Reverse Polarity on 555 timer???

This is the place for any magazine-related discussions that don't fit in any of the column discussion boards below.
N9AOK
Posts: 22
Joined: Fri Aug 01, 2003 1:01 am
Location: Worthington, IN
Contact:

Reverse Polarity on 555 timer???

Post by N9AOK » Fri Aug 22, 2003 8:17 am

Can a 555 timer take 10V reverse on its power/Grnd pins?<p>I'm building something and need to know if I have to include a protection diode?<p>I cant tell from the data sheet but it looks like it will be OK as long as the transistors inside can take it.<p>thanks for any help
Todd Snyder

dave8976
Posts: 40
Joined: Sun Dec 09, 2001 1:01 am
Location: Toronto Canada
Contact:

Re: Reverse Polarity on 555 timer???

Post by dave8976 » Fri Aug 22, 2003 2:14 pm

I have mistakenly applied a reversed 10 volts to the 555 timer several times without any damage. But I am not sure on the long term effect with continued applicaiotn of a reverse voltage. For the cost and space of a diode, I would recommend adding it as a precaution. Dave

N9AOK
Posts: 22
Joined: Fri Aug 01, 2003 1:01 am
Location: Worthington, IN
Contact:

Re: Reverse Polarity on 555 timer???

Post by N9AOK » Fri Aug 22, 2003 4:12 pm

I just hate to loose that 0.7volts since this is running off a 9V battery and as it drops voltage through out its life it will just end circuit functionality that much sooner.<p>My worries are only about inadvertant touching of the 9v battery backwards on there.<p>Thanks for you help....I think I may take the risk since it looks like its not an automatic smoker with backward polarity..<p>Todd

Chris Foley
Posts: 146
Joined: Mon Jan 13, 2003 1:01 am
Location: Chicago IL
Contact:

Re: Reverse Polarity on 555 timer???

Post by Chris Foley » Fri Aug 22, 2003 6:28 pm

Hi, Todd. You probably do want to add a protection diode. Try a Schottky -- real-world small current voltage drops across Schottky diodes are in the 0.15V-0.25V region (10 mA or so). By the time a 9V battery gets down to the area where this would make a difference (5V for a regular 555), your 9V battery is practically cashed, anyway.<p>It just costs one diode to make it right. If you're facing a "gotcha", you might as well solve it -- it'll just be more of a headache later when you have to fix it.<p>Chris

N9AOK
Posts: 22
Joined: Fri Aug 01, 2003 1:01 am
Location: Worthington, IN
Contact:

Re: Reverse Polarity on 555 timer???

Post by N9AOK » Sun Aug 24, 2003 6:05 pm

Chris,
Actually, I've decided to use a lm317 now to keep a steady voltage for my circuit. I'll be pulling pulses of up to 500ma at times.<p>So, now I guess the question is will the 317 handle reverse 10V for short times and if not then can the schottky diode you describe handle those kind of currents? I'll go read up on them now. <p>Thanks for your input..<p>Todd

Chris Foley
Posts: 146
Joined: Mon Jan 13, 2003 1:01 am
Location: Chicago IL
Contact:

Re: Reverse Polarity on 555 timer???

