Teensy’s and OpAmps

@Davee,

Thanks for the response. I am using a separate power source to provide the signal (the white power supply in the video), so it is not the Teensy. What would be the next step? I am going to build the schematic above (the one with the transistors and the three op amps) and will see how that compares.

As always, MANY thanks!

Jon

Hi @jonweisw,

   I assume you are using a PWM output from Teensy as the source for the variable voltage input to your circuit, as in your opening message you said " The controller has the ability to supply PWM 3.3v signals at 10mA max.", and continued to describe the unit you needed to drive, etc..

PWM stands for Pulse Width Modulation. This is a technique for producing 'variable' voltages from a digital circuit.

Simplistically. for a processor like the ESP32, a PWM output, (provided it is not connected to a low impedance load,) will be at (nearly) 3.3V or (nearly) 0V, at any given time. To produce "1.65V", it will repeatedly output 3.3V for a short time, and then 0V for the same short time. Hence, the "average" voltage will ((0 + 3.3)/2) = 1.65V.  Thus, it is Pulse Width Modulated, because the wider the "On" time, compared to the "Off" time, then the nearer the average voltage will be to 3.3V.

If the device being driven by this waveform has sufficient 'inertia', such then it will only show the 'average' result, then the result will be as required, but if the device can 'follow' the pulses, then it will display the pulsed nature. The 'inertia' can be 'real', if the device is a mechanical device such as a solenoid, or 'virtual', such as an LED.

For example, PWM is often used as a dimmer control on LEDs, where the power is switched at (say) 10kHz. To a human eye, the 'vision persistence' means we can only see individual flashes up to a rate of (say) 20 times per second, so it appears the LED is constantly lit, but a high speed video camera can capture it switching on and off.

I am  sure the Internet will have many articles describing PWM in more detail ... I noticed this one gave a simple introduction:

https://www.pjrc.com/teensy/td_pulse.html

In addition Bill (@dronebot-workshop) has discussed PWM on a number of occasions, including this article on driving motors:

https://dronebotworkshop.com/dc-gearmotors-pwm/

Note, it should now become obvious that if you have access to an oscilloscope, then examining the Teensy output signal could be very useful to help you decide how to proceed!

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As I do not know the parameters (time/frequency) of the PWM from your Teensy board, which will be largely defined in software, I do not know whether your 'output device' is trying to follow some of the pulses. Hence, my first suggestion is to test your circuit that is 'definitely' a variable DC voltage, not a PWM waveform.

I do not know whether your 'output device' will continue to twitch and jerk with the DC source, (because this is not the problem), or will become steady. But as with all the best detective stories, it is a case of examining each suspect in turn, and eliminating or incriminating them on the basis of evidence. This is my first suspect ... it may not be my last ... but I'll leave the others for another installment, if required!

I hope this makes a little sense.  I am sorry you are meeting a lot of new concepts, and this can be hard to begin with, but they will all become familiar in time.

Best wishes, Dave

@Davee,

Thanks for taking the time to respond. Yes, the end result will use the Teensy to power the signal source side of the circuit. But for testing, I am using a bench DC power supply which, I’m fairly confident, does not rely on PWM generated voltage. I will double check that (it’s a Siglent 3300). 

In the meantime, I tried it again today and oddly it is significantly smoother. Not 100% perfect, but about 95% there. Hooked it up to the Teensy and it works similarly.

Jon

Hi @jonweisw,

   OK, sorry.

Although virtually all contemporary commercial power supplies will be based on a variation of PWM, by design, they will limit the range of timings, and include filtering, to ensure the output has only a small residue ripple from the switching action, superimposed upon a 'solid' DC level. For the very cheap, switch mode modules available from the usual Internet vendors, that ripple might typically be about 50mV, and for a reputable source like a Siglent, I would expect any ripple to be orders of magnitude lower. Hence, if I had known you were using such a source, I would not have mentioned it.

By contrast, when someone is programming a microcontroller like a Teensy, they may pick virtually any timing values, and often do not include any hardware filtering, so unless I know exactly what the hardware and software is, I put it on the principal suspect's list, until proved innocent.

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Referring back to your previous message, you said "Also the gauge itself uses +15V, -15V and +/- as a reference voltage (+/- referring to the connection between the two power supplies). Could this be a problem related to the amperage issue you referred to above?"

I wasn't sure what you meant by that. I agree that the current demand values on the pair of power supplies looks very suspicious, but I am less clear what is causing it.

Unfortunately, I don't know anything about the internal circuit of the tachometer itself. I am not even clear as to why there are two pointers. The aircraft industry during the period of roughly 1900 to 1980 devised some ingenious electro-mechanical instruments to meet the requirements of the various tasks, which are unique to that industry. But, as I have never used or designed instruments from that period, I am struggling to know if there are any particular points I should be aware of. For now, I am assuming it is essentially a 'disguised' voltmeter for the range -10V to + 10V, with a fairly low input impedance (say 200 Ohm), and obviously scaled in engine RPM, rather than volts - but this may be far too simplistic. Any advice or pointers to relevant information could be helpful.

Best wishes, Dave