Transistor as a switch? - Page 1
Forum section: electronics / beginners

author: Cacao
author_rank: Contributor
timestamp: August 01, 2017, 12:44:25 pm
content: 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

author: Codebird
author_rank: Regular Contributor
timestamp: August 01, 2017, 12:52:43 pm
content: 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.

author: Cacao
author_rank: Contributor
timestamp: August 01, 2017, 01:18:38 pm
content: Quote from: Codebird on August 01, 2017, 12:52:43 pmCan this switch ckt be a lone ckt?https://ibb.co/gt2RUk

author: Ian.M
author_rank: Super Contributor
timestamp: August 01, 2017, 01:31:42 pm
content: 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.

author: Cacao
author_rank: Contributor
timestamp: August 01, 2017, 02:01:23 pm
content: Quote from: Ian.M on August 01, 2017, 01:31:42 pmsorry 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

author: Ian.M
author_rank: Super Contributor
timestamp: August 01, 2017, 02:18:47 pm
content: 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!

author: newbrain
author_rank: Super Contributor
timestamp: August 01, 2017, 05:31:58 pm
content: 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.

author: danadak
author_rank: Super Contributor
timestamp: August 01, 2017, 10:18:40 pm
content: 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.

author: Brumby
author_rank: Unknown
timestamp: August 02, 2017, 12:26:58 am
content: 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 youwantto do and what youneedto do. Some things you maywantto do might not be things you need to do - likewise, some of the things you mightneedto 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)

author: Ian.M
author_rank: Super Contributor
timestamp: August 02, 2017, 12:43:48 am
content: Quote from: danadak on August 01, 2017, 10:18:40 pmAnother question to add to Danadak's list for you:What is the maximum voltage drop you can tolerate across the transistor switch when ON?

author: djnz
author_rank: Regular Contributor
timestamp: August 02, 2017, 08:48:07 am
content: Quote from: Codebird on August 01, 2017, 12:52:43 pmUnless you use the loosely specified VGS(on) of a MOSFET as the cutoff voltage and the MOSFET as your comparator + switch.Not robust, but perhaps it can be done?

author: Cacao
author_rank: Contributor
timestamp: August 06, 2017, 10:49:28 am
content: 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

author: Gyro
author_rank: Super Contributor
timestamp: August 06, 2017, 06:34:11 pm
content: 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.