Please help me with High Low voltage cutoff with time delay

Thread Starter

sakbd.2010

Joined Sep 19, 2010
39
Hi chung,

What I mean that If I get two different 12x2 transformer,

Then I can build two different power supply. One for the Circuit to power, another for sensing voltage.

Both are completely different supplies no connection in-bitween them...i.e. no GND connection.

What actually the reason is two core transformers are 3 times more expensive here then single core i.e. single winding or center tap(center tap are nothing but a piece of wire connecting the center of the winding).

Since building two supplies is no problem, i would like to go in that path...reason economical.
 

eblc1388

Joined Nov 28, 2008
1,542
What actually the reason is two core transformers are 3 times more expensive here then single core i.e. single winding or center tap(center tap are nothing but a piece of wire connecting the center of the winding).
This cannot be true as there is very little different between center-tap and two winding secondary. There is no net increase in material or weight.

Instead of taking out one connection as center-tap, it just break into two connections.

Almost all center-tapped transformers can be "changed" into double windings without much difficulty if the tapping is done outside the physical winding layer which they usually do.
 

Thread Starter

sakbd.2010

Joined Sep 19, 2010
39
Hi chung.,

Lets just say most small transformers for electronics use are imported here in BD(Bangladesh), and got high tax...this makes them more expensive then finished products. Local made ones are no good, at least not yet, but they are cheap as got no import duty...


Anyway...I shall try today and tomorrow lets see what I can do.

Meanwhile.,

Lets say I got a transformer of 24x2 with two secondary windings with 4 connections. So each side and one of center wire shall give me,

24vac volts totol... Lets just say we got wire 1, 2, 3, and 4.

1. We connect one side i.e. wire 1 and 2 to power main circuit we get 24vac for that. and

2. We use remaining wires 3 and 4 we get rest of the 24 volts for the sensing circuit.

Now, What should be the value of the "Cx" Capacitor ?

Again Instead of 24 vac each side now we could get one with 12 volts in each side..what capacitor size we need to use(Your Cx value) ?

Let me know.
 

eblc1388

Joined Nov 28, 2008
1,542
Lets say I got a transformer of 24x2 with two secondary windings with 4 connections. So each side and one of center wire shall give me,
24V per winding is too high, You'll need a 12Vx2, non center-tapped transformer.

24vac volts totol... Lets just say we got wire 1, 2, 3, and 4.

1. We connect one side i.e. wire 1 and 2 to power main circuit we get 24vac for that. and
Yes, except 12Vac instead of 24Vac.

2. We use remaining wires 3 and 4 we get rest of the 24 volts for the sensing circuit.
Again 12V instead of 24V.

Now, What should be the value of the "Cx" Capacitor ?

Again Instead of 24 vac each side now we could get one with 12 volts in each side..what capacitor size we need to use(Your Cx value) ?

Let me know.
Cx value is 470uF for 12V transformer. 24V per winding is too high.

Here is the response of the voltage sense circuit using 470uF as Cx.

I just use the AC peak value in simulation as it didn't matter if rms is used or not. I have to divide the voltage down by pri/sec transformer ratio anyway.

 

Attachments

debe

Joined Sep 21, 2010
1,389
Dont quite know why this project has become so complex. Here in Australia a magazine called Silicon Chip published a project called a Brownout Protector, this used a 240v/12v transformer as a supply voltage for the circuit & for sensing under & over voltage. It used a LM358 dual op amp switching a relay to control the 240v ac. Daryl
 

Thread Starter

sakbd.2010

Joined Sep 19, 2010
39
Hi debe,

No, it has not died, only that chung finished designing the circuit.

I'm trying to get the transformer chung was intended for the circuit. As soon as I can get it I shall assemble the circuit. Also, I will hv to go out of town for a few days and as soon as I can finish it I shall try and report back the status here.
 

Thread Starter

sakbd.2010

Joined Sep 19, 2010
39
Hi chang,

I hv constructed the circuit.

