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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.

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#1

Re: Simple Switching Circuit

08/11/2017 12:47 AM
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#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.

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#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.

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#4
In reply to #3

Re: Simple Switching Circuit

08/13/2017 1:04 AM

GA!

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#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.

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#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?

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#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.

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#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...

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#11
In reply to #7

Re: Simple Switching Circuit

08/28/2017 5:52 PM

There does not seem to have been a simple answer so far, so this is my attempt, Galivan, I have used transistors and relays....

It uses two identical NTC thermistors in bridge circuit. The type I base design on are interchangeable (0.2 Celsius match) so if they are same resistance, you can be sure they are at same temperature and can be replaced without adjustment. The type below is from www.atcsemitec.co.uk, type SP.

If RL1/1 contact is open and all is at 25'C, NTC1 & NTC2 same resistance, then the bridge is balanced. Voltage A to B is zero.NTC1 is "room" and NTC2 is "hotbox". If NTC2 is hotter than NTC1, then voltage at A will be higher than voltage at B. At 25'C, A-B volts vary about 30 mV for 1 'C difference

However, if RL1 contact is closed, voltage A can exceed B only when NTC2 hotbox is above 50'C with pot at Max (5V), NTC1 staying at 25'C. Pot adjusts difference below 50 'C as required. Resistors can be increased to use a higher DC supply voltage to bridge. Log pot is better than linear because NTC are not linear.

To avoid self heating of thermistors, which are matchhead size, the current thro' NTC must be kept low - values shown give about 0.3 'C max rise. The relay contact must be a telephone type, with twin silver contact points, or a mercury reed relay, for reliable switching of the low voltage or a FET could be switched in place of RL1/1. A second contact of RL1 or another relay with parallel coil could switch fan.

If relay can be de-energised when Volt A greater than Volt B, then it will stay de-energised until the hotbox is cooled to room temp or near (adjusting 15k resistor feeding NTC1 or shunting NTC1 with resistor can adjust NTC1-NTC2 temperature offset to vary "near".

The following circuit for a differential amplifier can sense the bridge imbalance, by connecting input 1 and input 2 to A and B of bridge- I think it is 1 to A to work, if not swap over! Gdif is the gain. Probably easiest way to switch relay is use an FET fed from amplifier output. Q1 and Q2 should be same type of transistor, gain matched for best performance. Circuit diagram from "Art of Electronics" text book by Horowitz and Hill ISBN 0-521-37095-7.

The supply voltages and resistor values of the amplifier can be altered to more convenient values (values in diagram give gain of about 40, A-B difference to output). Ceramic bypass capacitors at Q1, Q2 base are advisable to avoid radio frequency pick-up trouble.

I will be away for a while.

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#12
In reply to #11

Re: Simple Switching Circuit

09/08/2017 7:15 PM

67:

You are awesome.

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#13
In reply to #12

Re: Simple Switching Circuit

09/11/2017 5:58 PM

Gavilan,

I got my SP type thermistors from rapid.co.uk for about $1 each [they still have 3.0kohm at £0.8], but I see that mouser.com have similar 232AP-2, which is 0.5'C tolerance as P/N 954-232AP-2 at $2.7 each.

It would be better to use an opto-coupler output in place of the Schottky diode, switched by the relay contact, for lower volt drop of a few tens of millivolt. The FET output type of opto-coupler would be even better. I got some opto-couplers from an obsolete computer phone-line modem [computer PSUs use them too] - datasheetarchive.com are good for finding the pin-out & performance from the part number.

67model

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#14
In reply to #11

Re: Simple Switching Circuit

11/09/2017 2:10 PM

What kind of unipolar transistor do you want to use here?

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#15
In reply to #14

Re: Simple Switching Circuit

11/10/2017 5:44 AM

The symbol in the diagram I reproduced in post #11 is for a common bipolar NPN transistor. I mentioned FET but not unipolar transistor. Device manufactureres and vendors do not describe the various types of FET as unipolar. I think I meant an enhancement mode MOSFET as a follower to the differential amplifier. This is just one way of getting the differential amp output to switch a relay.

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#9

Re: Simple Switching Circuit

08/22/2017 10:21 AM

I think the problem you describe indicates that you want heat from the solar collector when ever it is available. That could create a situation where on a moderate day, your shop would be too warm. The thermistor in the shop is your reference point. A difference between it and the solar collector is a relative relationship, but you don't want your shop overheating on a warm day. In that case you would need an override type of kill switch because that relative relationship is insufficient for your comfort.

Perhaps a couple of simple thermostats is all you really need. No IC's needed. They are simple switches that turn this into a simple control circuit.

Pick a couple so that when linked in series, TC(shop ~ 72°F) and TC(solar ~ 100°F) so that TC(shop) is closed when ever the shop is colder than say 72° but the TC(solar) does not kick on or Normally Open until 100° inside the chamber. They already have hysteresis built in so you don't have to worry about chatter.

Use your thermistors for something else. Look here for inexpensive and robust thermostats. You may find that you can run the fan directly through the thermostats and not require any external parts.

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#10
In reply to #9

Re: Simple Switching Circuit

08/25/2017 1:38 AM

Great answer ! But that's no fun.

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