Electronics Forum

by **fizzy123** » Mon Feb 17, 2014 10:50 am

Hi,

I am looking to setup a on/off switch, whcih closes the circuit when a set minimum voltage is exceeded. I am working with DC only. In this particular case, I want the circuit to be open below 9vdc. I am thinking of using SSR (solid-state relay) but not sure that what voltage comparator can be used to trigger it above 9v only. Any help will be appreciated.

Thanks,

PS: I am just a DIY enthusist and not an Electronics Engineer so please keep it simple. :mrgreen:

by **pebe** » Mon Feb 17, 2014 4:59 pm

You could use the circuit shown in the next topic, 'Need an electronic circuit for my project '.

Use a supply voltage of 12V or higher and set P1 to 9V.

by **fizzy123** » Mon Feb 17, 2014 6:24 pm

Pebe thanks for your reply. I have seen the circuit diagram and I am not very clear that how I am suppose to wire it up. What kind of comparator it is and which pins area attached to battery and which ones go to the relay input? Also do I need any other components?

by **pebe** » Tue Feb 18, 2014 8:14 am

I should have said it is the bottom of the two circuits - the one with the 741.

The 741 is an op-amp, but is used as a comparator in this case.The 12V supply connects to the two terminals on the RH side and powers the 741 and the relay. The battery that you are testing is connected to the input pins on the left hand side.

The operation of the 741 is that, if its negative pin is lower than its positive input, then its output pin will go high and turn on the transistor and relay. If the negative pin is higher then the relay switches off.

So set P1 to the voltage at which you want the relay to change over..

by **fizzy123** » Tue Feb 18, 2014 8:58 am

Pebe I have seen the circuit and since I do not have electronics background, I am still a bit puzzled that how it will suite my application.

I have attached a simple circuit of what I am trying to achieve here. If you can just tell me that what will be the missing component in the circuit and how to wire it then that will be great

So I have a 12Vdc battery that is connected to a controller, which can send 5v or 12v signal to the load. Load is set of High Power LEDs. I want the LEDs to turn on only when 12v is applied and not to respond to 5V. Therefore, I want to place a SSR between the controller and the LEDs. Only issue is that I am not sure that what component at input of SSR will be able to kill or supress that 5V. I cannot remove the controller in between so this setup is strictly as it is.

by **fizzy123** » Tue Feb 18, 2014 9:05 am

For some reason I am not able to attach the image here, therefore, please see the circuit on the link below.

http://farm8.staticflickr.com/7427/1260 ... 8394_o.jpg

by **fizzy123** » Tue Feb 18, 2014 10:28 am

The 5v is applied no more than 10ms so anything to delay the relay may simply resolve the issue. But delay must be short enough and must not be noticed.

by **pebe** » Tue Feb 18, 2014 3:10 pm

I am not sure what you want. Your first post implied that you had a voltage source and you wanted to operate a relay, depending on what that voltage was. I assumed it was some sort of under-voltage indicator for a battery..

Your later posts suggest that is a 12V source that pulses down to 5V for a 10mS period, and that you want the relay to ignore the pulse. Have I got it right? If so, how often do the pulses occur? And under what conditions would the relay change state?

A bit more info about the actual project would help, then I wouldn't need to guess.

by **fizzy123** » Tue Feb 18, 2014 3:21 pm

[quote="pebe"]I am not sure what you want. Your first post implied that you had a voltage source and you wanted to operate a relay, depending on what that voltage was. I assumed it was some sort of under-voltage indicator for a battery..

Your later posts suggest that is a 12V source that pulses down to 5V for a 10mS period, and that you want the relay to ignore the pulse. Have I got it right? If so, how often do the pulses occur? And under what conditions would the relay change state?

A bit more info about the actual project would help, then I wouldn't need to guess.[/quote]

Ok. This is for a car turn signal where original bulb is replaced by new LED system. The controller sends a bulb check signal (5v) to detect the bulb failure. This brief and small voltage application is enough to flash the LEDs. I want to remove this flashing.

The 5v pulse is sent every 30sec and it is no longer than 10ms. I want the relay to ignore this. The relay shall close the circuit when it detects 12v. the 12v pulse lasts upto a second. I hope my requirement is clear now

by **pebe** » Tue Feb 18, 2014 3:28 pm

So would a relay that is off at 0V and on at 4V do the trick?

by **fizzy123** » Tue Feb 18, 2014 3:34 pm

[quote="pebe"]So would a relay that is off at 0V and on at 4V do the trick?[/quote]

i don't understand how? Are you speaking in relative terms; i.e 0v state is 5V and 4V is 9v? :oops:

by **pebe** » Tue Feb 18, 2014 3:58 pm

No, I am speaking in absolute terms with the voltages quoted relative to the negative terminal/ground of the supply (as is usual).

If this is a turn indicator I assume the signal is normally off, ie. there is no 12V source signal so the input to the SSR would be 0V.
When the indicator is switched on there will be +12V signal applied to the SSR which reduces to 5V for 10ms every 30sec. So there will always be signal of more than +4V when the indicator is on.

In that case if the SSR can tell the difference between 0V and +4Vor more, it will be OK. Do you agree?

by **fizzy123** » Tue Feb 18, 2014 4:08 pm

[quote="pebe"]No, I am speaking in absolute terms with the voltages quoted relative to the negative terminal/ground of the supply (as is usual).

If this is a turn indicator I assume the signal is normally off, ie. there is no 12V source signal so the input to the SSR would be 0V.
When the indicator is switched on there will be +12V signal applied to the SSR which reduces to 5V for 10ms every 30sec. So there will always be signal of more than +4V when the indicator is on.

In that case if the SSR can tell the difference between 0V and +4Vor more, it will be OK. Do you agree?[/quote]

Yes normally it is off and 0V across the SSR. But if SSR responds to +4V or more than it will still respond to 5v? In absolute terms this difference shall be greater than 5V I think. That's why I said in my first post that relay should respond to +9v and above and ignore the +5v signal.

by **fizzy123** » Tue Feb 18, 2014 4:24 pm

I am also thinking on a different line to suppress the 5v signal. I tried to measure the trigger current for the SSR input and it appears to be around 6mA. Therefore, if I use a resistor in series at the SSR input (let's say 1K ohm), it will only pass 5mA at 5V and so SSR will not trigger. When 12V will be applied, the passing current will be 12mA and that will trigger the relay. Does this make sense?

by **pebe** » Tue Feb 18, 2014 4:33 pm

I thought you said you wanted it to switch on at +12V and ignore the +5V pulse, ie. stay switched on during the period when the voltage dropped to +5V .

If so, an SSR with a +4V input threshold, will switch off below +4V (0V is below +4V), and switch on above 4V (+5V or +12V).

Does that make sense?