Help explain why these components are here.

Thread Starter

pltom55

Joined Jul 13, 2017
4
Hi
can you please explain why the in electrolytic caps are in series. (C6 and C3)
and what is the purpose of D4, i know d3 is used to protect against incorrect voltage input.
and what is the purpose of c4

Thanks in advance
Tom
 

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SLK001

Joined Nov 29, 2011
1,543
They are DC blocking capacitors. D4 prevents too high of a signal getting to Q1. C4 is a high frequency bypass.
 

AnalogKid

Joined Aug 1, 2013
8,440
can you please explain why the in electrolytic caps are in series. (C6 and C3)
and what is the purpose of D4, i know d3 is used to protect against incorrect voltage input.
and what is the purpose of c4
C6 and C3 are DC-blocking, AC coupling capacitors. They let the signal through between stages while letting each stage be biased to a different DC operation point.

D4 clips large negative signal peaks to -0.6 V. Very large signal peaks can present a large negative
voltage across the Q1 base-emitter junction, weakening or destroying it.

C4 is called a Miller capacitor, after John Miller.

https://en.wikipedia.org/wiki/Miller_effect

It turns the transistor into an integrator, a form of low pass filter. Q1 is acting as a combination half-wave rectifier and current amplifier to drive the D2 LED and whatever the external load is. C4 filters off the high frequencies so the LED blinks in time with the music beat rather than flickering too fast to see as it follows the high notes.

ak
 

Jony130

Joined Feb 17, 2009
5,162
It is hard to say something about C6 and C3 exemptrthat they will pass AC only and block DC. But if the signal frequency is "low "they can help"detect" the edge (shorten the duration of the pulse).

D4 is needed to provide a closed loop pathe for C3 discharge current when Q2 turns-OFF and clips the voltage at the Q1 base to -0.6V. Widout this diode the circuit will only work once. The capacitor will charge but no way to discharge. For C6 capacitor R6 resistor( pull-up resistor) will provide this pathe.

D1 and D3 is a simple protection diodes.
 

ian field

Joined Oct 27, 2012
6,539
Hi
can you please explain why the in electrolytic caps are in series. (C6 and C3)
and what is the purpose of D4, i know d3 is used to protect against incorrect voltage input.
and what is the purpose of c4

Thanks in advance
Tom
The coupling capacitors have decreasing resistance with faster signal edges, I would assume the series resistors limit current peaks.

Its pretty much the same thing as ESR - which you'd want as low as possible in a SMPSU reservoir/filter cap, the designer of that circuit decided it may not be such a bad thing.
 

AnalogKid

Joined Aug 1, 2013
8,440
This looks like part of an infrared remote control extender or something like that. If U1 is an IR receiver, then its input is bursts of high frequency carrier at a low frequency rate, and the output is the low frequency square wave. This is amplified and limited by Q2. R3 and R5 limit the two transistors' base currents to approx. 1.0 mA and 0.8 mA. Q2 is a saturated switch. Q1 is a half-wave rectifier AM detector and lowpass filter. With the right corner frequency, LED D2 is on continuously as long as there is a valid IR signal input.

ak
 
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