Post by Chris Foley » Sun Aug 24, 2003 8:17 pm

The brief answer to your question is, try the 1N5817 schottky rectifier diode if you're worried about voltage drop. It has a PRV of 20V, it's good for 1A, and its V(f) is half that of the 1N4001 across the board.<p>Your basic problem, though, has to do with the voltage and current budget for a 9V battery. So, at what point are you going to be concerned about dropout voltage here? Let's look at your setup.<p>Before anything else, you should know that the 555 can be used with an unregulated voltage, as long as it remains stable over the one shot period or over the time period of oscillation. You might not need a regulator at all, just a good sized cap to keep Vcc stiff during that current spike. Also, remember that the standard 9V "transistor" battery doesn't do very well at high current pulses. For extended high-current pulses, you might be better off with a string of 6 C or D batteries.<p>The next thing you might want to consider is that many CMOS 555s such as the TLC555 are good down to 3V, as are many logic families, including the venerable 4000/4500 series classic CMOS. If you're looking at dropout problems, that's something to consider. They use a few less mA, too.<p>Assuming you need a regulated intermediate voltage that can give you half an amp, the LM317 still might not be a great idea for several reasons. Although the device itself uses only a fraction of a mA, it has a minimum load requirement of up to several mA in order to maintain its specs, especially if you use the minimum program current of 1mA. Even worse, the National LM317 data sheet only guarantees specs with the difference between Vin and Vout being 3V or greater. This would cause a headache, with or without any diode. Even if you want a 5V supply, you've bought into a chip that requires a minimum load of several mA, at least 1mA for itself, and requires 3V of headroom.<p>Hmmn. Minimize current draw because of battery, and be aware of dropout voltage to try to maximize battery life. Fortunately, they invented Low Dropout Regulators (LDO) for your problem. From less than a buck to a couple of bucks, you can get low dropout (less than a volt), low operating current (way less than 1mA), or both, in a variety of package types. Not knowing enough about your application, I can't recommend a specific one, but you should look at the National website for starters, and the DigiKey catalog for easy availability. I hope this has been of help.<p>Chris<p>[ August 24, 2003: Message edited by: Chris Foley ]</p>

N9AOK
Posts: 22
Joined: Fri Aug 01, 2003 1:01 am
Location: Worthington, IN
Contact:

Re: Reverse Polarity on 555 timer???

Post by N9AOK » Mon Aug 25, 2003 10:28 am

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Chris Foley:
The brief answer to your question is, try the 1N5817 schottky rectifier diode if you're worried about voltage drop. <p>I'll check that diode out.

Your basic problem, though, has to do with the voltage and current budget for a 9V battery. So, at what point are you going to be concerned about dropout voltage here?
<p>I'm flasing LED's for 8ms on about a 5% duty cycle and I'm pusing the LED's to about their high limit. I need to have a steady voltage to maintain brightness and not blow up the LED's with too much current.<p>Before anything else, you should know that the 555 can be used with an unregulated voltage,
Yes and I really wanted to do that but a steady voltage was important to me for the reasons mentioned above.<p>
The next thing you might want to consider is that many CMOS 555s such as the TLC555 are good down to 3V, as are many logic families, including the venerable 4000/4500 series classic CMOS. If you're looking at dropout problems, that's something to consider. They use a few less mA, too.

I did stumble across that and I have on my order list some 555 that are just what you describe. max output voltage 2.9volts but still able to be source with a broad range.<p>
Assuming you need a regulated intermediate voltage that can give you half an amp, the LM317 still might not be a great idea for several reasons. Although the device itself uses only a fraction of a mA, it has a minimum load requirement of up to several mA in order to maintain its specs, especially if you use the minimum program current of 1mA. Even worse, the National LM317 data sheet only guarantees specs with the difference between Vin and Vout being 3V or greater. This would cause a headache, with or without any diode. Even if you want a 5V supply, you've bought into a chip that requires a minimum load of several mA, at least 1mA for itself, and requires 3V of headroom.
<p>Well CRAP!! I did notice that I needed almost 2 volts headroom to get things stable. I'm probably OK in my application since only under load am I sensitive to the voltage. If things settle out quickly then I assume it wont be a problem.<p>
Hmmn. Minimize current draw because of battery, and be aware of dropout voltage to try to maximize battery life. Fortunately, they invented Low Dropout Regulators (LDO) for your problem. From less than a buck to a couple of bucks, you can get low dropout (less than a volt), low operating current (way less than 1mA), or both, in a variety of package types. Not knowing enough about your application, I can't recommend a specific one, but you should look at the National website for starters, and the DigiKey catalog for easy availability. I hope this has been of help.
<p>Thanks for your help. I really appreciate it. I'll see what I can find for 317 alternatives.
I'm not married to anything yet. <p><hr></blockquote>

N9AOK
Posts: 22
Joined: Fri Aug 01, 2003 1:01 am
Location: Worthington, IN
Contact:

Re: Reverse Polarity on 555 timer???

Post by N9AOK » Mon Aug 25, 2003 10:37 am

Here's my circuit. I was going to resize everything (bigger cap, smaller divider) so I could use a cheaper pot that the 2 meg one I would need with this.<p>http://www.homefires.us/bluemarble/PAGE ... STROBE.jpg<p>UV LED strobe<p>Other comments welcomed.<p>Todd Snyder
N9AOK

N9AOK
Posts: 22
Joined: Fri Aug 01, 2003 1:01 am
Location: Worthington, IN
Contact:

Re: Reverse Polarity on 555 timer???

