Electronic Switches

[Fight]

Hello to all
i attatch diagram the resistor short or open thru switches these switches can handle thru relay
i want these switch are electronic switches how control thru BJT or other switching device  

[Hero999]

If you don't want mechanical relays then you can get solid state relays.

What are you trying to do?

There's probably an easier way.

[Fight]

if own design a saimple solid state relay please send any diagram

[Fight]

i trying a fixed resistor to variable resistor using no of fixed resistor and control using switches
these resistor 1,2,4,8,16 in steps
mean using fixed resistor control digital mode

[Hero999]

Why not use a digital pot?

[Fight]

Because digital pot is not available in this country

[Fight]

i try to buy a digital pot but not meet digital pot
NO
DS1803
AD7376

if meet the digital pot in market than this lower position only 64 position digital pot

[Hero999]

Four resistors with switches across them would only give 16 positions which is much worse.

You can use analogue switches (e.g. the CD4066) but it's better to just by a proper digital pot. IC.

You also need to beware that the maximum voltage on the pot or analogue switch can't exceed the supply voltage.

[Fight]

if i use cd4066
1.  this ic Open or close the resistor like other sitches (mean short and maximum current flowing)

2.  if short than max current handle

3.  if short than max Voltage handle = 15V or greater if i use 5V supply for ic than switch 15V can handle or not

[Hero999]

An analogue switch is not a solid state relay, the maximum voltage at either side of the switch cannot be higher than the power supplied to the IC. This is because the switches are made of MOSFETs and the gate drive is not isolated.

The schematic is on the datasheeet.
http://www.fairchildsemi.com/ds/CD%2FCD4066BC.pdf

The same also applies to digital pots which are simply resistor arrays with an array off analogue switches bypassing them, similar to your schematic but with additional logic gates.

[Fight]

solid state relay can hadle high voltage and current
any other method using SCR and other method control as switch

[Hero999]

An SCR will turn on and remain on until the current is interrupted.

Solid state relays are much more expensive than just buying a digital pot IC.

If you use an SSR, you also need to ensure it has a MOSFET output, not TRIAC which is AC only or SCR which can cause distortion.

[Fight]

I Attatch Diagram
if i use digital pot by replacing R2 that ok or not
and if digital pot using in variable power supply by changing voltage and current than ok or not

please send digital pot ic nos that available in market 

[Hero999]

The  power supply voltage to the digital pot will be need to be > the output voltage of the regulator - 1.25V

You could power the digital pot IC from the input voltage to the regulator and it will be fine as long as the voltage rating is high enough.