CR4 - The Engineer's Place for News and Discussion ®


Previous in Forum: PCI to ISA   Next in Forum: Convert Ethernet to Serial RS 232 Modbus and Use Modscan Software, Possible?
Close
Close
Close
8 comments
Guru

Join Date: Sep 2007
Location: East Texas
Posts: 1252
Good Answers: 32

Simple Switching Circuit

08/10/2017 10:47 PM

Some number of years ago you folks helped me come to an inexpensive, simple, and robust circuit design used as a dusk to dawn switch for LED lighting. I used many of these circuits for both solar powered and grid powered LED lighting projects.

I now want to build a simple and robust DC switching circuit that puts forward bias to the base of a power transistor when a voltage drop is sensed across two identical thermistors.

One thermistor will be in a solar collector air box.

The other identical thermistor will be in my shop.

When the temperature is warmer in the solar collector air box than the air in my shop I want the circuit to forward bias the base of a power transistor. I will use the power transistor to activate a relay which will switch a fan on and off.

Any information that could be offered me would be greatly appreciated.

__________________
"The greater danger for most of us lies not in setting our aim too high and falling short; but in setting our aim too low, and achieving our mark." -- Michelangelo
Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".
3
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 18292
Good Answers: 1061
#1

Re: Simple Switching Circuit

08/11/2017 12:47 AM
__________________
Life is like riding a bicycle. To keep your balance you must keep moving. A.E.
Register to Reply Good Answer (Score 3)
Guru

Join Date: May 2006
Location: Placerville, CA (38° 45N, 120° 47'W)
Posts: 4399
Good Answers: 158
#2
In reply to #1

Re: Simple Switching Circuit

08/12/2017 12:10 AM

I do see one weakness in that circuit: since there is no feedback, there is no hysteresis, so when the two inputs are very close to each other, the output could chatter or oscillate. A very high resistance from the output to the positive input could prevent that.

__________________
Teaching is a great experience, but there is no better teacher than experience.
Register to Reply
3
Guru
Hobbies - DIY Welding - Wannabeabettawelda

Join Date: May 2007
Location: Annapolis, Maryland
Posts: 4036
Good Answers: 222
#3
In reply to #1

Re: Simple Switching Circuit

08/12/2017 1:29 AM

GA SE. This circuit is a very good start, but it needs a few things to keep it 'happy' in the real world.

First, we should add a little hysteresis to keep the amplifier/comparator from oscillating. What can happen at the time that the comparator changes state, the change in current tends to cause a little voltage droop which can cause the comparator to fall back to its previous state and start all over again. And again, and again. Below is a link to Analog Devices white paper on hysteresis. This is one case where positive feedback in electronics is actually a good thing. The 1Kohm and 1 MegOhm resistors will take care of the hysteresis. You may want to experiment a bit with the value of the 1 MegOhm resistor to see what deadband you get with the switching.

http://www.analog.com/en/analog-dialogue/articles/curing-comparator-instability-with-hysteresis.html

I would also replace the PNP transistor with an N-channel MOSFET, like a BUK9Y22-30B.

https://www.digikey.com/product-detail/en/nexperia-usa-inc/BUK9Y22-30B,115/1727-4615-1-ND/2531005

This MOSFET has a Vgs of +/-15V and handle 30 amps of current with proper heatsinking. That will consume less current because you won't be driving any base current. Make sure the Vgs (gate to source) rating is at least your supply voltage. There are many others from any number of sources that would work just fine as well.

Use twisted wires to connect to your thermistors. Old telephone wire or a pair from a CAT-whatever Ethernet cable is good.

A DC fan on the circuit is going to add a bunch of noise to the bus. Help filter out the noise by adding a ten ohm resistor in the positive connection to the op-amp/comparator and a 1 uF ceramic capacitor across the power terminals of the amp. That will help prevent supply noise from affecting the comparator. Likewise for the reference voltage and the thermistors. A ferrite bead would also be a good idea to add into the positive supply rail between the motor/relay and the 'electronics'.

Add a 50K resistor from the MOSFET gate to ground to make sure that when the comparator goes low, the 50K will help bleed charge off the gate to keep it turned off. This can be an issue when you use a single supply with a comparator.

I did this revision to this sketch entirely with MICROSOFT PAINT!!! http://cr4.globalspec.com/blogentry/28499

Good luck with your quest.

