5V required for PWM signal only

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Hi All

 

I working on a small project, controlling a FAN(blower type). The Fan operates with a 5V PWM signal or a 0-5V analog signal.

 

I will be using the EMC2301 - Thermal Management - Closed Fan Controllers (microchip.com), with a Pull-up resistor on the PWM pin to 5V.

 

The remaining parts PCB runs of 3.3V.

 

The Voltage prior to the 3.3V step-down DC-DC  will be 12-24V.

 

I would like to avoid using a step-down for 5V, as it's only needed for a PWM signal.

 

Would using Zener diode with a series resistor be the best option?

 

 

 

 

 

 

This topic has a solution.

Thanks

Regards

DJ

Last Edited: Tue. May 4, 2021 - 10:33 PM
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with a Pull-up resistor on the PWM pin to 5V.

You have to ensure the pwm pin is set for open drain mode on the chip

Each output driver can be configured to operate as an open-drain (default) or push-pull driver and can be configured with normal or inverse polarity (see Register 6-8).

 

not sure if this applies to the pwm pin as well (doesn't sound like it, but the wording is poor), you will have to look

For the 5V tolerant pins that have a pull-up resistor (SMCLK, SMDATA, TACH and ALERT), the pull-up voltage must not exceed 3.6V when the device is unpowered.

 

Would using Zener diode with a series resistor be the best option?  ----yes the series resistor to 12v is your 5V pullup, which is clamped to 5v by the zener.   If the zener is missing or blown , then the chip will get 12V and blow too.

So--use some care!

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Sun. May 2, 2021 - 03:47 PM
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Thanks for the reply.

 

Yes it is set to open drain, i have tried on demo board, with 100 ohm(best result) pull up.

 

Only the PWM pin is 5V tolerant.

 

How often do Zener diodes get blown, maybe it will be better to place a cheap LDO. This would mean heat on PCB for few mA?

 

 

 

 

 

 

 

 

Thanks

Regards

DJ

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Suppose "few mA" means 5mA. Then regulator will dissipate: (12V - 5V)*5mA = 7V * 5mA = 35mW. That is pretty modest.

 

By the way, there is no need for LDO, Remember, LDO = Low Drop Out which refers to small input-output differences. 7V is not small. 7805 would do that just fine. 7805 would not even need any heat sink at 35mW.

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

Last Edited: Sun. May 2, 2021 - 04:06 PM
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This would mean heat on PCB

Energy is energy, regardless of how you generate / expend it...

 

The resistor for the Zener will dissipate heat.

Or the voltage regulator will dissipate heat.

 

Either way you need too get from 12 V down to 5 V at whatever current the output signal needs.

(Fortunately, as a control signal, that should be a lot less than 5 mA, (which I understand is just an example)).

 

JC  

Last Edited: Sun. May 2, 2021 - 05:16 PM
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ka7ehk wrote:

Suppose "few mA" means 5mA. Then regulator will dissipate: (12V - 5V)*5mA = 7V * 5mA = 35mW. That is pretty modest.

 

By the way, there is no need for LDO, Remember, LDO = Low Drop Out which refers to small input-output differences. 7V is not small. 7805 would do that just fine. 7805 would not even need any heat sink at 35mW.

 

Jim

 

Hi Jim

Yes few mA is all i need at 5V.

 

Ok, i will use a simple linear regulator. 

Thanks

Regards

DJ

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The FAN must have an internal 5V supply, are there no way to take it from there.

Perhaps the input in the fan controller has protection diodes so  a weak (220K or so) 12V pull up will put 5V on the line.(5.5V)

 

And perhaps stupid but have you checked the level of the input if it float perhaps it's high (so no input mean full speed).  

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How often do Zener diodes get blown

They usually wont...unless you burn off too much power  (from too low a resistance)...common zener values are 500mw & 1W...you can expect to use about 1/3  to 1/2 of that without reaching the danger zone or needed to worry much at all about "better" cooling.