As I said before I really tried but could not get a 2 secondary transformer, so I'm using two completely seperate transformers. One for the circuit and one for sense voltage. Powering the circuit voltage reads 12.60/13.00(after relay operates) and idle(before timer times out) reads 14.20/15.00 volts.

To drive the relay I'm using one P-channel IRF 9530(12a, 100v).

The sense circuit reads 18.20 to 19.20 depending on a/c supply. I'hv tried configure the circuit with a 10k vr and a 5vdc PSU. But no matter what I do it always activates the relay i.e. stays in cutoff mode.

I'hv checked the voltages at the LM393:

Comparator 1: Multimeter fixed at Output1 and In1(-) reads 12.20 to 12.70vdc and In2(+) reads 10.50 to 11.02 vdc.
Comparator 2 : Multimeter fixed at output2 and In2(-) reads 0.50/0.55 vdc and In2(+) reads -0.20/-0.10 vdc.

TL431 : Both sides of TL431 Reads 0.20/0.21 vdc.

Sensing power supply : Produce about 18.20 to 19.20 vdc depending on a/c supply that varies from 230VAC to 240VAC.

After Building the circuit, I thing the comparator 2 gets low as the sensing voltage is lot higher then comparator Vcc comparator voltage is at 12/15 vdc but sensing voltage is 18/19vdc, a difference of 4 to 6 volts.
From the above voltage figures can you suggest me any modification or do I need to change any component.

I tried changing the TL431, LM393 With a different one and NE555N with another, but the circuit always activates the cutoff. It takes about 30/35 second to activate relay if powering after an hour or so, if
powering time differs no more then few minutes, then relay activates immidiately. This activate on normal voltage operation.

Pls. let me know if you need to know further readings..
 

eblc1388

Joined Nov 28, 2008
1,542
Power your circuit by AC and measure the dc voltage at those points. The BLACK lead(negative) of the meter must be connected to common 0V for the measurement, as shown.

Post the measurement results here.

If you get 0.2V at point D, then the TL431 is either bad or not connected properly.

Adjust the variable resistors so that point E = 2.4 to 2.45V and point F = 2.55 to 2.6V.

 

Attachments

Thread Starter

sakbd.2010

Joined Sep 19, 2010
39
Hi chung,

Someone gave me some very good idea about transformer.

If I get two 12x2 center tap transformer and

1. Connect the Primary side in series.
2. Connect the Secondery side one half of each transformer in parallel then,

I get transformer that can handle upto 440 vac as all transformer here are 220 vac. Also each 12 volts becomes 6 volts and two 6 vac(output) together gives 12vac(Secondary side). I think great for the bad electricity supply of this country.

Now If I connect two varistors(MOV) in series with the primary windings then it should work, what you think ?

Now What should be the value of the MOV's for line voltage of 230/240 vac ? Please let me know.


Thanks for your info. No electricity right at this moment, I shall take the readings as soon as supply electricity is back.
 

Thread Starter

sakbd.2010

Joined Sep 19, 2010
39
Hi,

Here is what it shows :

A = 7.14 v

B = 7.08 v

C = 12.24 to 12.28 v

D = 12.20 to 12.28 v

Took above after setting,

E = 2.41 v

F = 2.58 v

All measurement taken after connecting meter -ve to the common ground...

Please note current supply vac now at 224 to 226 volts
 
Last edited:

Thread Starter

sakbd.2010

Joined Sep 19, 2010
39
OK, thanks,

Now c & d Reads,

C = 8.80 v

D = 2.53 v

TL431 connected OK, but I hv taken it's ground from the 100uf capacitor -ve pin, Now I got them corrected. Is "c" point value OK ?

Also U hv not commented on the transformer, can u please help.

Also, would like to connect LEDS for Both Point E and F(High and Low) voltage situation, also One for Relay activation...can you please tell me how to connect the LEDS and resistors...thanks.

I'hv just set the inverter Voltage to 320 vac and power the circuit with this voltage BUT it did not activate...I shall hv to see further into configuring the circuit..
 

eblc1388

Joined Nov 28, 2008
1,542
TL431 connected OK, but I hv taken it's ground from the 100uf capacitor -ve pin, Now I got them corrected. Is "c" point value OK ?
Yes.

