Linear moving switch, decade counter, chaser

[Tom.G]

Especially after you lose track of where Pin 1 is!

 

[artis]

So I made the 32 bit chaser. It uses a 555 timer in astable mode and 4 74hc595N chips.

Just one tiny problem, I have connected my N mosfets directly to the 595 outputs, for whatever reason I cannot get hem to shut off when the outputs go low.
I measured that each 595 output when OFF measures about 18 ohm to ground.

So either I need pulldown resistors for each gate or I have managed to thermally damage one of my 32 fets while soldering. These are the small TO-251AA devices where you solder the backplate - drain.
I was careful but who knows.
So I guess I'm asking whether a 595 output can effectively drive a fet without a pulldown resistor or does it need one?

 

[Baluncore]

What is the part number of the MOSFETs you are using?
What are the supply voltages to the 555 and 74hc595?
What is power supply to the LEDs?
What limits LED current?

 

[artis]

What is the part number of the MOSFETs you are using?
What are the supply voltages to the 555 and 74hc595?
What is power supply to the LEDs?
What limits LED current?

Fet's are IRLR8256TRPBF - TO-252AA type package

The supply voltage given that the 595 max is 6 volts is about 5.7v from a 6v battery , same voltage goes to 555 timer. When 595 output goes high I observe around 5.5v on gate which is sufficient for these small fets to turn on and they are ON the problem is - all the time...

Each LED connected to each 595 output is in series with 750ohm to ground.

So it's 595 output ---LED---resistor----GND.
The gate is attached between the 595 output and LED , so directly to the 595 output which should discharge the gate charge to ground when it goes LOW.
Or so I thought.

 

[DaveE]

I'm asking whether a 595 output can effectively drive a fet without a pulldown resistor

Yes. HC logic can pull hard to ground with it's MOS output.

 

[DaveE]

Each LED connected to each 595 output is in series with 750ohm to ground.

So it's 595 output ---LED---resistor----GND.
The gate is attached between the 595 output and LED , so directly to the 595 output which should discharge the gate charge to ground when it goes LOW.

What? Can you draw a schematic, take a photo and post it here?

 

[artis]

@DaveE

I made a quick drawing , I hope it should be clear how each output is arranged.

 

[artis]

When the 595 IC is powered up but in it's inactive state (all outputs LOW) then it measures roughly 18 ohms from each output to IC ground.
That should surely discharge a mosfet according to "my book".

It's the first time for me soldering these tiny DPAK mosfets, so I really don't know how easy it is to kill them with heat. But given my circumstances it seems I might have killed one or two.
I did not solder them for long, definitely no longer than 10 seconds, usually about 5 but the temp of the solder gun might have been high because I used my big soldering iron as I needed to bond the fet backplate to a rather beefy copper surface.

 

[DaveE]

@DaveE

I made a quick drawing , I hope it should be clear how each output is arranged.
View attachment 317797

You need a pull-up resistor at the drain/input. HC logic has essentially infinite input resistance. Your FET could be off with no current available to make it change it's voltage.

It's also not clear why you need discrete transistors for this. How about a 74HC04 connected to the resistor voltage? Or, if you don't need the inversion just connect any HC input to the resistor.

PS: That MOSFET is huge for a logic application. For discretes most EEs would use a SOT-23 package. They're smaller and easier to hand solder with exposed small terminals.

 

[artis]

You need a pull-up resistor at the drain/input. HC logic has essentially infinite input resistance. Your FET could be off with no current available to make it change it's voltage.

It's also not clear why you need discrete transistors for this. How about a 74HC04 connected to the resistor voltage? Or, if you don't need the inversion just connect any HC input to the resistor.

PS: That MOSFET is huge for a logic application. For discretes most EEs would use a SOT-23 package. They're smaller and easier to hand solder with exposed small terminals.

I think you misunderstood my poor drawing. I simply put the FET where it is because of drawing space limitations. It's an N channel FET and all I need is it to be controlled from the 74HC595 , so source to GND and gate to 595 output. I mean the mosfet can be turned on/off with a potential applied between S and G.
Why would I then need additional potential to be applied to drain?

As for the FET size, this was most comfortable to work with and also the last size that was not too big for what I needed.
I have tried sot-23, for me was an absolute pain to work with. Too small, definitely too small for my current application.But right now I'm trying to understand just this, before I start soldering transistors off to check them, whether I can safely assume a FET should be turned OFF by the HC595 when all it's outputs are LOW?

I think I can assume the 595 turns all FET°'s OFF when it's outputs go LOW?
If that is the case then my FET's shouldn't measure as shorted therefore I suspect damage to some FET's has occurred.

 

[Baluncore]

I made a quick drawing , I hope it should be clear how each output is arranged.

It is not clear.
Attached is the schematic I expected.