DO-41 leads can help suck away heat into the PCB copper (such as a gnd plane). 

 

A 15V zener at 50ma is 750 mW,,,so you can get a nice hot surprise even at "low" current.   Note a parallel (shunt) load reduces the power the zener has to burn off  (since it is getting the extra help in lowering the voltage).

 

The FAN must have an internal 5V supply, are there no way to take it from there.

Why must?  if it is a 12V or 24V fan, it could still have an input pin that responds to 5V logic levels (without a 5V supply in the fan).

Maybe the fan signal will work from 3.3V logic?  Where is the fan datasheet?

 

The Fan operates with a 5V PWM signal or a 0-5V analog signal

So the answer is likely NO, very most likely they just RC the input...if you apply an analog voltage, then the RC is along for the ride...if you provide a pwm, then it smooths it up to make the analog voltage that you could have applied.

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Sun. May 2, 2021 - 05:45 PM
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Please find the datasheet attached of the Fan/blower that will be used with this circuit.

 

When i measure the PWM wire without it being connected to my PCB ,there is 4.5V. 

 

What i have observed...

 

If i was to set my device(EMC2301) to open-drain, and then generate a PWM signal of duty cycle 50%, the fan does not preform as the specification, as the voltage on the PWM wire is about 0.6V. But once i add a pullup resistor of 100ohm to 5V, its starts preforming very close to the spec.

 

When the PWM is left floating, is makes the fan ran at 100%.

 

On regards to Zener diodes, i have full reel of MMSZ5231BT1G, would this work?

 

"So the answer is likely NO, very most likely they just RC the input...if you apply an analog voltage, then the RC is along for the ride...if you provide a pwm, then it smooths it up to make the analog voltage that you could have applied." Seems like it

 

Attachment(s): 

Thanks

Regards

DJ

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It appears exactly as I mentioned ...they also add a 10k pullup so the default floating value is 100%  ON   .Note you can get up to about 75%  full speed with a 3.3V PWM

 

The zener will be fine for your needs.  Make sure not to install it backwards (3 terminals like sot-23, are better prevent that).

 

Why not use AVR to create the pwm??!?---you don't need this extra EMCxxx chip at all (but then you'd need a 3 cent 2N3904 open collector transistor or fet for a 5V PWM, unless you run the AVR at 5V).   They're just trying to sell you a chip you prob don't need.

At >80 cents it costs almost as much as the AVR (or a good portion of it).  Anything to raise the BOM cost through the roof.

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Mon. May 3, 2021 - 12:14 AM
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Thanks

 

I can make sure that the MMSZ5231BT1G is placed the correct way. My preference is to use that part as i got few reels of them.

Thanks

Regards

DJ

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Djosi,

 

looking at the datasheet you might not need 5V at all.

what happens if you hook up a switching transistor or FET to the speed setting and just switch that?

All you have to keep in mind is that the control is inverse. So 100% duty cycle is 0% speed.

 

If you want to you can make a simple 5V circuit from a transistor and 2 resistors, Specially if your 12V is stable.

 

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hook up a switching transistor or FET to the speed setting and just switch that?

That is an interesting idea....

When  the transistor is open (off), the interior voltage will be 5V

When the transistor is closed (on), the interior voltage will be approx 0.5V

 

The cap will form the avg & it will be smooth, assuming the freq is pretty high (1 nF is pretty sad filter, seems rather small...maybe they meant 1 uF)

 

I'd toss the chip completely & just use an AVR with pwm & an open collector/drain NPN/NFET

 

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Using a transistor(2n3904), wouldn't that be as same the switching i am currently doing with open-drain. Previously i have tried this, and found that duty cycles do not match air flow as per datasheet.

 

My Vin will be AC-DC power supply outputting 12V,  please send more information about this circuit for 5V.

 

My PCB will have some space, therefore i can implement multiple options.

Thanks

Regards

DJ

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avrcandies wrote:

Haven't we seen this diagram before ... ?