Also U hv not commented on the transformer, can u please help.
Putting two transformers in series will not work in this case. The current from one transformer will affect the others.

Also, would like to connect LEDS for Both Point E and F(High and Low) voltage situation, also One for Relay activation...can you please tell me how to connect the LEDS and resistors...thanks.
See schematic below on how to add LEDs for indication. You would need to use only RED LEDs.

I'hv just set the inverter Voltage to 320 vac and power the circuit with this voltage BUT it did not activate...I shall hv to see further into configuring the circuit..
It is possible that you have damaged the 555 IC because of high supply voltage when the AC input is 320V.

That's is why I have used a 12V 7812 voltage regulator in the circuit.

 

Attachments

Thread Starter

sakbd.2010

Joined Sep 19, 2010
39
"" In your last circuit diagram, You hv removed one diode (1n4148) and replaced with a led, this is for both the comparators (low/high ) i.e. 1st. and 2nd. comparator, shall I be doing the same!!""

Nevermind(comment above), I hv replaced red LED at the comparator outputs..working just fine.

Also, I don't thing I'hv damaged 555, I did damaged them before, but I know they are 16v ic's so I always used them from 7812 regulator. Also If I use/rotate the vr (2nd/high side) it does activate the relay but doesn't seem to hv any effect while the AC side goes to 320 VAC, note checked regulator voltage i.e. Vcc for 555 is well within 16 vdc.

I'hv got another 555 timer ic, it's 1555 and sources confirmed it's an 18/20vdc timer and higher temp..replaced just now, and working just fine. But just wondering..if AC voltage goes beyond 400 vac, what shall happen at the regulated supply, will that shutoff(7812) or shall feed that high voltage to the timer i.e. for 400 volts dc can go as high as 25/26 vdc, and that surely damage the 555 ic.


This seems to be working pretty well now.
 
Last edited:

eblc1388

Joined Nov 28, 2008
1,542
I'm glad that your circuit is working.
But just wondering..if AC voltage goes beyond 400 vac, what shall happen at the regulated supply, will that shutoff(7812) or shall feed that high voltage to the timer i.e. for 400 volts dc can go as high as 25/26 vdc, and that surely damage the 555 ic.
The purpose of the 7812 voltage regulator is to keep its output at 12V, whether the input is 15V or 28V. It regulates and that means keeping the output voltage constant, at 12V for the 555. I don't know why you think the 220V AC supply in your country(or village) can go up to 400V because any electrical appliance will be most likely be dead as that happens.
 

Thread Starter

sakbd.2010

Joined Sep 19, 2010
39
Hi chung,

After some trial and error...I managed to use the circuit. But still got some problems.

If I use LEDS(no matter what color) instead of 1n4148 at the output pins of the comparators, it cannot detect differences in 240vac to 280/290vac...

If I use the simple diodes i.e. 4841's, the circuit can detect the changes and turn the relay on and it works...but do not exactly know when it's signalling high or low as no leds can be added.

Can you please see if I can add leds but also have the correct sensitivity of the comparators. it seems most LEDS got forward voltage drops within 1400 ~1700 whereas 1n4148's got 550 to 580, not sure if that's the cause. All leds I got(some 3mm some smaller) but got higher voltage drops stated above.

thanks
 

Thread Starter

sakbd.2010

Joined Sep 19, 2010
39
Hi chung,

Sorry for delay...

I hv modified the circuit...

It's working but not the way I want.

It can now differenciate 225/230 and 320/325 But still cannot detect changes bitween 230 and 290

For I hv checked it's high(2nd.) side comparator to detect changes of 225 to 320 the Point E reading is 2.90/93 dcv for 225/30 vac and when vac goes to 320 it moves up at 304/6 vdc..

while the circuit inbitween 2.95 to 303 it is around 290 vac but cannot detect the changes and stays showing no high....

thanks
 
Top