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Please help me with High Low voltage cutoff with time delay

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Power line fluctuation is very common in my country, so was looking to build a Hi/low cutoff circuit for my power inverter which do not have one.

I hv found this circuit that looks nice,



I hv built some circuit before and can breadboard one if needed.  But the one I'm interested in uses LM741 op-amp, but I got one LM393(comparator) available with me, so can someone please help me convert above circuit to use LM393(Comparator) Instead of LM741 op-amp.

I have already got the linear supply as was in above circuit detail. I was hoping to simplify the circuit with a single ic LM393 Instead of two 741's.

Also, I cannot find SL100 NPN transistor, But got some 2n2222a and BC547's, will they work instead of sl100 ?

Please help me build this circuit, as I need the inverter and hi/low protection for it, in fact writing this without line power i.e. backup power from the inverter.

My power line 230VAC 50 Hz

I have got one more question, How to protect the power supply circuit and transformer when the power line goes beyond 270(some times it does) any ideas ?? ....

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The circuit is horribly drawn.

I see not need for a 555 timer, three comparators could be used allowing you to use a single LM339.

The questions are what are the voltage limits and the time delay?

A simpler option is to use an RC filter so the delay is dependant on the magnitude of the over/under-voltage, rather than a fixed delay. It will behave in a similar manner to a fuse, a minor under/over-voltage will take a long time to trip, a more severe under/over-voltage will trip more quickly.

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hello hero999,

Thank you for your reply.

As I hv mentioned before my power line voltage 230vac at 50 Hz

What my plan was to build a small unregulated power supply that can feed the circuit and can monitor real time ac voltage.

My No cutoff zone is  : 170 vac ~ 270 vac.

Any voltage above or below shall have the logic(circuit) cutoff the power through the relay, which would be used to supply power to my appliance/inverter.

Regarding the delay : Was hoping for a fixed time delay for 30sec to 1 minute.  If after delay, line voltage came back within the "no cutoff zone" then power should be restored(relay off ) otherwise hold power cut for another delay period..

LM393 or LM339 : I see what u'r saying...my best bet is to use a LM339 which can control the entire logic.  if we use LM393 then shall probably need another ic as a timer...please let me know if i'm confusing you.

Hope I could make myself clear.

appreciate your help..

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The trouble with that is you need a to do some experimentation, unless you use a calibrated voltage measuring transformer. Using a general purpose transformer won't give you a predictable result because the regulation factor will be poor, for example a transformer with a primary voltage of 240V and a secondary voltage of 12V will give more than 12V unloaded, it could be as much as 15V. The only way is to use a variac on the primary and measure the voltage on the rectifier when the circuit is connected to it, then determine the component values.

You also need to get a mains transformer which can operate of over 270V without overheating. What's the maximum voltage you want connected to this device? I know you said a cut of of 270V but can it be connected to higher voltages?

You might be all right with a standard mains transformer if you oversize it.

The solution is just a windows comparator connected to a timer.

Here's a link to a comparator tutorial which discusses window comparators and all the calculations.

Attached is an example. The relay will be on when the Vin is between 5V and 10V and off when it's outside that range. The turn on time is nearly instantaneous. The turn off time is equal to 1.1C*R6.

Vs is the supply voltage to the circuit and can be the same as Vin.
Vin is the voltage you're measuring.
Vref is a fixed reference voltage. It can be from a zener diode, a reference IC or taken from Vs, via a porential divider (just add another resistor in series with R3) providing Vs is regulated.

U1A and U1B form the window comparato, see the tutorial I linked to above for more information.
U1C is just a logic inverter, the + input could be connected to Vref rather than R3 and R4, in fact any voltage which will certainly be 3V under Vs or over about 50mV. The output discharges C1 which resets the timer,
U1D is a Schmitt trigger combined with Tr1 to boost the output enough to drive the relay. It functions as a timer with a delay dependant on R6 to R9 and C1. It's better to not play with the values of R7 to R9 and just stick with altering R5 and C1. Don't use too larger capacitors for C1 otherwise it could destroy the output transistor on U1C and it will take longer to reset.

U1 must be a comparator with open collector outputs. It won't work with a quad op-amp such as the LM324. The LM339 is perfect.

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Hello hero,

Thanks and appreciate your response.

