555 tips, fixes, and cures

[otaio]

Dear all,

Many of you will know that the leap from paper to project is rarely straightforward.

Sometimes circuits are published which neglect to mention that they have not been tested in the real world or that the circuits provided are for indicative purposes only.

You are not alone if you find this frustrating.

I and no doubt others would be interested to hear about the sort of things you should check for in the event of a 555 project not working.

There are many things which you might think obvious because of your experience and knowledge but might not occur to a novice such as me.

This forum provides an opportunity for you to share your knowledge.

This may range from simple advice such as check your circuit for solder bridges or whiskers to more complex stuff such as a description of a symptom and what you did to cure it. You might even wish to share your top ten 555 trouble shooting tips.

Regards
Bob B

(Yes, I have a simple 555 project but for the life of me can't see why it works sometimes and not others when I supplementary components such as diodes across relay coils).

 

[pebe]

You say '..works sometimes and not others when I supplementary components such as diodes across relay coils.'

One possible problem may be you are not using a fast enough diode across the coil. The o/p and trigger pins are adjacent and the trigger pin has a very high input Z. So capacitive coupling can cause erratic triggering. I use 1n4148, but that may not be good enough if you are using a high current relay.

 

[ElectroNerd]

it sounds like you have not used a cap on the reset pin of the 555 timer. a 1uF will do the trick. it is also good practice to put a smoothing cap on vcc+ to prevent spuriouse triggering. hope this helps. you can read more on 555 circuits by going to 'google.com' and searching for '555 timer ic tutorial'...... have fun, you nerd!!!

 

[melone]

What do you mean "works sometimes"? When it "doesn't work", what happens? If you do a search on this site, I have answered the question of why diodes are needed across relay coils on a couple of different occasions. If you have a specific question, just ask, we will help!

Good Luck and keep us posted!

 

[otaio]

Pebe

Thank you for your comments regarding the diodes. A very good tip and not at all obvious. In my application I use a 1N001 in series with the output pin (pin 3). The relay coil (12volt 360 ohm) is then shunted with another 1N001. The coil is across pin3 and ground.
Vcc is about 17 volts.
Output on pin 3 -- when active -- 15.3v and across coil 14.6 v

Your comments are appreciated.
Regards
B

 

[otaio]

Electronerd

Your comment regarding the reset pin is noted. In my application I have 'tied it' to Vcc via a 4.7k resistor.

I have put a 100 miroFarad capacitor across pin1 and 8 as near to these as is possible with 0.1" matrix board.

Your suggestion about seeking a tutorial was a very good one and I recommend:

http://www.williamson-labs.com/480_555.htm

Thanks for your suggestions.
B

 

[otaio]

Melone,

Thanks for your post.

I take your first tip... be precise in your use of language when defining the conditions of your query.

Your second one... do research around your area of your query.

Third tip is don't be afraid to ask for help.

Well here goes:

Problem:
Relay doesn't open when relay control circuit opens.

Circuit onditions:
DC rail voltage 16.4V
DC coil nominal 12V; resistance 360 Ohms

Circuit arrangement 1:
Vcc 16.4V feed to a reedswitch which is in series with a 470 Ohm resistance which is in series the relay coil which is connected to ground. No shunt diode.

Test conditions 1

Description of problem:
a) On closure of reed switch, relay operates normally open contacts closes.
b) On the reed switch opening the normally open contacts referred in a) remain closed.
c) physically breaking the circuit results in the closed contacts opening.

Circuit arrangement 2:
Vcc 16.4V feed a piece of wire in series with a 470 Ohm resistance which is in series with a connecting block which is in series with the relay coil which is connected to ground. No shunt diode.

Description of problem:
a) On completing the circuit by connecting the wire with the 470 ohm series resistance into the connector block the relay operates, the normally open contacts close.
b) On unconnecting the wire with the series resistance the normally open contacts referred in a) remains closed.
c) physically breaking the circuit results in the closed contact opening.

Circuit arrangement 3:
Vcc 16.4V feed a piece of wire in series with no series resistance in series with a connecting block which is in series with the relay coil which is connected to ground. No shunt diode.

