Pwm and frequency questions ?

[curry87]

Couple of newbie questions about this pwm,frequency,duty cycle:

Whats the difference between pwm,frequency and duty cycle ?


Do infrared leds which have been modulated at 38khz for example use a different amount of current than if they had been connected 100 duty cyle all the time to the power rail using the same limiting resistor?


Does using pwm on the base of a bi transistor to modulate the load(IRLED) effect the fully on saturation requirements of the bi transistor ?


Does changing duty cycle effect frequency ?

 

[KeepItSimpleStupid]

IR communication also uses PCM pulse code modulation.

Suppose you had a square wave of a particular frequency and the on and off time was the same. That is a 50% duty cycle. PWM or pulse width modulation changes the duty cycle. If the on time was 10% of the period and the off time wass 90%, then you have a 10% duty cycle. The frequency doesn't change because the repetative event still occurs x times in a given time y. Frequency is x events per second.

Your second question is best answered by using power, not current. Power is the integral of I*V dt over a given time. Thus, a 50% duty cycle uses half the power as something on 100% of the time. The peak cuurent for the LED would be the samein both cases.

PCM/PPM, Pulse Position Modulation on the other hand uses a single fixed width pulse but changes the off time. IR communication can use a mix of these techniques to send a code, henceit can be called Pulse Code Modulation.

Does using pwm on the base of a bi transistor to modulate the load(IRLED) effect the fully on saturation requirements of the bi transistor ?

No.

 

[curry87]

So if a infa led is modulated at frequency xkhz and the duty cyle is 50 % i can:
A.Put twice the amount of max current though it ?
b.If i used a transistor to drive it the max limits of the trans can be be doubled because the load(infaled) is only on half the time ?

 

[ronv]

Normally that is true but....... you need to check the data sheet on the led for peak current and the transistor for maximum collector current or safe operating area.

 

[audioguru]

The chip in an LED is very small so it heats very quickly. I have some 5mm red LEDs that have a max current rating of 40mA when the ambient is 25 degrees C or less. The peak current rating is 200mA only when the pulse is 10% at 1kHz so the pulse durations are only 100us.

Remote controls for TVs blast very narrow 1A current pulses through the IR LEDs.

 

[Nigel Goodwin]

So if a infa led is modulated at frequency xkhz and the duty cyle is 50 % i can:
A.Put twice the amount of max current though it ?
b.If i used a transistor to drive it the max limits of the trans can be be doubled because the load(infaled) is only on half the time ?

Pretty well - but that's not how it's done.

Generally you drive the 40KHz at 50% duty, BUT you only send a fairly small number of cycles, then a space to let it cool down. In this way 1A drive currents are easily achieved.

 

[KeepItSimpleStupid]

Here is aa particular IR data sheet: https://www.electro-tech-online.com/custompdfs/2010/11/QED234IRLED.pdf

Continuous current is at 100% duty cycle.

Peak current is at a specify period, hence frequency, and pulse width.

 

[Nigel Goodwin]

Here is aa particular IR data sheet: https://www.electro-tech-online.com/custompdfs/2010/11/QED234IRLED-1.pdf

Continuous current is at 100% duty cycle.

Peak current is at a specify period, hence frequency, and pulse width.

The datasheet doesn't mention any of those things, just contiuous current (1.5A), and peak current (100mA).

 

[audioguru]

Correction: For the Fairchild IR LED:
The max continuous current is 100mA and the max peak current is 1.5A.
Note 5 on the datasheet says, "5. Pulse conditions; tp = 100 μs, T = 10 ms".

 

[Nigel Goodwin]

Correction: For the Fairchild IR LED:
The max continuous current is 100mA and the max peak current is 1.5A.
Note 5 on the datasheet says, "5. Pulse conditions; tp = 100 μs, T = 10 ms".

Well damn! - only 1% duty cycle then.

 

[curry87]

Does changing the duty cycle of a frequency effect the voltage that is measured by the multimeter if so why?

 

[Nigel Goodwin]

Of course it does, that's the reason for doing PWM.

The reason is pretty obvious, the more something is ON the more power is tranferred to the load.

Say you have 12V, if it's ON all the time, the output is 12V, if it's OFF all the time the output is zero volts. So guess what happens if it's ON 50% of the time and OFF 50% of the time.

 

[KeepItSimpleStupid]

A "real" multimeter will read 6V when presented a 50% duty cycle of a 12 V waveform. A "real" multimeter is defined as one that is TRUE RMS and has the required frequency response and a 0% error.

A run of the mill multimeter will have a poor frequncy response and is "Average responding", RMS reading which means it can only measure sine waves correctly.

One way to think of this is that the AC voltage value measured should be the same as the same amount of DC to heat an object to the same temperature in the same amount of time.

The "run of the mill multimeter" usually precision rectifies the AC signal and averages it with a capacitor; pretends it''s a sine wave and multiples it by a constant.