First I must say..You are the man..as from your reply I can guess you are an expert in this side of the terrirory..though this kinds of situation may not be the case in your country.

Also, I'm not very keen to this circuit things...(I did built some on veroboards with schemetics). As I started learning from books and forums such as yours on the internet...believe me I'hv learnt a lot.  But for a short time it is overwelming..So, Please bear with me...you may have to spoon feed me, though I shall try to grasp ideas as much and as quickly as possible.

Transformer : As you say the circuit needs very little current. If I get a 230vac 12x2 3amp center tapped tranformer, will that be oversize enough ? , let me know. The transformer must hv to withstand voltages ranging from 230 to 350/400 volts. Most of the times may be 230 to 330 volts. I hv built one rectifier circuit with a 230vac 12x2 1 amp transformer.  My inverter can be set to supply voltage ranging from 250vac to 320vac..I tried 320vac with 250 ma dc fan connected(with the PS), it gets hot but sustained 2 hours of backup time.

The different voltages and power supply: Vs as per you must be regulated...so I can build one side of the powersupply rectifier with a fixed regulator LM7812 and feed the circuit. But when the relay will operate..it shall take the voltage down so I must get separate supply for Vin, don't u think? .  Also Vin is the voltage that I'm
going to measure...so this cannot be regulated...so I shall use no regulator just a 2 capacitors.. say 400uf and 0.1uf and use the other side of the transformer with common ground(center wire), But if I do not regulate it, as per u the voltages would not be 10v instead it would be 14v to 15v. In order to feed U1B comparator a 10vdc I can change the devider R1 and R2.

Vref/U1C : to get fixed 5vdc, I hv seached google. their a many #'s(zener doide) but most are 5v 1/2 watt, will it suffice?, or I need more power i.e. 1 watt, what u'r openion. Here U1C is the delay timer, am i right ? changing c1 capacitor I can change the timing, 100uF, is for what time delay ?

U1D Relay driver and tr1: U1D is a Schmitt trigger and relay driver. I would like to use a 12vdc relay, rated 10a at 250vac and 30vdc, is that enough to switch on the a/c mains of the inverter ? let me know. Now what can be TR1, I got following PNP Power transistors with me, BD140, BD242, B649A(2SB649), A614Y -can any above work as tr1, or do i need higher powerd P-ch mosfets, let me know ?

variac and comparator U1A/B : I do not hv one(variac) nor can i get one. I do know it can work like a VR/Preset and vary voltage . But I can set the voltage limits with a few Variable resistors/Presets connected at the two end of window comparator, may be R2 and R4 as 100k VR/Presets? Let me know the right choices, if any. This way I can set the 10vdc for corresponding 270vac supplied at Vin and 5vdc for 170vac.

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Sorry, I don't spoon feed people but I'll give you some more information.:

Have a look for control panel transformers rated to 380V or 400V or look into using two standard 230V transformers in series with the secondaries in parallel. You'll obviously have to scale the rated voltage down to ensure it's still enough at the lowest input voltage range, so if it's a 24V transformer, with a 400V primary, the secondary voltage will be just 12V, if the primary voltage is 200V.

The LM7812 is acceptable but you'll need to make sure the primary voltage remains within an acceptable range as the primary voltage is varied. The LM7812 requires a minimum voltage of 14V to regulate properly and it has a maximum voltage rating of 35V. The rectifier voltage will be equal to the AC peak voltage minus the rectifier losses:
VDC = VAC√2 - 2

There will also be some ripple on the DC side so the capacitor needs to be large enough to ensure the minimum instantaneous voltage is above the lowest voltage required by the regulator.
C = (10I)/V
V = maximum ripple voltage.
C = required capacitance in

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  • 2 weeks later...

After lots of struggle and work...managed to get the circuit complete.

Here the comparator U2A and U2B works just fine.

At around R2=5.05 vdc, high side LED turns on but relay does not operate.  But If I get this to over 12.5 vdc relay turns off..not sure why.

At around R2 = 3.10 vdc low side turns on but it also tries to turn off the relay but fails and constantly tries to turn it off in every 30/20 seconds delay.

for the Q1 I'm using a PNP darlington a614y (KSA614Y) and also tried 2n3906 same effect cannot turn off relay

for LM339 I'm using HA17339.

