Question about DC source switching

[swanann2000]

Hi everybody!

I want to switch between two DC sources (24V & 36V). These sources may supply a quite high current (0.5 A):


+------+
+24 Vdc o-|-o\ |
| \ |
| o-|-o Output (max. 0.5A)
| |
+36 Vdc o-|-o |
+------+
|
|
+------+
| |
| uC |
| |
+------+

The purpose of this cirucit is to transmit a low baud rate serial communication
signal (~10 KBaud) over a supply line:


V(t) |
| (Serial data)
36V -| +-+ +---+ +---+
| | | | | | |
24V -|----+ +-+ +---+ +----------- (DC)
|
|
|
|
+-------------------------------------> time

I've tried a very simple switch with a NPN & a PNP plus some resistors and a
diode, but the result hasn't been as expected, and the efficiency is quite low.
Besides, a 1 KOhm load resistor is needed in order to increase the switching ti-
me (from 36V to 24).
I would prefer to limitate the emission of the circuit, that is, the switching
shouldn't radiate too much.
Any one can help me? Any proposal is welcome

Thanx in advance!

 

[flatulent]

use fdm

You would be better off using a frequency division multipex system. At each end split the line into high and low frequency outputs. (Inductor to the power path and capacitor to the signalling path.) Use fsk on a vco at the sending end and a PLL at the receiving end for the data.

 

[swanann2000]

Thanks flatulent for you reply!!

I don't want/need a fsk modulation (this would complicate my design), actually, my design suports this low baud velocity (9600). I was thinking in a Mosfet conmutation with some second order circuit to attenuate the switching noise.

What do you thing about this?

Thanks a lot!!!

 

[flatulent]

try this

How about putting a regulator on the load end to eliminate the noise. Then use a series zener and a FET to short it out. This will give you the two voltage levels. Then at the load end use a voltage divider to get the levels down and then put it to a comparator.

 

[swanann2000]

Thanks a lot flatulent,,!!!!!!

But, how could be the schematic of the switching source?,
Actually at the end load, as you say , I put a TRT as a comparator to read the transmition...

a lot of Thanks!!!!

 

[flatulent]

schematic

The attachment is one way to make your functions.

 

[swanann2000]

Hi Flatulent!!!

Thanks for the schematic!!!!

My design is like yours, using npn & pnp bipolar transistor, but the switching induces a lot of noise of radiated emi, and it requires a load in the line.
I was thinking to implement it with power mosfets to reduce emi (or a ressonant circuit).... i don't know....

Thanks!!

 

[flatulent]

try this

Here are other options.

Run a second wire for the signalling.

Slow down the switching by putting a capacitor across the base-emitter terminals of the two transistors.

Put a series resistor and shunt capacitor at the input of the regulator.

 

[swanann2000]

I supose that you don't like the option of the mosfet.
Do you know where i could find the topology of the design?,

Thanks a lot!