Register to Reply Good Answer (Score 3)
Guru

Join Date: May 2006
Location: Placerville, CA (38° 45N, 120° 47'W)
Posts: 4399
Good Answers: 158
#4
In reply to #3

Re: Simple Switching Circuit

08/13/2017 1:04 AM

GA!

__________________
Teaching is a great experience, but there is no better teacher than experience.
Register to Reply
Guru

Join Date: Sep 2007
Location: East Texas
Posts: 1252
Good Answers: 32
#5

Re: Simple Switching Circuit

08/16/2017 12:36 AM

Think this would work?

I don’t like using ICs for three reasons. I don’t see so good, I don’t solder so good, and I don’t understand how they work unless I can see the equivalent circuit and it is real simple.

What I like about your linked design is the variable speed; but I don’t understand how it works.

After giving this more consideration I’m going to play around with a simple resistive balance as the base control signal for a common Power Resistor.

The bridge for the balance will be two thermistors; one PTC and one NTC. As the temperature gradient changes between the air inside the collector (NTC) and the air in the shop (PTC) the net resistance of the parallel bridge will change as two resistors in parallel. Using the proper values for the base resistor and the two thermistors it should be possible to switch fans on only when the temperature in the solar collector is higher than in the shop. AND, I think I will understand how it works which is an important part of all this.

I tried to show the circuit but this is all I could get up on the post. I made it in paint converted it to a jpeg and uploaded it as a photo and this is what I got.
Any suggestions on how I can show the circuit.

__________________
"The greater danger for most of us lies not in setting our aim too high and falling short; but in setting our aim too low, and achieving our mark." -- Michelangelo
Register to Reply
Guru

Join Date: May 2006
Location: Placerville, CA (38° 45N, 120° 47'W)
Posts: 4399
Good Answers: 158
#6
In reply to #5

Re: Simple Switching Circuit

08/16/2017 1:34 AM

It is no harder to solder on simple ICs than on small transistors. If you don't have one, get a pair of jeweler's loupes.

https://www.amazon.com/HEADBAND-MAGNIFIER-JEWELRY-GLASSES-BINOCULAR/dp/B014PVVCTM/ref=lp_9361539011_1_2?srs=9361539011&ie=UTF8&qid=1502861272&sr=8-2

I think you want both temperature sensors to have the same slope (either both NTC or Both PTC. You could use either, depending on how your circuit is arranged.

When you converted to jpeg, did you specify the highest possible resolution?

__________________
Teaching is a great experience, but there is no better teacher than experience.
Register to Reply
Guru

Join Date: Sep 2007
Location: East Texas
Posts: 1252
Good Answers: 32
#7
In reply to #6

Re: Simple Switching Circuit

08/16/2017 8:16 PM

Thanks for the comment D.K.

The reason I am considering the use of both an NTC and PTC is to prevent the fan from turning on when the shop temperature is higher than the solar collector box temperature.

I am still experimenting with different resistances for the base resistor and two thermistors. I am using two resistive pots in place of thermistors to determine the ohmic values of on and off for my TIP31A control transistor.
This monkey is on the learning curve. When I come to a solution it may not be eloquent; but it will work.

I want to thank you guys for your input. As usual I may not have followed the presented path but it helped me to see another one.

__________________
"The greater danger for most of us lies not in setting our aim too high and falling short; but in setting our aim too low, and achieving our mark." -- Michelangelo
Register to Reply
Guru

Join Date: May 2006
Location: Placerville, CA (38° 45N, 120° 47'W)
Posts: 4399
Good Answers: 158
#8
In reply to #7

Re: Simple Switching Circuit

08/17/2017 12:21 AM

You're welcome. Unfortunately, I'm getting ready to go up to Oregon to see the eclipse. I'll be back in a week. If you haven't got it worked out by then, I can probably help some more...

__________________
Teaching is a great experience, but there is no better teacher than experience.
Register to Reply
Register to Reply 8 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".
Copy to Clipboard

Users who posted comments:

Brave Sir Robin (1); dkwarner (4); Gavilan (2); SolarEagle (1)

Previous in Forum: PCI to ISA   Next in Forum: Convert Ethernet to Serial RS 232 Modbus and Use Modscan Software, Possible?

Advertisement