The relay is an ordinary 12v 400 ohm i.e. Part # JZC-20F(4088)

attached is the exact diagram of the circuit.

Can anyone help me correct the problem so that relay turns off as soon as LED D3 OR D4 Turns on.



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OK, Did solved the low side problem.

Changed r1 to 15k and as soon as below 2.45 vdc relay turns off...

But for high side window comparator lights up at 5.10~5.20 vdc but relay do not turn off.

Can Anyone please help solve this problem.

Do I need separate transistor to turn relay off for high side...please help



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Your circuit is poorly drawn and contains an obvious error. Compare it to my circuit connection by connection and you'll see what I mean.

Here's my revised version of the circuit. The one I posted previously will work but it didn't show the power connections to the comparator IC and I got the values of R7 and R6 the wrong way round which makes calculating the delay more difficult than t = 1.1*R6*C1

I suggest you copy my circuit exactly and get it working before modifying it to work with the voltages you want it to. If that's difficult you could try building the window comparator section, then adding the timer and relay.

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I did not find any mistakes other then,

Your circuits U1C is connected to 5vdc instead of the R3 & R4 divider...this is done as per your post,

"U1C is just a logic inverter, the + input could be connected to Vref rather than R3 and R4,..."

Your circuits R5 for U1B is a preset, set at 50k(mine is r7 50k) instead of 100k, just now corrected to 100k...trying different values as you asked in your post...

If any other mistakes Pls. point me to it, as my limited knowledge cannot find it...sorry...


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I'hv just corrected your r6 and r7..switched the registors 100k and 330k...

hv no change in behavior..

Also tried again connecting your U1C "+" connection to the r3 & r4 devider...delay to deactivating relay for the low side went longer about 30s.

High side of the window comparator lights up but did not switch off the relay...

for your info...I really did not change your circuit much..otherthen adding some leds for me to find the exact voltage where the window is showing high low and when the relay is turing on/off..

In my poorly drawn circuit I did added the power supply that's about it.  The power connection of the IC339 done correctly to pin 3=vcc and pin 12=GND.  no problem there.


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I know what u are saying...

And thank you for your help, this is the first ever complete circuit i hv ever drawn.

Well they were tied together.  You hv r5 = 100k, I hv got two r5's (r7 & r8 of my circuit) with two 100k registors.  I hv separated them only because I hv added two LEDs their to figure when the window comparator is going low or high.  Shorting pin 1 and 2 does nothing but lights both high and low LEDS(leds d3 and d4 of my circuit) together.

I hv just shorted the two pins and now both of the LEDS d3 and d4 lights up for both the high and low condition and goes off when no high/low condition is present.

It did not change the behavior of the circuit i.e. still high side relay not working, in fact it wasn't working the first time as they were tied together i.e. output of U1A and U1B of your circuit.

Interestingly the Low side comparator still working the same.

In the Vin and Vref. side I hv only changed the registors to comply with the voltage requirements of the circuit.  With two 100k registors I wasn't getting 5v and 2.5v from the supply they were out of bounds at best.  Now, with current settings Vref. is showing exact 5 volts and 2.5 volts and I can change them separately if needed.  Also Vin is at exact 5.01 volts while the line voltage is normal(not exceeding limits in both low or high side), and I can set Vin divider from 1.5 volts to 19 volts unregulated.

Please note that High side works...if and only if when Vin goes beyond 12.05/12.60(depending on supply voltage) at that voltage the high side lights off along with relay light goes off i.e. relay turns off, But it should turn off as soon as it reach over 5 volts(where it shows high via the LED) as we'r dividing the actual voltage and feeding the window comparator.

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OK, Now I figured out.

By connecting both the output of the window comparator You are creating an OR function...as both output got open collector type of output.

It must work ..as it is the logical thing...

Sorry..I now connected the +ve connection of inverter U1C TO +5vdc..it was connected to  2.5vdc

It is now working in both ways as both the U1A & U1B swinging in the Higher then +5vdc and Lower(U1B) THEN +5vdc and completing the truth table of the logic circuit.

It wasn't working at +2.5vdc as it is 1/4 not 1/2 the vin.

thanks..but I wanted to show the lights for both the high and low separately..Guess we can use some 1n4148 signal diodes for the LEDS...don't know how to figure that part.  Can u please help in this regard.

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