 

and doesn't it suggest that the fan already has the pullup internally?

 

EDIT

 

Here: https://www.avrfreaks.net/commen... - again by djoshi.

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Last Edited: Mon. May 3, 2021 - 09:53 AM
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Yes earlier post 

 

Sorry to repost like this, but this questions is more about generating the 5V. I want to avoid using power supply if possible,

Thanks

Regards

DJ

Last Edited: Mon. May 3, 2021 - 10:47 AM
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 is more about generating the 5V. I want to avoid using power supply if possible,

So why did you wait until #9 to post & show there is an internal pullup to 5V?????

 

Note in general, an external 5V signal can drive faster than a 10K pullup RC signal (since external drives through 1K). 

However, the very small cap, makes this  rather moot; it is fast either way.
 

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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avrcandies wrote:
So why did you wait until #9 to post & show there is an internal pullup to 5V?????

Seems to be OP's modus operandihttps://www.avrfreaks.net/commen...

 

frown

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I plan to use Zener 5.1V, with a resistor between 350-400 ohms, i will need to see what i have available in my stock, but use 0603 mostly. Would it be better to use larger package maybe 1206?

 

Thanks

Regards

DJ

This reply has been marked as the solution. 
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Look at the power dissipation ratings

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 resistor between 350-400 ohms

That's ridiculous--you don't have a calculator?  you'd be running almost 60ma and burn everything up

 

 

Use maybe 4K if you insist on 0603.    Maybe 2500 ohm if you use 1206

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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I aim to use a 4.7K 0603. Got a lot them in a reel.

Just out of curiosity has anyone used EMC2301?

Thanks

Regards

DJ

Last Edited: Mon. May 10, 2021 - 12:23 AM
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Have the pity for 0603, can you.

 

Try again PWM, because **it is** the solution, as avrcandies in #10 said.
Be sure not to have to small PWM frequency (Specification indicates 20kHz), so 32kHz plus 3k3 plus 2N3904 must be adequate.
Anything else will be overkill.
 

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grohote wrote:

Have the pity for 0603, can you.

 

Try again PWM, because **it is** the solution, as avrcandies in #10 said.
Be sure not to have to small PWM frequency (Specification indicates 20kHz), so 32kHz plus 3k3 plus 2N3904 must be adequate.
Anything else will be overkill.
 

 

I will keep both options on the PCB. I had selected the EMC2301 as it also packaged fault detection, but i guess i can simply set the PWM channel on uC and make some for of lookup table to compare the tact against PWM to determine any fault in the fan.

 

 

Thanks

Regards

DJ

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I had selected the EMC2301

Sometimes they make chips to give you something to spend money on that you probably could do yourself.   But then engineering time isn't cheap & they (supposedly) did a lot of legwork for you.

In some cases an extra 80 cent chip means nothing, in other products a 20 cent part is make or break, raise or layoff.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Sound correct.

 

Well  i am going to keep both options on the PCB. While PCB is getting made, i am going to try getting PWM option working.

 

I am working on new PCB, that will have the SAMD21JA with WINC3400. I am very new to SAMD21A so trying get my head around it.

 

Thanks

 

 

Thanks

Regards

DJ

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I did not want to open a new post as this is still related to PWM and Fan Control

 

There are some 2 wire fans which I would like to test.

 

So I could remove the 5V pullup and connect that PWM to a MOSET to control the 2 wire FAN.

Thanks

Regards

DJ

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djoshi wrote:

So I could remove the 5V pullup and connect that PWM to a MOSET to control the 2 wire FAN.

I haven't read the whole thread but

 

If your PWM pin is in open-drain configuration, and you remove the 5V pull up resistor from it, you won't be able to drive the MOSFET from PWM to control the 2 wire FAN.

Last Edited: Fri. May 14, 2021 - 11:42 AM
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Yes, i agree but i can change it from open-drain to a standard 3.3V pulse.

 

 

Thanks

Regards

DJ