Ripple Current ?

[PCBONEZ]

Re: Ripple Current ?

It helps by preventing the arc when the switch opens from being closed with full current passing through.

When you close switch the load causes current to change much slower than opening the circuit and that helps in minimizing the arc. Charging a small 0.1uF cap isn't going to be very significant compared to powering up the load.

Technically charging the cap will make arcing worse when the switch closes but the 'hurt' there is much less than the 'help' when the switch opens.
Overall you are ahead.

.

[Joe Rampolla]

Re: Ripple Current ?

Hi PCBONEZ,

Thanks again for the kind help!

Is there anything that can be done when using a reed switch on an AC circuit with AC passing through the reed's contacts, to preserve them? I have one situation where I notice a brief spark within the glass tube. On a similar DC circuit, the snubber diode does the trick. Perhaps the best solution is just to convert the circuit to DC. (I use AC with the Lionel type 3 rail O gauge trains.)

Thanks!

Take care, Joe.

[PCBONEZ]

Re: Ripple Current ?

They do have circuits for that but off the top of my head I can't think of anything that would be small and simple.

Best solution may be to get reed switches for an amp rating way over what you are pulling through the circuit.

[Joe Rampolla]

Re: Ripple Current ?

Hi PCBONEZ,

Very much appreciate all the help and info! When I get this circuit built, I will post all the results of my experimenting. I ordered some parts last night and should have it done within a couple weeks or less. The best part of this is the learning. It's great fun! Thanks for helping me understand the circuit. It really helps when I can wrap my head around it.

Take care, Joe.

[Gianni]

Re: Ripple Current ?

Hi Gianni,
Thanks for the information about the relay contacts! I am learning something new all the time!

Take care, Joe.

Only reading today I see I made a mistake... I was talking about relay contacts while you were asking for reed switch... probably yesterday I was a little bit drunk
Anyway PCBONEZ helped you out.

Ciao
Gianni

[Joe Rampolla]

Re: Ripple Current ?

Hi Gianni,

Actually, that information about the relay contacts was very informative!
There are no mistakes in life, only learning experiences!!!!!!!

Take care, Joe.

[Joe Rampolla]

Re: Ripple Current ?

Hi PCBONEZ, Gianni, and Folks,

Well, I couldn't get the circuit to work with R2 at 30K, but when I reduced the resistance to 20K, it worked great. I eliminated R1 since it caused the cap to discharge too quickly. In my new diagram, R2 is now R1. I found that I get approx 45 seconds of "on" time for every 1000uF of capacitance. I used 2 1000uF 35 V caps in series.

Would all TIP31C transistors behave the same? Both the TIP31C transistors I tried wouldn't work with the 30K resistor. Is 20K enough to protect the transistor? Have I created a problem, in the long run, when I eliminated the R1 resistor in the original diagram?

Thanks!

If this circuit is robust, it is all I will ever need for the layout where I can use a momentary trigger of a relay and have the relay stay energized for a certain period of time, without the more complicated timing chips. (I have wired relays to latch, but then I needed a second relay to break the circuit, when I needed precise "on" and "off" timing, like at a railroad crossing.) Just altering the capacitance controls the timing with this simple circuit.

I will be testing it through many cycles to see how it holds up.

It was a great learning experience!!!!!!! First time I understood transistors!

Take care, Joe.

[Joe Rampolla]

Re: Ripple Current ?

Hi Folks,

I made a mistake in my post above. I should have said that the 2 caps I used are in parallel, not in series. I corrected the wording in the diagram.

Take care, Joe.

[Joe Rampolla]

Re: Ripple Current ?

Hi Folks,

Well I tired a few other things because I felt that the resistor I eliminated might have some protective effect for the circuit. By making R1 400K ohm (with R2 at 20K), it only shaves off about 4 seconds of "on" time, as compared to it not being there at all. So the circuit is in its original design.

Take care, Joe.

[Gianni]

Re: Ripple Current ?

Hi Joe

I don't think R1 has any protection function in the circuit, in the original circuit was there just to change the time.

Usually you find a resistor between B-E of transistor, just to avoid any false trigger due to parasitic charge, spike or electric noise. Sometimes you can find just a small ceramic cap.
In your case I think it is not necessary because TIP31C has a low BETA, this means that with small current from base it sinks small current from collector, so probably it is not able to feed the relay.
This is the reason why you had to lower R2 value, in the original circuit the transistor was 2SC2002 , it has a typical beta of 200 and min 90 while TIP31C has a min BETA of 20, I don't see the typical value.
Just as an example, with a BASE current of 1ma, in the worst cas 2SC2002 is able to sink 90mA while TIP31C only 20mA; that's why you need a lower BASE resistor to have a higher base current to sink the same current.

If you want some more explanation read Experiment11 and the explanation to light a LED (at the bottom of page), they explain step by step how to dimension resistor when you are using a transistor.

On your circuit I will change only C1 with a 10uF or 22uF if the cables from wall transformer are long.

Ciao
Gianni

[Joe Rampolla]

Re: Ripple Current ?

Hi Gianni,

Many thanks for the information! I found a tutorial on the internet that showed how to test a NPN transistor, and they used a 10K resistor as R2 with a 9 V power supply. When the circuit didn't work, at first I thought that I damaged the transistor when I soldered to it. I did use a heatsink (alligator clip) when I soldered. I had a 20K resistor, so that was the reason I used that value for R2.

I have been testing the circuit for many cycles now. I was watching for some heating of the transistor, but it stays cool. It works well.

Years ago I substituted a Darlington transistor for a transistor in a simple flashing circuit that used a flashing LED to pulse the transistor. I was just shooting in the dark, but it worked. It is a terrible feeling when you can't get something to work. Early successes built my confidence. I only started to get into the electronics in my late 30s, didn't even know how to solder.

When I get the chance, I will buy some 20uF caps for C1. I will be making several of these circuits.

Thanks again!

Take care, Joe.

[Joe Rampolla]

Re: Ripple Current ?

Hi PCBONEZ, Gianni, and Folks,

Well, the circuit is working reliably through many cycles!

I want to thank all of you for your kind help!

If I didn't have you guys to bounce ideas around with, I probably would never have tried the circuit!

I have passed this circuit on to a couple other hobbyists who wanted to do the same thing.

Take care, Joe.