Convert 12V/24V logic signal to 5V #2

Thread Starter

AlphaGeek

Joined Sep 6, 2020
4
Yeah, One of the was mine. Where did it go? Is it unacceptable to reply to old stuff? I was interested in discussing the way people design voltage dividers. It seems my very first post ever on this site got deleted? Not feeling very welcome.



Hi AlphaGeek.
Welcome to AAC.

I will Create your own Thread.

Mod.
 

MrChips

Joined Oct 2, 2009
22,097
Hi @AlphaGeek,

Welcome to AAC!
There are some forum etiquette to follow on AAC forums.
A thread belongs to the TS (Thread Starter) the member who started the tread. It is considered bad form to use someone else's thread to advance your own queries. It is best to start a new thread.

It is also not a good idea to post in a thread that is more that 6 months old unless one has very pertinent information to add, and not to ask more questions.
 

Thread Starter

AlphaGeek

Joined Sep 6, 2020
4
Web forums aren't a medium I'm accustomed to (I am most familiar with Facebook), so I'll try again. It was just a general comment/blatant troll.

I see a lot of voltage dividers designed with much lower resistor values than make sense to me. Yes, with a BJT, you need to allow enough current to bias the device, but I often see examples where folks will make a voltage divider with two 1K resistors when two 10K resistors would possibly work as well. I'm curious opinions on power consumption from design decisions - specifically low resistor values in voltage dividers - which can produce waste heat for (what to me seems) no purpose. This would seem especially important for designs intended to operate from a battery. Yes, I know we're talking about fractions of milliamps here sometimes.
 

MrChips

Joined Oct 2, 2009
22,097
You need to pay attention to source-to-load ratio, or vice versa.
The general rule of thumb is to make the load-to-source resistance ratio 10 or greater, i.e. the load should not take more than 1/10 of the source current.

For example, if the load draws 10mA, then the source should supply 100mA or more.
In this way, your voltage at the load will not deviate by more than approx. 10%.
 

dl324

Joined Mar 30, 2015
11,894

Thread Starter

AlphaGeek

Joined Sep 6, 2020
4
@AlphaGeek, don't know if you were able to read my response before posts were deleted and/or moved around.

https://forum.allaboutcircuits.com/threads/convert-12v-24v-logic-signal-to-5v.144428/post-1553053

I gave the heads up because some of us were recently tricked in to responding to an old post that got resurrected by a new member. Since I had gotten bitten recently, I paid attention to the dates of the earlier posts in the thread you posted to.
I actually did see it. Thank you.
 

soyez

Joined Aug 17, 2020
51
Yeah, One of the was mine. Where did it go? Is it unacceptable to reply to old stuff? I was interested in discussing the way people design voltage dividers. It seems my very first post ever on this site got deleted? Not feeling very welcome.



Hi AlphaGeek.
Welcome to AAC.

I will Create your own Thread.

Mod.
Don't feel that way you are absolutely welcome
 

BobaMosfet

Joined Jul 1, 2009
1,229
Web forums aren't a medium I'm accustomed to (I am most familiar with Facebook), so I'll try again. It was just a general comment/blatant troll.

I see a lot of voltage dividers designed with much lower resistor values than make sense to me. Yes, with a BJT, you need to allow enough current to bias the device, but I often see examples where folks will make a voltage divider with two 1K resistors when two 10K resistors would possibly work as well. I'm curious opinions on power consumption from design decisions - specifically low resistor values in voltage dividers - which can produce waste heat for (what to me seems) no purpose. This would seem especially important for designs intended to operate from a battery. Yes, I know we're talking about fractions of milliamps here sometimes.
You are asking a good question- something many lesser knowledgeable don't ask- and they should.

In the DC domain, a voltage divider does 2 things- it divides a single voltage level ratiometrically (or 'fractionally') and the sizing of the resistors control the max current output on the output side. You need to know how much current the load is going to take, and this determines the resistor size for at least one of the resistors. The other resistor can be a much higher value as it is not passing current, it is simply being used for the voltage ratio aspect.
 

OBW0549

Joined Mar 2, 2015
3,566
I see a lot of voltage dividers designed with much lower resistor values than make sense to me. Yes, with a BJT, you need to allow enough current to bias the device, but I often see examples where folks will make a voltage divider with two 1K resistors when two 10K resistors would possibly work as well. I'm curious opinions on power consumption from design decisions - specifically low resistor values in voltage dividers - which can produce waste heat for (what to me seems) no purpose. This would seem especially important for designs intended to operate from a battery. Yes, I know we're talking about fractions of milliamps here sometimes.
Your instincts are correct. There's no point in making the resistors in a voltage divider any smaller than they need to be; if they're a lot smaller, the only result is wasting power and, if a device is battery powered, shortening the time until the battery needs recharging.

Picking an appropriate size for the resistors in a voltage divider is largely a judgement call, and comes down to how much load current (and load current variation) is involved. Many people use a rule of thumb that the divider resistors should be chosen such that the current through the divider resistors is at least 10X the maximum load current that will be drawn from the divider, thus ensuring that the divider voltage will not vary more than about 10% from no load to full load.
 

soyez

Joined Aug 17, 2020
51
Don't feel that way you are absolutely welcome
  1. Step 1: Take All Components As Shown Below and in Pictures. Components required - ...
  2. Step 2: Connect Input Power Supply. 7805 voltage regulator contain three pins.In which we have to give power supply on pin-1 and pin-2. ...
  3. Step 3: Output Power Supply. ...
  4. Step 4: Wiring Is Completed. ...
  5. Step 5: Checking.
 
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