555 timer reverse polarity circuit

[CDRIVE]

I came back here to delete a nasty post that I made but it seems the Mods already did that for me.

Thank you!

Chris

 

[CDRIVE]

After giving this a bit more thought I've updated the schematic (LEDs included) but kept the SPDT relay. I really didn't like using an extra set of relay contacts just to light some LEDs, so this version drives the LEDs from the 555's Output pin. This doesn't mean that you're committed to a SPDT relay. A DPDT relay will work to. The extra contacts won't hurt anything.

The Green LED is on when the Output is (-) Negative. The Red LED is on when the Output is (+) Positive.

Good luck.

Chris

 

[Electrobrains]

...The Green LED is on when the Output is (-) Negative. The Red LED is on when the Output is (+) Positive.

Hint: You could put those LEDs on the other side of the relay (directly on the output) and save a resistor and get a 100% true indication of the output state. With a well chosen resistor, even the voltage level on the output could roughly be seen.

 

[CDRIVE]

Hint: You could put those LEDs on the other side of the relay (directly on the output) and save a resistor and get a 100% true indication of the output state. With a well chosen resistor, even the voltage level on the output could roughly be seen.

No, I couldn't. I dismissed connecting the LEDs to the Output because he's maintaining constant current through his load. Yes, this means that his +- supply can't maintain a constant 30V but this isn't important to him, the current is.

 

[CDRIVE]

welshdragon, I don't know if having the LEDs being powered by your current source is that important to you but if it is the LED's can be put in series with the output pin of the relay or in the + & - legs of the supply. The LEDs would have to be shunted with an appropriate value resistors though.

If you want this I will need the Max and Min currents that will be fed to your electrodes.

 

[welshdragon]

welshdragon, I don't know if having the LEDs being powered by your current source is that important to you but if it is the LED's can be put in series with the output pin of the relay or in the + & - legs of the supply. The LEDs would have to be shunted with an appropriate value resistors though.

If you want this I will need the Max and Min currents that will be fed to your electrodes.

G'day
Sorry for the delay in answering you, i have been in London at the Olympics.

The current across the electrodes are kept at a constant of .29ma. The voltage at the electrodes ranges from 31.5 v down to about 20v minimum.

Hope this helps
Thanks

 

[CDRIVE]

G'day
Sorry for the delay in answering you, i have been in London at the Olympics.

The current across the electrodes are kept at a constant of .29ma. The voltage at the electrodes ranges from 31.5 v down to about 20v minimum.

Hope this helps
Thanks

,29mA = 290uA or is that a typo?

 

[welshdragon]

,29mA = 290uA or is that a typo?

Sorry that was meant to be 29 milliamps

re: your latest schematic, i couldn't find C4 Electrolytic 50vdc on the diagram, could this be a mistake in the notes or am i missing something.

 

[CDRIVE]

Sorry that was meant to be 29 milliamps

re: your latest schematic, i couldn't find C4 Electrolytic 50vdc on the diagram, could this be a mistake in the notes or am i missing something.

C4 no longer exists. It was used in the earlier schematic that employed a 7812 voltage regulator.
https://www.electronicspoint.com/at...everse-polarity-circuit-555-1minhi1minlo-.jpg

This was removed when 555 power reverted back to a wallwart but not updated in the notes.

I'll update the schematic and put shunted LEDs in series with your +- supply. This should give you a direct LED indication as ElectroBrain suggested.

 

[CDRIVE]

Hopefully this will be the last version of this. The LEDs were moved into the constant current legs of the supply as ElectroBrain suggested.

************* Notes **************
All Resistors are 1/4W
All Disc Caps are rated >= 25VDC
C2: Electrolytic >=25VDC
K1: Can be any SPDT relay with a coil resistance >= 400 Ohms
and contacts rating >= 500mA
http://www.mpja.com/12VDC-SPDT-Essex-Relay/productinfo/18454+RL/

 

[welshdragon]

Hopefully this will be the last version of this. The LEDs were moved into the constant current legs of the supply as ElectroBrain suggested.]

Thank you for your time and patience in helping me greatly with this my first project.
I will be ordering the components today and will post back when i have completed the build.

Many thanks to all
regards
Chris

 

[CDRIVE]

Hi Chris,

Just so you know.. Electrolytics are typically +-20%. The larger they are (capacitance) the more leaky they are. The first few time cycle will probably be longer than subsequent cycles because the cap will be forming itself and also because of the nature of the 555, After the first trigger the charge cap will never be discharged to zero. The 555 re-triggers when C2 discharges to 1/3 Vcc. The discharge transistor is turned off when the C2 recharges to 2/3 Vcc.

So, to summarize; don't rely on the first few cycles to determine if your getting the 60 sec cycles you want. Let it run for a while. If it's too long you can reduce the value of C2.

Good luck,

Chris

 

[DontLetTheSmokeOut]

Chris,
What is the purpose of D1 and R5? I've not seen that configuration on any 555 timer circuits before.

I ask because when I simulate it, change the duty cycle for out to something like 90/10. If I remove them it looks just like what you show.

Thanks for the schematic,
dave

 

[CDRIVE]

Chris,
What is the purpose of D1 and R5? I've not seen that configuration on any 555 timer circuits before.

I ask because when I simulate it, change the duty cycle for out to something like 90/10. If I remove them it looks just like what you show.

Thanks for the schematic,
dave

D1 and R5 are used to improve 50-50 symmetry. If a get time I'll post some plots with and without D1 & R5. As a side note I chose 400uF over a smaller cap like 100uF because it would force R5 to be about 4Meg; not too common. There's also another issue when very long time constants are required. Since Electros leak, the larger R is the more effect the leakage issue will be for an expected T=RxC. This may be up for debate though.

Chris

 

[CDRIVE]

OK, it's Sat at the watering hole but I wanted to get these plots posted before I head out.

Note: C2 Leakage is not plugged into the the plot parameters.

Later.
Chris

 

[DontLetTheSmokeOut]

I figured out my problem. For some reason LTSpice put the resistor at 2m instead of what I typed, 2M. I can't get it to take M so I used 2000k and it works.

thanks,
dave

 

[CDRIVE]

No problem. By the way, it's been quite a while since I used LT Spice but since it won't accept "M" did you try "Meg"?

Chris

 

[mypearl]

Man I have been searching for this for 6 months or better, same application! Can I ask a question CDRIVE?

My power supply is a mastech HY3003DX, which allows me to take advantage of constant flow circuitry within the unit. I set it for 20ma and it adjusts the voltage for me. What if any does this change on your circuit on post #30? I am very thankful for your help.

 

[DontLetTheSmokeOut]

It shouldn't matter. He also noted on the schematic that it was for use with a constant current source of 28mA.

Looking at your power supply, I don't think it does -30V to +30V, but 0V to 30V. You will need a little different circuit to get that to work. You'll need the use of a dpdt relay. This is exactly what I'm trying to make

dave

Man I have been searching for this for 6 months or better, same application! Can I ask a question CDRIVE?

My power supply is a mastech HY3003DX, which allows me to take advantage of constant flow circuitry within the unit. I set it for 20ma and it adjusts the voltage for me. What if any does this change on your circuit on post #30? I am very thankful for your help.

 

[CDRIVE]

If you're referring to the whole circuit here ..
https://www.electronicspoint.com/at...verse-polarity-circuit-555-1minhi1minlo-3.jpg

you can just remove the shunt resistors across the LEDs.

Chris