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From: Grant on 7 Aug 2010 16:37
On Sat, 07 Aug 2010 08:47:31 -0500, John Fields <jfields(a)austininstruments.com> wrote:

>On Fri, 6 Aug 2010 21:17:56 -0700 (PDT), stratus46(a)yahoo.com wrote:
>
>>On Aug 6, 6:59 pm, ravi <ravi2n...(a)gmail.com> wrote:
>>> How can design analog SPDT switch which switches VCC between +3.3V and
>>> 5V depending on application? I've an available pin on micro which can
>>> select the VCC. There are some ICs available to do this job but I
>>> would prefer to do it FET or Transistor. Anybody has idea or circuit?
>>>
>>> Thanks
>>
>>I wold use an adjustable regulator like an LM317 and use the FET
>>switch to add a second resistor in parallel for the lower voltage so
>>that the voltage will never exceed the higher of the two selections.
>>The resistor calculation is slightly more complicated but you can
>>handle it.
>
>---
>The problem with that approach, if the OP is trying to switch between
>a 5V and a 3.3V supply, is the LM 317's dropout voltage, which will
>vary from about 1.3V at 75C with 20mA out, to about 2.2V at 75C with
>1.5A out:
>
>http://www.national.com/ds/LM/LM117.pdf

Unless you're switching between regulating down from a higher
voltage, then the LN317 approach is useful. But OP asked to
switch between two voltages, not control a regulator. So
obviously an SDPT switch is either two p-channel or pnp devices
forming high side switch, or perhaps an 'hc405{2,3} if the load
is in the mA range.

Grant.
From: Jamie on 7 Aug 2010 19:09
ravi wrote:
> How can design analog SPDT switch which switches VCC between +3.3V and
> 5V depending on application? I've an available pin on micro which can
> select the VCC. There are some ICs available to do this job but I
> would prefer to do it FET or Transistor. Anybody has idea or circuit?
>
> Thanks
Feed the 5V through a couple or 3 diodes for the low side and bridge
this link with a PMOS high side to deliver the full 5? Depending on the
selected diodes, they can drop your voltage on the average of .3 to .6
volts each.

I don't know how stable your circuit must be but, its a simple
solution and the diodes will give you some kind of reg over a R
net work. How ever, if you know your load is constant, then use a
R inplace of the Diodes..

etc..

Just a thought..

From: default on 8 Aug 2010 07:41
On Fri, 6 Aug 2010 18:59:05 -0700 (PDT), ravi <ravi2neha(a)gmail.com>
wrote:

>How can design analog SPDT switch which switches VCC between +3.3V and
>5V depending on application? I've an available pin on micro which can
>select the VCC. There are some ICs available to do this job but I
>would prefer to do it FET or Transistor. Anybody has idea or circuit?
>
>Thanks

Is the application the availability of two different supplies and
something makes it desirable to have the ability to switch between
them, or a necessity to HAVE two different supplies?
--
From: John Fields on 8 Aug 2010 09:03
On Sat, 7 Aug 2010 13:38:54 -0500, "Jeff Johnson"
<Jeff_Johnson(a)Hotmail.com> wrote:

>
>
>"ravi" <ravi2neha(a)gmail.com> wrote in message
>news:dd623d43-6c24-4300-afd4-944764ccc40c(a)y11g2000yqm.googlegroups.com...
>> How can design analog SPDT switch which switches VCC between +3.3V and
>> 5V depending on application? I've an available pin on micro which can
>> select the VCC. There are some ICs available to do this job but I
>> would prefer to do it FET or Transistor. Anybody has idea or circuit?
>>
>
>Must easier than you might think! A simple mosfet will do the trick.
>
>
>
>VL---MOSFET---+---Vout
> |
> R
> |
> VH
>
>If the gate of the mosfet is off then Vout = VH. If the mosfet is on then
>Vout = VL.
>
>If VL = 3.3V and VH = 5V then applying 0V to the gate will give Vout = 5V
>and supplying 5V to the gate will hopefully give you Vout = 3.3V.
>
>There are some issues with this as the mosfet R_DSON and R form a voltage
>divider. If you cannot get a low enough turn on voltage then Vout may be
>significantly degraded. Also if you need to source appreciable current then
>R may end up being to low. R can be replaced with a p-ch mosfet with similar
>effect.
>
>VL---MOSFET1---+---Vout
> |
>VH---MOSFET2---+
>
>looks very similar to a SPDT switch.

---
Except that it won't work because of the MOSFET's body diode.
---

>You can do similar things with BJT's.