Description of problem:
a) On completing the circuit by connecting the wire into the connector block the relay operates, with the normally open contact closing.
b) On unconnecting the wire, the normally open contacts referred in a) opens.

I would be interested in any explanation why the contacts remain closed when the coil is closed with a circuit that has resistance but not when the circuit in series with the coil has neglible resistance.

Bob B:–)

 

[pebe]

otaio,

If you mean a 1N4001, then it's a power diode and it's too slow for that application. You don't need the series diode.
But ALWAYS use a shunt diode across a coil.

You may also want to check max Vdd for the particlar 555 you are using. You're using 16.5v. In particular, the Philips NE555 is only rated to 15v. Could you be getting some form of latch-up in the output beacuse of that? Check the voltage across the relay when it should be off.

If you are not using the reset pin then connect it direct to Vdd without the resistor.

 

[otaio]

Pebe,

Thank you for pointing out my error. It is of course a 1N4001.

The 555 is configered as a monostable pulse generator which in the real world application will be triggered once a day.

Can you point me to an internet resource which gives guidance of the selection of diodes for particular applications?

I note your advice about checking the elcetrical characteristics for the 555 you use as they can vary considerablr. An excellent tip for the novice.

Finally I summarise the essence of your final tip is if you don’t use the reset function then tie the pin to the supply rail.

Once again thank you for your comments they are both interesting and helpful.

B

 

[otaio]

Dear all,

It strikes me that technical discussions are helped if everyone can view a circuit diagram. Is there a way of including such an item in a posting and if so how do you go about doing it?

B

 

[gjones33]

Hello Otaio,

For reliable operation d.c operated relays should have a small air-gap between the magnetic pole pieces otherwise the magnetic field may not collapse after the current has been removed. A piece of sellotape over the pole piece should suffice and this will have no effect whatsover on the closing of the relay.

Cheers,
G

 

[otaio]

G -- Hi!

A very practical tip worth noting.

It sounds as if you have practical experience of DC relays misbehaving.

Sadlyly the relay in question is a subminature encapsulated one.

Can you point me in the direction of an internet resource which discussions the theory and application of dc relays?



B

 

[buzz41]

Just a tad O/T, but mentioned was made that a 1N4001 series diode won't work as a catch diode across a relay.

Relay coils typically have self resonant freqs in the KHz range. And, I know the 1N4001, etc. has long storage times. I also know that ANY diode directly across a relay coil will usually increase relay release time significantly, so a direct diode shunt can not be used if fast relay operation is necessary.

But, have I been missing something important all these years about these diodes? So far, so good with these and similar diodes.

 

[pebe]

Hi buzz41,
The point I made to otaio about fast diodes related to his circuit where he was getting unpredictable results. The back EMF generated by a relay coil when switched off is dependant on the rate of change of current in the coil.

The 555 o/p is good to 200KHz so is able to cut off the current within a few microsecs. As I understood it, a slow clamping diode would allow the -ve end of the coil to go well above the +ve supply rail before the diode could act. About 25 years ago when I first started using 555s, I read an article which recommended a high speed diode should be used when the 555 was driving a relay direct. This was because the trigger pin is adjacent to the o/p pin and it has a very high input impedance (it takes only 500 nanoamps) so capacitive coupling of a large spike could cause false triggering. For that matter, all the input pins must be very close on the silicon chip, so the same would apply to them. That is what I thought could be happening to otaio's circuit, and is why I always use a signal diode in that position.

On the wider issue of diode clamps. Their purpose is to protect the driving transistor from excess voltage when the relay current is cut off. Without a diode the max voltage could be exceeded and there is probably enough energy in the coil to pop the transistor. A 1N4001 is quite adequate for those situations.

As an alternative to the diode, it is possible to slow down the rise time at the collector by fitting a 47n capacitor between collector and base. As the transistor tries to cut off, the cap feeds the rising collector volts back to the base which tries to keep the transistor switched on. The result is the collector slowly rises in a time determined by the cap and the base feed resistor.

I hope this helps.

