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PWM detector?

Hello all,

I am designing an automotive product (operates 9 to 16VDC) that will
drive a regulated high current 10 amp load with a NPN low side drive
transistor. It will maintain average current to the load via PWM
(400Hz 1% to 99% duty cycle) and sense resistor.

My question, I want a way to shut the load down in the event the PWM
controller flakes out and gets stuck high. This would be a bad thing
since the load would remain on 100% and possibly be damaged. So, what
I am asking is...

Is there a cheap and easy way to detect the presence of a PWM signal
and allow operation, but, in the event the PWM gets stuck on for X
amount of periods, turn the output stage off?

I tried coupling the output FET to the PWM drive circuit through a
capacitor. However, no matter what value cap I choose, I compromise
either high duty or low duty cycle drive. Is there a solid state
solution that will not affect the waveform?

Thanks

Gerb
 
M

MooseFET

Jan 1, 1970
0
Hello all,

I am designing an automotive product (operates 9 to 16VDC) that will
drive a regulated high current 10 amp load with a NPN low side drive
transistor.  It will maintain average current to the load via PWM
(400Hz 1% to 99% duty cycle) and sense resistor.

My question, I want a way to shut the load down in the event the PWM
controller flakes out and gets stuck high.  This would be a bad thing
since the load would remain on 100% and possibly be damaged.  So, what
I am asking is...

Is there a cheap and easy way to detect the presence of a PWM signal
and allow operation, but, in the event the PWM gets stuck on for X
amount of periods, turn the output stage off?

I tried coupling the output FET to the PWM drive circuit through a
capacitor.  However, no matter what value cap I choose, I compromise
either high duty or low duty cycle drive.  Is there a solid state
solution that will not affect the waveform?

Retriggerable oneshot.
 
Retriggerable oneshot.







- Show quoted text -- Hide quoted text -

- Show quoted text -

Ok, you have my attention.

Would this work as an edge trigger? I guess what i am getting at,
would it work reguardless of the duty cycle of PWM?

I'll look into it.

Thanks.

Gerb
 
S

Spehro Pefhany

Jan 1, 1970
0
On Sun, 30 Nov 2008 17:39:00 -0800 (PST), the renowned
Hello all,

I am designing an automotive product (operates 9 to 16VDC) that will
drive a regulated high current 10 amp load with a NPN low side drive
transistor. It will maintain average current to the load via PWM
(400Hz 1% to 99% duty cycle) and sense resistor.

My question, I want a way to shut the load down in the event the PWM
controller flakes out and gets stuck high. This would be a bad thing
since the load would remain on 100% and possibly be damaged. So, what
I am asking is...

Is there a cheap and easy way to detect the presence of a PWM signal
and allow operation, but, in the event the PWM gets stuck on for X
amount of periods, turn the output stage off?

I tried coupling the output FET to the PWM drive circuit through a
capacitor. However, no matter what value cap I choose, I compromise
either high duty or low duty cycle drive. Is there a solid state
solution that will not affect the waveform?

Thanks

Gerb

An alternative solution might be a Polyfuse or something similar...
which could (depending on how sensitive the load is) also protect
against output stage failure.


Best regards,
Spehro Pefhany
 
P

Phil Allison

Jan 1, 1970
0
<[email protected]
I am designing an automotive product (operates 9 to 16VDC) that will
drive a regulated high current 10 amp load with a NPN low side drive
transistor.

** OK - so the switch is a BJT.
I tried coupling the output FET to the PWM drive circuit through a
capacitor.


** Now the switch is a MOSFET.

????????????????



...... Phil
 
J

Jamie

Jan 1, 1970
0
Hello all,

I am designing an automotive product (operates 9 to 16VDC) that will
drive a regulated high current 10 amp load with a NPN low side drive
transistor. It will maintain average current to the load via PWM
(400Hz 1% to 99% duty cycle) and sense resistor.

My question, I want a way to shut the load down in the event the PWM
controller flakes out and gets stuck high. This would be a bad thing
since the load would remain on 100% and possibly be damaged. So, what
I am asking is...

Is there a cheap and easy way to detect the presence of a PWM signal
and allow operation, but, in the event the PWM gets stuck on for X
amount of periods, turn the output stage off?

I tried coupling the output FET to the PWM drive circuit through a
capacitor. However, no matter what value cap I choose, I compromise
either high duty or low duty cycle drive. Is there a solid state
solution that will not affect the waveform?

Thanks

Gerb
Put a current sense in line with either the C/E and have it operate
a comparator or some current sense transistor config to operate a
555 timer which will trip the Threshold input. The
discharge output can then be used to disable some circuit..

Tailor an RC L circuit from the current sense for the Threshold
input so that it reaches 2/3 Vcc voltage in the time it would
take for 2 time periods of a PWM..

How this works, if the pulses stop and it locks on or the output
transistor shorts, constant current will appear (Ice), and
force the Threshold on the timer to trip thus putting the 555 into a
latched state which will also set the discharge pin to low.
You will need to reset the timer with a power up or button to restart it.


http://webpages.charter.net/jamie_5"
 
An alternative solution might be a Polyfuse or something similar...
which could (depending on how sensitive the load is) also protect
against output stage failure.

Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
[email protected]             Info for manufacturers:http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com- Hide quoted text -

- Show quoted text -

I have not checked in a few years, but, I was unable to find a
Polyfuse that could handle this high of a current (can sustain peaks
of 25A for brief duration).

I did not mention this earlier, but, the temperature extremes I will
be working with might rule a Polyfuse out. The circuit must operate
in an environment -40C to 125C.

Gerb
 
J

James Arthur

Jan 1, 1970
0
Hello all,

I am designing an automotive product (operates 9 to 16VDC) that will
drive a regulated high current 10 amp load with a NPN low side drive
transistor. It will maintain average current to the load via PWM
(400Hz 1% to 99% duty cycle) and sense resistor.

My question, I want a way to shut the load down in the event the PWM
controller flakes out and gets stuck high. This would be a bad thing
since the load would remain on 100% and possibly be damaged. So, what
I am asking is...

Is there a cheap and easy way to detect the presence of a PWM signal
and allow operation, but, in the event the PWM gets stuck on for X
amount of periods, turn the output stage off?

I tried coupling the output FET to the PWM drive circuit through a
capacitor. However, no matter what value cap I choose, I compromise
either high duty or low duty cycle drive. Is there a solid state
solution that will not affect the waveform?

Thanks

Gerb

Is the output device a low side NPN, or a MOSFET ? (You mention
both.)

Anyway, this gets pretty close to what you're asking for:

C1
R1 | |----------
|| ___ |
---||--o-----o--|___|---| |<-. Q1
|| | | | |
| | | |--+
| .-. |
D1 - | | R2 ===
^ | | GND
| '-'
| |
=== ===
GND GND

(created by AACircuit v1.28.4 beta 13/12/04 www.tech-chat.de)


As long as C1 * R2 is several 400Hz time-constants, PWM
goes straight through and the charge on C1 is replenished
each 'off' time through D1.

If PWM drive hangs high, C1 discharges and the output stage
turns off. It's failsafe.

To work up to 99% duty C1 has to be able to charge through
D1 100x faster than it discharges through Q1 and R2. That's
easy for a MOSFET; driving a 10A bipolar switch might need
an active device or even a full stage instead of D1, but
the same idea applies.

Cheers,
James Arthur
 
J

John Devereux

Jan 1, 1970
0
Hello all,

I am designing an automotive product (operates 9 to 16VDC) that will
drive a regulated high current 10 amp load with a NPN low side drive
transistor. It will maintain average current to the load via PWM
(400Hz 1% to 99% duty cycle) and sense resistor.

My question, I want a way to shut the load down in the event the PWM
controller flakes out and gets stuck high. This would be a bad thing
since the load would remain on 100% and possibly be damaged. So, what
I am asking is...

Is there a cheap and easy way to detect the presence of a PWM signal
and allow operation, but, in the event the PWM gets stuck on for X
amount of periods, turn the output stage off?

I tried coupling the output FET to the PWM drive circuit through a
capacitor. However, no matter what value cap I choose, I compromise
either high duty or low duty cycle drive. Is there a solid state
solution that will not affect the waveform?

You have had various suggestions, but do you actually want to do this?
If it is a high average power that is dangerous, perhaps you should
simply detect this directly. For example is a continuous 99% duty
really that much safer than continuous 100%?

In which case a simple RC filter and comparator would work, assuming a
basically resistive load:


o--------[R]------|\
| | \
----- | ---- Alarm
----- | /
| -|/
GND |
REF
 
Hello all,
I am designing an automotive product (operates 9 to 16VDC) that will
drive a regulated high current 10 amp load with a NPN low side drive
transistor.  It will maintain average current to the load via PWM
(400Hz 1% to 99% duty cycle) and sense resistor.
My question, I want a way to shut the load down in the event the PWM
controller flakes out and gets stuck high.  This would be a bad thing
since the load would remain on 100% and possibly be damaged.  So, what
I am asking is...
Is there a cheap and easy way to detect the presence of a PWM signal
and allow operation, but, in the event the PWM gets stuck on for X
amount of periods, turn the output stage off?
I tried coupling the output FET to the PWM drive circuit through a
capacitor.  However, no matter what value cap I choose, I compromise
either high duty or low duty cycle drive.  Is there a solid state
solution that will not affect the waveform?

You have had various suggestions, but do you actually want to do this?
If it is a high average power that is dangerous, perhaps you should
simply detect this directly. For example is a continuous 99% duty
really that much safer than continuous 100%?

In which case a simple RC filter and comparator would work, assuming a
basically resistive load:

 o--------[R]------|\
              |    | \
            -----  |  ---- Alarm
            -----  | /
              |   -|/
            GND  |
                REF

--

John Devereux- Hide quoted text -

- Show quoted text -

First off let me thank everyone for their poropsals. A few good ideas
came to light that will be useful in the future.

However, you are absolutely right John. It is the power to the load
over time that is damaging. Not neccessarily the exact duty cycle.

My original thought was to shut things down should the micro flake out
and peg the output high 100% on for prolonged durations. But, if the
micro did the same thing, say at even 95%, the same net result would
occur.

Thank you for letting me look at the problem from a different angle.
Your proposed solution makes things perfectly clear.

Gerb
 
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