with this? Thanks.

View attachment 117275

The circuit in post #1 is OK apart for one important issue, you cannot use a single 220R, you must have individual resistor for each optocoupler. If each optocoupler needs 10mA then put 270R in series to each diode. If the total current is more than 25mA then use a transistor or a buffer because the uC can source only 25mA.
The reason is that LEDs inside optocoupler are not identical and in parallel they don't share the current equally.

For the 24VAC you can use a single diode instead of a bridge and add a cap to ground after the diode.
 
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Gary_SMi

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The circuit in post #1 is OK apart for one important issue, you cannot use a single 220R, you must have individual resistor for each optocoupler. If each optocoupler needs 10mA then put 270R in series to each diode. If the total current is more than 25mA then use a transistor or a buffer because the uC can source only 25mA.
The reason is that LEDs inside optocoupler are not identical and in parallel they don't share the current equally.

For the 24VAC you can use a single diode instead of a bridge and add a cap to ground after the diode.

Thanks for that, will make the changes. At the moment the 5V section is not being energised at all. Have placed bridge and opto circuit for the first 2 channels but when I apply the 24V ac to either of the channels it seems to switch both OC' s on, any suggestions?

20150509_232856_resized.jpg

As well as the fact that the fets are not driving current but measure the full 12V with a multimeter !?!

Ok channel 2 fet has somehow been damaged with this circuit
 
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Vbase

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I can't see the bridge rectifiers, I guess they are on the other side of the board. The 5 outputs of 24Vac have common at the PLC and you common the negative side of the bridges on the board, you are shorting half of the bridge. This could have damaged the bridges.
Try replacing the bridges in those 2 channels with a diode for each input, the anode to the PLC. The diode function as a rectifier and as a gate.
If that works add resistor for each OC as in post #5 , add also 22uF from the cathode of the rectifier diode to ground.

- - - Updated - - -

2813602500_1431210635.jpg
 

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Thanks, yes the bridges are on the reverse side. Also, haven't indicated there is a LED on the output of the OC in line with the 10K resistor. The gate of the FET is directly to the OC. Seems there is a damaged FET from my attempts, would you consider the OC output and drive circuit to be ok?

- - - Updated - - -

Thanks, yes the bridges are on the reverse side. Also, haven't indicated there is a LED on the output of the OC in line with the 10K resistor. The gate of the FET is directly to the OC. Seems there is a damaged FET from my attempts, would you consider the OC output and drive circuit to be ok?

Hi the input side seems 100% now with the diodes and caps but just blowing the fets on the output, 3rd one gone now?

- - - Updated - - -

Using ch1 fet getting extremely hot can only run for short time, seems to work with the 24Vac input trigger but not the 5v trigger although the led on the oc output does illuminate but not quite as bright as with the 24V
 

Vbase

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When you get the circuit working as you wish it to then you can call it OK. It could be done in many ways and every engineer would have favor another way.
I would have used a transistor instead of an optocoupler because you don't need isolation of the PLC 24V, I see in the picture a wire from the ground of the PLC to the ground of your mosfets and uC. I can send you a drawing if you wish.

The LED in series to the 10K is in the wrong place. The resistor has to be between the gate and the source of the mosfet. The job of the resistor is to pull the gate source voltage to 0 when the OC is off. The LED stops the gate discharging at about 1.5V, most mosfets are off with gate voltage of 1.5V unless they are 'logic mosfets', it is unsafe to leave the gate at 1.5V. It would have been better to have the LED between the OC and the gate.
 

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When you get the circuit working as you wish it to then you can call it OK. It could be done in many ways and every engineer would have favor another way.
I would have used a transistor instead of an optocoupler because you don't need isolation of the PLC 24V, I see in the picture a wire from the ground of the PLC to the ground of your mosfets and uC. I can send you a drawing if you wish.

The LED in series to the 10K is in the wrong place. The resistor has to be between the gate and the source of the mosfet. The job of the resistor is to pull the gate source voltage to 0 when the OC is off. The LED stops the gate discharging at about 1.5V, most mosfets are off with gate voltage of 1.5V unless they are 'logic mosfets', it is unsafe to leave the gate at 1.5V. It would have been better to have the LED between the OC and the gate.

Hello Vbase, it would be appreciated if you could send a drawing of a better way to do this fets still getting hot, many thanks
 

Vbase

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It is very possible that the mosfets are hot because they don't switch off completely. Try linking the LEDs in series to the 10K. Also without the cap the mosfet is switching the load at 50Hz.
This is the circuit I recommend.
The uC doesn't need buffering, the current is only 2.5mA.
https://obrazki.elektroda.pl/3829681800_1431224262.gif
 

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It is very possible that the mosfets are hot because they don't switch off completely. Try linking the LEDs in series to the 10K. Also without the cap the mosfet is switching the load at 50Hz.
This is the circuit I recommend.
The uC doesn't need buffering, the current is only 2.5mA.
https://obrazki.elektroda.pl/3829681800_1431224262.gif

Thanks Vbase, this looks like a much more efficient way of doing things will give it a try in the week :)
 

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Thanks, yes the bridges are on the reverse side. Also, haven't indicated there is a LED on the output of the OC in line with the 10K resistor. The gate of the FET is directly to the OC. Seems there is a damaged FET from my attempts, would you consider the OC output and drive circuit to be ok?
Probably not if the LEDs are preventing Vgs from returning to zero. In other words, you simply don't manage to show a complete schematic. How can you expect specific help in this situation?

Using ch1 fet getting extremely hot can only run for short time, seems to work with the 24Vac input trigger but not the 5v trigger although the led on the oc output does illuminate but not quite as bright as with the 24V
Another point is that you didn't say a word about the actual load. MOSFETs can run hot for quite different reasons, e.g. in on-state because load current is too high for a specific Rdson or Vgs is too low, also in off-state by too high residual current, or during switching, e.g. because inductove loads are switched without clamping circuit. The reasons can be well distinguised by watching circuit behaviour and measurements.
 

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The load is 3A inductive dc motors (fets still get hot with the pic input but not the 24vac inputs, have reworked resistor values and removed led's etc, Flywheel diode across load also makes no difference. The fets are MTP2955E, 8A. Will try circuit from Vbase recommendation.
 

Vbase

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When switched by uC the mosfet gets hot because it is only half on. The reason is no enough current through the led of the OC, see post #5. It can be cured with individual resistor from the uC to each OC. If the LED isn't dim with uC drive it indicates the drive to the mosfet is ok.
 

Gary_SMi

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Thanks Vbase made board using transistor method as outlined in your drawing and works 100%. Initially went the OC route to provide isolation to/from the PLC network but if unnecessary, this works perfectly and more efficient & economical, with Uc and 24Vac inputs either/or/both. Much appreciated :)
 

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Hello Vbase, am needing to add some functionality to your circuit as outlined in the attached pic. The problem I am having is that the fet banks are triggered when bank enable goes high even when card enable is low? Do you have any suggestions on a better way to accomplish this?

 

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