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555 output to a Schmitt trigger.

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Plasma

Mon Nov 18 2019, 04:39AM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
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Posts: 268

Hi I've got a 555 IC with the output connected to a Schmitt trigger. When measuring from the 555 to ground on the DC setting shows 7.5DcV, from the 555 to 12Vcc measures 4.7DcV.
When measuring from the Schmitt output, to ground is 4.7DcV to 12Vcc is 7.5DcV.
Measuring on ACV shows nothing between those two outputs, but shows 3DcV.
It still displays 48khz.

Why is there no reading on the AC setting, shouldn't the inverter flip the polarities?
Can it still function to turn on two N MOSFETs? Fully?
Is this normal?

klugesmith

Mon Nov 18 2019, 06:10PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Pictures, please, to help us read your mind. Any reference to your posts in other threads should here be made explicitly.

Show at least 3 connections to your 555 and four connections to your Schmitt trigger gate. (Is it inverting? What part number?)
Drawing doesn't need to include wires to meter, if it shows one unique name for every node that meter might be connected to.

The behavior of multimeters in AC Volts mode depend a lot on the meter, even if there's no "true RMS" feature.

To help us classify your meter, please put it in AC volts mode and report the following tests:
1) probe across the AC output of a plain mains-frequency transformer.
2) repeat with probes reversed.
3) probe a DC source such as a 9-volt battery.
4) repeat with probes reversed.
Then put the meter in DC volts mode and tell us what it says for the same numbered set of connections.
Bonus points for AC and DC mode measurements, with & without probe reversal, of transformer output in series with battery.

You might feel like skipping some steps because you already know the answer, so it would be a waste of time.
If you act upon those feelings then I am done with trying to help you here.

After those observations, we can talk about the meaning of voltage and frequency readings at 48 kHz.

Plasma

Mon Nov 18 2019, 06:58PM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
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Posts: 268

Ac supply 9AcV a= 10.74
B=10.73
Ac battery 9 V both 0
Ac supply plus 9V battery
As=10.72
Bb=10.68
Ca=10.64
Db=10.66
Batter y DC setiings
A=9.58
B=9.58

The Dmm is true RMS, the 555 output goes to a inverting gate, with the readings coming from the output inverted gate. Both IC are connected to Vcc and Vss, 555 is working(unless I fried it trying to fix a bridge that only works with DC motors.

Hence the reason why I'm following Suleiman suggested bridge design, just replacing the npn for FETs.

klugesmith

Mon Nov 18 2019, 07:45PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Thanks for the quick, if confusingly terse, answers.

They put your meter's AC volts behavior in the "fancy" class instead of the "cheap" class (e.g. sub-$10 DMM's).
It properly rejects the DC component of applied voltage.
We can bet it's AC-coupled, so if you measured the meter's input resistance with another meter it would be infinite.

A possible weakness of true-RMS meters is the analog RMS-to-DC conversion IC.
They unavoidably have a longish settling time (seconds, for the 3rd and 4th significant digits).
But what about accuracy at 48 kHz? What does your meter's spec sheet say about that?

Not clear if your observations using DC volts mode showed proper rejection of mains-frequency AC,
when you had the battery and transformer in series.
I assume that reversing the probes changed the sign indicated by meter, a detail skipped in the report.

If voltage measurements between input and output of Schmitt trigger gate are still perplexing,
I'd start by measuring their static high and low levels. Use 555's _input_ pins to force the output logic state.
Then when you go back to 48 kHz, consider the rise times, fall times, duty cycle, and phase delay. Guess if necessary.
DC measurement of the live output can be a very good indication of the duty cycle.

klugesmith

Mon Nov 18 2019, 08:59PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Hey Plasma, would your observations make more sense if the duty cycle is far from 50%?

It's hard to get 50% from a 555.
One simple way is to run the 555 twice as fast, and clock a flip-flop IC that's configured to divide by two.
Then you automatically get 50%. As a bonus, you get complementary outputs with very low skew.
But the flip-flop outputs are probably pretty weak for directly driving big MOSFETs.