 

[otaio]

Buzz 41,

Thank you for your interesting contribution.
You introduce a term with I’m not familiar. ‘diode storage time’. What is the definition of this term and how is this information used in practice?
Again thanks for taking the time to submit to this thread.

B

 

[otaio]

Pebe
Your discussion about the use of signal diodes as a high speed clamping diode is both interesting and well made.

Thank you also for the confirmation of the importance using a ‘clamping diode’ across a relay coil and how in slow switching operations -- such as mine -- a 1N4001 would be adequate. Another execellent tip.
B

 

[melone]

Actually, you can decrease the release time and still use a fast switching diode by simply adding a series resistor. The problem with swithing diodes is that there aren't worth anything if you have any real current flowing through them. Therefore, when you attempt to clamp large current spikes, which can occur when dealing with relay coils, then you definitely risk the possibility of blowing up a fast acting diode.

The other problem of a simple diode clamp has also been pointed out, long release times. Therefore, use the diode as a simple DC block and let a resistor disapate your energy. This system will only work if your circuitry can handle a voltage spike. Remember, your diode is only acting as a DC block, not a clamp. By increasing the value of the R, then release time decreases. However, with the decrease in release time, an increase in voltage is also seen. Depending on your circuit, this might be acceptable.

Good luck and keep us posted!

 

[otaio]

Dear all,

Thank you all for the tips. As a result of your interest and advice I think I have found the source of the problem.

The problem was not the relay.

The symptoms appeared when I added a 'shunt diode' across a relay coil. The configeration of my circuit meant that in doing this action I was in effect placing the diode also in parallel with the trigger circuit of the 555 thereby negating its effective action.

Remove the shunt diode the 555 monostable triggers as designed.

Insert into circuit the 555 monostable does not trigger at all.

I still wish to have protection of the shunt diode and have now put this in series with a 10k resistance and the 555 triggers as intended.

I don't know what effect the addition of a series resistance will have on the efficacy of the protective properties of a shunt diode and I would be interested to receive comments/explanations or analysis.

Regards

Bob B

 

[melone]

Are you sure that you had the diode oriented correctly? It sounds like you had your diode shunting the 555 timer output to ground. Therefore, your diode clamped the output voltage of your 555 timer to .7V. Your relay won't work because the diode is shunting all of your current, and most of your voltage straight to ground. Make sure that your diode is placed in an anti-parallel configuration. This means, that the diode will not conduct when a positive voltage is applied across your relay coil.

I think that this your problem, and your 10K resistor lends credance to this line of thinking. A 10K resistor will limit the current to a fairly small value (exact value based on the voltage you are actually working at, but probably 5V b/c of the 555 timer -> 5/10K=10uA). Therefore, your 555 can source that much current without any problem. This also means, that the voltage coming out of the 555 is high enough to trigger your transistor that drives your relay (I hope that you are not trying to drive a relay directly from a 555 timer output).

If you simply have a diode across the coil, then you have essentially shorted the 555 output to ground. Therefore, you will never allow the relay to turn on.

Hopefully, this helps, if not, please let me know where I went wrong!

Good luck and keep us posted!!!

 

[otaio]

melone

Thankyou very much for your analysis. I can confirm that the diode is the correct way round.

The problem seems to be caused by the shunt diode being in parallell with the trigger circuit.

The trigger circuit goes from positive rail through a signal diode which is in series with a capacitor which is in series with a normally closed contact which is wired to ground.

Because of the way the circuit is laid out the relay coil shunt diode is in effect in parallel across the signal diode/capacitor, contact.

At the diode / capacitor junction a lead is taken to pin 2 of a 555.

The circuit is designed so that the contact opens. At the end of the process the contact closes causing the 555 to trigger on a negative edge.

By putting a 10k resistance in series with the relay coil shunt diode the circuit works as intended. Use a diode on its own (IN918 or 1n4001) then the 555 does not trigger.

It easier to draw than describe.

Interested to receive your further thoughts.

This has triggered an interest in wave shaping circuits and I have found two sites worth a visit:

http://www.tpub.com/neets/book9/37.htm

http://www.enel.ucalgary.ca/People/Haslett/Enel565/Modules/diodes/diodes.html

Regards
Bob B