1. Restrict your supply voltage to +5 volt using a LDO regulator. HC does not like higher voltages.

2. The MOSFET is "Logic level" gate, so it is OK, but very high current for the job of driving a LED.

3. The 74HC logic has sufficient voltage swing to control the gate of the MOSFET without pull-up/down resistors.

4. Restrict the LED current with R1 = ( 5 volt - LED Vfwd ) / ( LED current ).

 

[DaveE]

Yes, if Vgs < Vgs(th) (read the data sheet, of course), and if current will flow from drain to source from your ohmmeter (polarity is important here, this is for NMOS), then you have killed that device.

Regardless, you'll need that pull-up resistor to drive an HC input with a MOSFET.

 

[DaveE]

This post may help https://www.physicsforums.com/threads/heavy-duty-transistors.997686/post-6434695

 

[DaveE]

I think you misunderstood my poor drawing. I simply put the FET where it is because of drawing space limitations.

Um.. What? What's the point of posting an incorrect schematic? I don't care where you put it, I care how it's connected to other things.

I think I can assume...

Assumptions are the major source of confusion when troubleshooting things that don't work. Measure it.

 

[Tom.G]

If the schematic you supplied is how the circuit is really wired, there are a few problems.

Please see:
https://electronics.stackexchange.com/questions/443623/driving-led-with-an-n-channel-mosfet

Cheers,
Tom

 

[artis]

I'm sorry I confused you. Although I did not show a connection the original drawing seems to indicate the FET drain is connected to the 595 but it's not.
The circuit is real simple. The 595 has 8 outputs. Each of the 8 outputs is connected to a LED + terminal and a mosfet gate, the LED - terminal is connected to a 750 ohm resistor which is then connected to ground. This is the same for all 32 LED's.

So the 595 drives the LED's so that I can visually easily confirm the pattern that I set but the 595 outputs, the same outputs also drive the gate's of the FET's.
Since the FET's are N channel their sources are connected to the same GND as the 595 chip, so that the chip can drive them , and it does, I observe the gate voltage.

The FET drain and source itself is part of a different circuit and has nothing to do with the 595 logic. the 595 just drives the FET's.
Just like you would have a relay where the contacts are driven by the field created within the relay coil which is attached to a separate circuit.

I made a correction in the drawing and show a line at an angle in the FET drain, indicating that it doesn't go to the 595 chip.
I apologize for any confusion.

 

[Baluncore]

I'm sorry I confused you.

Partial circuits are difficult to debug because undocumented components have unspecified implications across the arbitrary bulkheads and firewalls.
If the MOSFET load is inductive, the circuit will require a flyback diode to catch the voltage spike, or the MOSFET drain will be subjected to high voltage that will destroy the device insulation.

Each time the digital output of 74HC switches, it must charge the 1nF capacitance of the power MOSFET from the logic supply. To prevent those current spikes making the logic supply noisy, it will require some current limit, such as a series gate resistor. Clocked logic is particularly susceptible to power supply noise.
Ask how much current can the 74HC output source or sink? What is the 74HC output impedance? How much capacitance is needed across the +5 volt supply to handle those parallel switching currents?

High power MOSFETs require high-current gate-drive to charge the capacitance. To minimise switching current, minimise the gate capacitance by selecting a MOSFET that is sufficient, but does not have an excessive current capacity.

Series gate resistors slow down the MOSFET transition time. That may lead to higher device power dissipation. Lower gate resistance requires higher current gate drive circuits between the 74HC and the MOSFET. Expect to use at least 12 ohms of gate resistance to prevent ultrasonic oscillation during the transition.

 

[artis]

I did debug my circuit. The problem was as expected , one of the 32 fet's had died with it's D-S shorted but gate left intact therefore in a circuit of 32 parallel fet's it wasn't easy to locate the dead one and I had to manually take off all of them until I found the culprit.

Since I had not previously worked with smaller size and rating fet's I did not know that I have to be extra careful with the heat of soldering. I was careful but apparently not enough.
Most likely cause given the circuit wasn't even working is that the channel of the FET got damaged due to heat. Soldering has never ever been a problem for me but that could be because I have always mostly repaired or built stuff with larger form factor devices.
The ones where I used small packages I also used a small iron but my current application requires a larger iron because the surface area where to FET has to connect is larger.
The smaller iron quickly loses heat and sticks to surface.
Anyway I will perfect my skill in the future.Anyway I will change out the FET's , test the rest and see how it goes, then will come back and tell.
I appreciate your help!

 

[Baluncore]

Is the load inductive?
How many loads are there?
Why are the MOSFETs all in parallel?

 

[artis]

Is the load inductive?
How many loads are there?
Why are the MOSFETs all in parallel?

I plan to make a new thread about the device I've made , it's an experiment out of curiosity , I will explain there, you are welcome to join it when it's up. @DaveE will also probably add some valuable commentary hopefully as well as others