Author Topic: Transistor as a switch?  (Read 2473 times)

0 Members and 1 Guest are viewing this topic.

Offline CacaoTopic starter

  • Contributor
  • Posts: 26
  • Country: ph
Transistor as a switch?
« on: August 01, 2017, 12:44:25 pm »
I plan to use transistor as a switch for my supply. Here is the situation: I have two 9v battery that supplies different ckt, but I need to place a switch that can cutoff the supply of both batt whenever one or both of it drops it's voltage rating to 4v. Will transistor suffice in this situation? Thank you in advance :D
 

Offline Codebird

  • Regular Contributor
  • *
  • Posts: 161
  • Country: gb
Re: Transistor as a switch?
« Reply #1 on: August 01, 2017, 12:52:43 pm »
Almost. A transistor is required for the switching, but it can't actually sense the supply voltage on its own. What you need is:
- A voltage reference (a zener will do).
- A comparator.
- Two transistors (one per circuit)
- Whatever resistors it takes to tie it all together.

I'd suggest the LM393 for the comparator, as open-collector makes it easy to OR the outputs together. One of us will probably be able to draw up a rough schematic if you give us all the details, it's a pretty simple circuit.
 

Offline CacaoTopic starter

  • Contributor
  • Posts: 26
  • Country: ph
Re: Transistor as a switch?
« Reply #2 on: August 01, 2017, 01:18:38 pm »
Almost. A transistor is required for the switching, but it can't actually sense the supply voltage on its own. What you need is:
- A voltage reference (a zener will do).
- A comparator.
- Two transistors (one per circuit)
- Whatever resistors it takes to tie it all together.

I'd suggest the LM393 for the comparator, as open-collector makes it easy to OR the outputs together. One of us will probably be able to draw up a rough schematic if you give us all the details, it's a pretty simple circuit.

Can this switch ckt be a lone ckt?
https://ibb.co/gt2RUk
« Last Edit: August 01, 2017, 01:25:31 pm by Cacao »
 

Online Ian.M

  • Super Contributor
  • ***
  • Posts: 12807
Re: Transistor as a switch?
« Reply #3 on: August 01, 2017, 01:31:42 pm »
How are the batteries interconnected?  If the answer is 'they aren't' and it isn't permissible to add an interconnection, you'll also need two optocouplers.

The image you linked to is useless - it isn't a schematic and doesn't make sense as a block diagram.
 
The following users thanked this post: Cacao

Offline CacaoTopic starter

  • Contributor
  • Posts: 26
  • Country: ph
Re: Transistor as a switch?
« Reply #4 on: August 01, 2017, 02:01:23 pm »
How are the batteries interconnected?  If the answer is 'they aren't' and it isn't permissible to add an interconnection, you'll also need two optocouplers.

The image you linked to is useless - it isn't a schematic and doesn't make sense as a block diagram.

sorry for the lack of details. Im still all planing the device. I will add a block diagram once I'm finish. Thank you for your time :D
 

Online Ian.M

  • Super Contributor
  • ***
  • Posts: 12807
Re: Transistor as a switch?
« Reply #5 on: August 01, 2017, 02:18:47 pm »
You (and we) need the block diagram near the start of the design process, not at the end.
Without the details, no-one can actually help you, unless they waste lots of time trying to guess what you need to do (not what you want to do) + you get lucky and realise their idea is applicable to your problem! :horse:
 
The following users thanked this post: Cacao

Online newbrain

  • Super Contributor
  • ***
  • Posts: 1714
  • Country: se
Re: Transistor as a switch?
« Reply #6 on: August 01, 2017, 05:31:58 pm »
As they say, help us help you!
Recommended reading: http://xyproblem.info/.

Once the problem is known, it will be easier to say whether "Transistor as a switch" is a good solution or not.
Nandemo wa shiranai wa yo, shitteru koto dake.
 
The following users thanked this post: Cacao

Offline danadak

  • Super Contributor
  • ***
  • Posts: 1875
  • Country: us
  • Reactor Operator SSN-583, Retired EE
Re: Transistor as a switch?
« Reply #7 on: August 01, 2017, 10:18:40 pm »
Do you want to switch low side or high side of battery ?

What is current load of each battery ?

Each battery share same ground but have different loads ?

Accuracy of V to switch at ?


Regards, Dana.
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 
The following users thanked this post: Cacao

Offline Brumby

  • Supporter
  • ****
  • Posts: 12288
  • Country: au
Re: Transistor as a switch?
« Reply #8 on: August 02, 2017, 12:26:58 am »
As already stated, there are a lot of questions that need to be answered for anyone to give you some clear pointers.

For starters (as mentioned above) we need to know the relationship between the two circuits and whether they can be interconnected.  If you don't know whether or not they can be interconnected, don't guess - just set out what you can so we can help you get answers.  Your given diagram sort of starts along this line - with the sources and loads ... but the switch portion doesn't add anything useful.

Another thing is to be careful about making design decisions too soon ... and understanding the difference between what you want to do and what you need to do.  Some things you may want to do might not be things you need to do - likewise, some of the things you might need to do may not be things you want to do.

Sometimes people are reluctant to share the full details about their project - and while that is understandable, it might be necessary to share a little more than you are comfortable with to get the answers that you need.  Some members here might ask more than seems necessary, but it might be for a reason that you had not even considered.  Either that, or from simple curiosity.  (We are a curious bunch  ;D )
« Last Edit: August 02, 2017, 12:29:34 am by Brumby »
 
The following users thanked this post: Cacao

Online Ian.M

  • Super Contributor
  • ***
  • Posts: 12807
Re: Transistor as a switch?
« Reply #9 on: August 02, 2017, 12:43:48 am »
Do you want to switch low side or high side of battery ?

What is current load of each battery ?

Each battery share same ground but have different loads ?

Accuracy of V to switch at ?


Regards, Dana.
Another question to add to Danadak's list for you:

What is the maximum voltage drop you can tolerate across the transistor switch when ON?

 
The following users thanked this post: Cacao

Offline djnz

  • Regular Contributor
  • *
  • Posts: 179
  • Country: 00
Re: Transistor as a switch?
« Reply #10 on: August 02, 2017, 08:48:07 am »
... A transistor is required for the switching, but it can't actually sense the supply voltage on its own...

Unless you use the loosely specified VGS(on) of a MOSFET as the cutoff voltage and the MOSFET as your comparator + switch. >:D

Not robust, but perhaps it can be done?
 
The following users thanked this post: Cacao

Offline CacaoTopic starter

  • Contributor
  • Posts: 26
  • Country: ph
Re: Transistor as a switch?
« Reply #11 on: August 06, 2017, 10:49:28 am »
I would like to thank you all for your time and response. I'll update my post as soon as I tidy up my thoughts on my project. Again thank you guys :D
 

Offline Gyro

  • Super Contributor
  • ***
  • Posts: 9410
  • Country: gb
Re: Transistor as a switch?
« Reply #12 on: August 06, 2017, 06:34:11 pm »
A few more question to add to your list...

- How much current is this circuit allowed to consume? You obviously want this to be very low in comparison to the circuit you are driving. It will also affect component choice, eg. voltage reference current, comparator choice, transistor vs mosfet etc.

- What happens when the batteries reach 4V? Most monitor circuits will continue to draw some current, if the batteries are rechargeable (I'm assuming not with that voltage) then it could damage them. If primary batteries then it might make them leak if forgotten for too long.
Best Regards, Chris
 
The following users thanked this post: Cacao


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf