Anyone have experience with an RC driven temporary "ON" transistor switch?

[EasyG0ing1]

Hello,

The idea is to use a transistor to activate a latching relay once a voltage is applied but to prevent a constant voltage from being applied to the relay coil, I thought it might be a good idea if it were possible to make sure that the voltage applied to the coil is temporary even though the source voltage could be "ON" for long periods of time.

This is the circuit that I came up with. I have not tested it yet because I don't have the relay yet, though I am looking for anyone who might have experience with this kind of a setup to hopefully offer some advice, words of wisdom, etc.



Thank you,

Mike

[petehall347]

best to add a flyback diode across the relay coil

[stj]

555
is better

[EasyG0ing1]

555
is better

You mean it's better to use a 555 instead of the RC circuit? Can you show me an example of this?

[harp]

I think that your R1 must be a way to higher then 1k to C1 have sense. It is paralel with it... But dont know your preferences, what is the time interval for ON state?
I have some schematics for 555 one shoot, if it ok to your needs.

[EasyG0ing1]

555
is better

Would you be referring to something like this?

[EasyG0ing1]

I think that your R1 must be a way to higher then 1k to C1 have sense. It is paralel with it... But dont know your preferences, what is the time interval for ON state?
I have some schematics for 555 one shoot, if it ok to your needs.

Well, the idea is that as soon as voltage is applied, the transistor goes into saturation but only for about 1 second ... maybe 2 or anywhere in between, but then after that time, the transistor shuts off even though power is still applied. Also, I calculated the values of R1 and C1 using t = RC where 33k times 47µ = appx 1.55 seconds.

[harp]

Well, the idea is that as soon as voltage is applied, the transistor goes into saturation but only for about 1 second ... maybe 2 or anywhere in between, but then after that time, the transistor shuts off even though power is still applied.

Yes, then use only one 555 from circuit... the circuit is trigger by grounding input, and if it stay grounded it maintain only one single initial pulse of 1 second duration.

[EasyG0ing1]

Yes, then use only one 555 from circuit... the circuit is trigger by grounding input, and if it stay grounded it maintain only one single initial pulse of 1 second duration.

I built this circuit



And indeed when I measure across the 10k resistor, I get 5 volts for a short time, and then it goes to 0. However, when I remove power from the circuit altogether, the voltage across the 10k goes close to 5 volts and then slowly dissipates ... it takes about 5 seconds for this to happen. How can I make sure that the 10k stays at 0 volts when I cut power to the circuit?

[harp]

I dont know what you mean... accross 10k... on collector (respect to ground) is normally 5V if transistor is not powered on.
Are you simulating this circuit, or you actualy built it on testboard?
Have you large capacitor on power rail with you try to control circuit?

You shall measure between 555 pin#3 and ground.
When you designed a new circuit, allways start from basic circuit, and gradualy develop with stable modules, so, get rid of transistor at the moment, and on output connect 470 ohm resistor in series with one LED to ground.

See:
http://www.fileconvoy.com/dfl.php?id...ce601e3f503beb

[stj]

https://ohmslawcalculator.com/555-monostable-calculator

[truclacicr]

The first circuit makes no sense. The transistor's base will never see more than 0.1V, so it will never turn on.

You need to interchange the positions of R2 and C1, and insert R1 (plus a 1.8V zener?) into the base rather than between base and ground.

[sam_sam_sam]

https://ohmslawcalculator.com/555-monostable-calculator

Thank for sharing this calculator for the 555 timer ic chip I could have used this a couple of times in the past

[EasyG0ing1]

I dont know what you mean... accross 10k...

I mean I measured voltage across the 10k resistor... negative lead of the volt meter on the collector and positive lead on the other side of the resistor.

[EasyG0ing1]

I dont know what you mean... accross 10k... on collector (respect to ground) is normally 5V if transistor is not powered on.
Are you simulating this circuit, or you actualy built it on testboard?
Have you large capacitor on power rail with you try to control circuit?

You shall measure between 555 pin#3 and ground.
When you designed a new circuit, allways start from basic circuit, and gradualy develop with stable modules, so, get rid of transistor at the moment, and on output connect 470 ohm resistor in series with one LED to ground.

See:
http://www.fileconvoy.com/dfl.php?id...ce601e3f503beb

I built the circuit on a breadboard. Using a bench power supply for power source.
The capacitance values are as shown in the schematic, all ceramic capacitors. When I put a load resistor on pin 3 to ground, the circuit works as I described. It goes high for a short time, then it goes to zero volts. However, when I remove power from the circuit, the voltage at pin 3 goes high again and slowly drains down to zero.

My suspicion is that because pin 4 is tied to VCC, it leaves pin 3 in an "on" state and whatever is causing it to clamp down to zero after power on is simply no longer holding when power is removed from the circuit. Being able to bring pin 4 to ground at power down would probably stop it, but I don't know how to do that.

[petehall347]

think you need a diode across the 10k much the same as if it was a relay .. but i am often wrong with such things

[harp]

It goes high for a short time, then it goes to zero volts. However, when I remove power from the circuit, the voltage at pin 3 goes high again and slowly drains down to zero.

From where? Investigate what is source in this moment?
If you put LED like I say, is it act also like that? Your real load, relay definitely dont do that, there is no power for that.
But I want you to realize that in general electronic control and regulation circuit is not intended to be designed to control (operate) withot power, or in lack of it. 555 pin#4 cant operate correctly without powered ic already. If you have some power on board all the time, that must be using to control circuit, the signals and sense must be apart of that. Maybe to consider redesign circuit?

[harp]

I mean I measured voltage across the 10k resistor... negative lead of the volt meter on the collector and positive lead on the other side of the resistor.

If transistor is closed (blocked), on collector (one side of resistor) and on positive rail (other side of resistor) you have 5V on both point respect to ground, so, between them there is no difference in potential at all - 0V. Thats why you dont measure voltage accross resistor, in general.

[stj]

use film caps for the timing, ceramics can drift by upto 40% with temperature, film is 5%

[eccerr0r]

truclacicr had it correct in post #12.

The first circuit if it were drawn right is (almost) typical of power on reset circuitry, no need for a 555 if all one needs is POR.

Changes as said is to swap the 33K and 47µF capacitor. Lose the 1k resistor, not needed and interferes with operation. Adding a diode with anode to GND and cathode to the base of the transistor will help speed up cycle time.

BTW you can and should test with a LED or light bulb or something before you get the relay, it should pulse once on power up and stay off until you remove and reapply power.