---
This seems to work:

Version 4
SHEET 1 880 724
WIRE 224 16 -144 16
WIRE 224 96 224 16
WIRE 48 144 0 144
WIRE 160 144 128 144
WIRE 0 192 0 144
WIRE 224 224 224 192
WIRE 400 224 224 224
WIRE 224 256 224 224
WIRE 0 304 0 256
WIRE 48 304 0 304
WIRE 160 304 128 304
WIRE 400 336 400 224
WIRE -144 432 -144 16
WIRE 0 432 0 304
WIRE 224 432 224 352
WIRE -144 624 -144 512
WIRE 0 624 0 512
WIRE 0 624 -144 624
WIRE 224 624 224 512
WIRE 224 624 0 624
WIRE 400 624 400 416
WIRE 400 624 224 624
WIRE -144 704 -144 624
FLAG -144 704 0
SYMBOL pnp 160 192 M180
SYMATTR InstName Q1
SYMATTR Value 2N3906
SYMBOL pnp 160 256 R0
SYMATTR InstName Q2
SYMATTR Value 2N3906
SYMBOL voltage 224 416 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 3.3
SYMBOL voltage 0 416 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value PULSE(0 5 0 1e-6 1e-6 .1 .2)
SYMBOL voltage -144 416 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value 5
SYMBOL res 384 320 R0
SYMATTR InstName R1
SYMATTR Value 100
SYMBOL Digital\\inv -64 256 R270
WINDOW 3 8 104 Invisible 0
SYMATTR InstName A1
SYMATTR Value trise 1e-7 tfall 1e-7 vhigh 5V
SYMBOL res 144 128 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R3
SYMATTR Value 1000
SYMBOL res 144 288 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R5
SYMATTR Value 1000
TEXT -88 656 Left 0 !.tran 1


Whether the OP is going to get something that works for him is going
to depend on the load current and what's available in terms of
Vce(sat) at that current.

JF

From: John Fields on 8 Aug 2010 11:57
On Sun, 08 Aug 2010 08:03:10 -0500, John Fields
<jfields(a)austininstruments.com> wrote:

>On Sat, 7 Aug 2010 13:38:54 -0500, "Jeff Johnson"
><Jeff_Johnson(a)Hotmail.com> wrote:
>
>>
>>
>>"ravi" <ravi2neha(a)gmail.com> wrote in message
>>news:dd623d43-6c24-4300-afd4-944764ccc40c(a)y11g2000yqm.googlegroups.com...
>>> How can design analog SPDT switch which switches VCC between +3.3V and
>>> 5V depending on application? I've an available pin on micro which can
>>> select the VCC. There are some ICs available to do this job but I
>>> would prefer to do it FET or Transistor. Anybody has idea or circuit?
>>>
>>
>>Must easier than you might think! A simple mosfet will do the trick.
>>
>>
>>
>>VL---MOSFET---+---Vout
>> |
>> R
>> |
>> VH
>>
>>If the gate of the mosfet is off then Vout = VH. If the mosfet is on then
>>Vout = VL.
>>
>>If VL = 3.3V and VH = 5V then applying 0V to the gate will give Vout = 5V
>>and supplying 5V to the gate will hopefully give you Vout = 3.3V.
>>
>>There are some issues with this as the mosfet R_DSON and R form a voltage
>>divider. If you cannot get a low enough turn on voltage then Vout may be
>>significantly degraded. Also if you need to source appreciable current then
>>R may end up being to low. R can be replaced with a p-ch mosfet with similar
>>effect.
>>
>>VL---MOSFET1---+---Vout
>> |
>>VH---MOSFET2---+
>>
>>looks very similar to a SPDT switch.
>
>---
>Except that it won't work because of the MOSFET's body diode.
>---
>
>>You can do similar things with BJT's.
>
>---
>This seems to work:

---
Snip...

Even better:

Version 4
SHEET 1 880 724
WIRE -176 -32 -304 -32
WIRE 288 -32 -176 -32
WIRE -176 0 -176 -32
WIRE 288 96 288 -32
WIRE -176 144 -176 80
WIRE -112 144 -176 144
WIRE 224 144 -32 144
WIRE 288 224 288 192
WIRE 416 224 288 224
WIRE -176 256 -176 144
WIRE 288 256 288 224
WIRE -80 304 -112 304
WIRE 48 304 0 304
WIRE 112 304 48 304
WIRE 224 304 192 304
WIRE -304 432 -304 -32
WIRE 48 432 48 304
WIRE 288 432 288 352
WIRE 416 432 416 224
WIRE -304 624 -304 512
WIRE -176 624 -176 352
WIRE -176 624 -304 624
WIRE 48 624 48 512
WIRE 48 624 -176 624
WIRE 288 624 288 512
WIRE 288 624 48 624
WIRE 416 624 416 512
WIRE 416 624 288 624
WIRE -304 704 -304 624
FLAG -304 704 0
SYMBOL pnp 224 192 M180
SYMATTR InstName Q1
SYMATTR Value 2N3906
SYMBOL pnp 224 256 R0
SYMATTR InstName Q2
SYMATTR Value 2N3906
SYMBOL voltage 288 416 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 3.3
SYMBOL voltage 48 416 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value PULSE(0 5 0 1e-6 1e-6 .1 .2)
SYMATTR InstName V2
SYMBOL voltage -304 416 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value 5
SYMBOL res 400 416 R0
SYMATTR InstName R1
SYMATTR Value 100
SYMBOL npn -112 256 M0
SYMATTR InstName Q3
SYMATTR Value 2N3904
SYMBOL res -192 -16 R0
SYMATTR InstName R2
SYMATTR Value 1000
SYMBOL res -16 128 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R3
SYMATTR Value 1000
SYMBOL res 208 288 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R4
SYMATTR Value 1000
SYMBOL res 16 288 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R5
SYMATTR Value 1000
TEXT -264 656 Left 0 !.tran 1

JF

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