Here is a paper-only educational exercise for you.

Part 1: waveform switching between 0 and +12 V, alternating 15 us Low and 15 us High.
DC component = average voltage = +6 V.
AC component alternates between -6 V and +6 V, with middle = average = zero.
If rise and fall times are negligible, the true RMS value of AC component is 6 V. (why?)
The true RMS value of composite waveform, which most meters can't measure directly, is 12 V divided by sqrt(2).
The heating of a hypothetical load resistor to ground would be 1/2 as much as you would get from 12 volts DC, because it's only "on" half the time.

Part 2: now the waveform switches between 0 and +12 V, but it's Low for 20 us and High for 10 us.
What is the value of DC component?
What are the AC component's low, high, middle, and average voltages?
What's the true RMS value of AC component?
What's the true RMS value of composite waveform? (This one should be easy.)

Plasma

Mon Nov 18 2019, 11:19PM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268

The measurement of Ac supply and battery on DC setting is 9.58DcV, forgot to check that.
The Dmm goes from 10Hz up to 10MHz not sure on the error percent, but 48.78khz gets displayed.
The duty cycle has been 44/66/99% at that frequency.
Measuring a unused gate output of the Schmitt trigger shows 12.18DcV, to ground
The Schmitt trigger to ground, shows 4.45DcV that should make it 0.365% duty cycle?

So the reason it display low AcV between both output because of root mean squared =0
And the DcV is average of duty cycle from max and min voltage.

Thanks

radiotech

Tue Nov 19 2019, 09:55AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

I do not know a DMM that can measure AC voltage between 10 Hz and 10 Mhz.

( Even my Fluke 87 can't do that. It was about $600 when new.

Fluke 87 bandwidth 20 kHz with low pass filter; 3 dB @ 1 kHz)

Do you have access to an oscilloscope ?

Plasma

Tue Nov 19 2019, 04:06PM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268

Hi no I don't have a ossiclopescope but plan to get one that plugs into a computer.
The Dmm is mid range, but doesn't have inductance measurement, need to get a Ac/DC clamp Dmm would be good

050045

Sulaiman

Tue Nov 19 2019, 04:34PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I have a Fluke 87, but most of the time I use my little Aneng AN8002
(because buying a new AN8002 is cheaper than buying the two fuses for the Fluke 87 :)
The AN8002 has true rms up to about 1 kHz only.
but
if you make a full wave rectifier bridge (e.g. using 4x1N4448 or 1N4148),
with a coupling capacitor to the bridge, and a smoothing capacitor across the output of the bridge,
you can use the d.c. range (of any dmm) to measure a.c. up to around 100 MHz, vhf frequencies.
It will of course be peak reading, but still useful.

Unfortunately, to really see what is actually happening,
and to get an understanding of an electronic circuit,
an oscilloscope IS required.
Probably the greatest investment (other than learning, time and effort) required for the hobby.
I am completely out of touch with currently available options,
... looking on eBay I might go for a usb 'scope such as

because I have a laptop computer available.
if you do not have a PC then maybe a small hand held 'scope such as

but please do a lot of research before you commit to a very important investment.
I consider TRIGGER stability and flat BANDWIDTH the most important things,
but no matter what bandwidth you get, you will always want more later.

2Spoons

Tue Nov 19 2019, 10:37PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

Yep, A scope is a really worthwhile investment. Even an old analog CRT one - those are probably better spec than a lot of the cheap PC scopes for the same price.
Definitely go for as much bandwidth as you can - I chose bandwidth over channels when I bought mine (300MHz 2-ch vs 100MHz 4-ch) and I'm glad I did.

Also don't underestimate the value of using simulators for simple circuits - eg Simetrix (my fav), LTSpice, TI Tina etc. They're free and can tell you an awful lot before you touch a soldering iron.

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