Post by N9AOK » Mon Aug 25, 2003 10:49 am

This is the timer I was looking at<p>TS3V555IN<p>http://www.mouser.com/index.cfm?handler ... tid=320941<p>Oh, it really is a bummer when you open up your package of timer you just bought to find out you got little tiny SMT versions instead of the nice big DIPs I really needed...Glad they were only a quarter...LOL

User avatar
Joseph
Posts: 656
Joined: Mon Dec 10, 2001 1:01 am
Location: USA,World
Contact:

Re: Reverse Polarity on 555 timer???

Post by Joseph » Mon Aug 25, 2003 7:30 pm

Another approach would be to use the diode as a crowbar. That way, the diode is only a factor if the battery is inadvertently hooked up backwards. Then it would forward bias and short out the backwards voltage. You could add in a polymer circuit protector which will automaticaly reset when the short is removed. http://www.allelectronics.com/cgi-bin/category.cgi<p>I remember that 555 timer circuits suffer from changing timing periods, especially lengthening when the voltage is reduced. Maybe you can utilize this property to increase the pulse width to compensate for lowering battery voltage. Also, you could probably use a small transistor to draw a current from the 555's timing ramp in some way proportional to the supply voltage.

bodgy
Posts: 1044
Joined: Tue Dec 04, 2001 1:01 am
Location: Australia
Contact:

Re: Reverse Polarity on 555 timer???

Post by bodgy » Mon Aug 25, 2003 8:11 pm

Instead of a Schottky you could use a MOSFET - with internal protection diode.<p>Gate to +V Source to -V Drain to circuit return<p> + G positive side of circuit<p> - S D- negative side of circuit.<p>The voltage drop will be negligable if a low FET with low RDS(on) is used - if the battery is connected the wrong way around the FEt will not switch on and the internal protection diode will spring into action.<p>Colin
On a clear disk you can seek forever.

N9AOK
Posts: 22
Joined: Fri Aug 01, 2003 1:01 am
Location: Worthington, IN
Contact:

Re: Reverse Polarity on 555 timer???

Post by N9AOK » Tue Aug 26, 2003 9:01 am

Thanks, this is a Great idea.<p>Anyone have an idea on how to make this circuit Fade in and out?
I was trying to figure out a way to apply a triangle wave to the base of the transistor (superimposed onto the strobe action).
I can get a triangle wave wtih op amps but how can I superimposed (mix) that signal with the pulses I now have? !pictures please!<p>My thoughts were to use "superpostion" to just "add" a new signal to the base of the transistor using another transistor but how to couple it has me confused.<p>Thanks
Todd Snyder<p> <blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Joseph Meisenhelder:
Another approach would be to use the diode as a crowbar. That way, the diode is only a factor if the battery is inadvertently hooked up backwards. Then it would forward bias and short out the backwards voltage. You could add in a polymer circuit protector which will automaticaly reset when the short is removed. http://www.allelectronics.com/cgi-bin/category.cgi<p>I remember that 555 timer circuits suffer from changing timing periods, especially lengthening when the voltage is reduced. Maybe you can utilize this property to increase the pulse width to compensate for lowering battery voltage. Also, you could probably use a small transistor to draw a current from the 555's timing ramp in some way proportional to the supply voltage.<hr></blockquote>

N9AOK
Posts: 22
Joined: Fri Aug 01, 2003 1:01 am
Location: Worthington, IN
Contact:

Re: Reverse Polarity on 555 timer???

Post by N9AOK » Tue Aug 26, 2003 9:06 am

Thanks for you help...
I've not used mosfets for anything yet so they arent' on my radar screen (not till now anyway).<p>I like this idea too. I'll check that option out too. Are Mosfets expensive?<p>Todd<p> <blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by bodgy:
Instead of a Schottky you could use a MOSFET - with internal protection diode.<p>Gate to +V Source to -V Drain to circuit return<p> + G positive side of circuit<p> - S D- negative side of circuit.<p>The voltage drop will be negligable if a low FET with low RDS(on) is used - if the battery is connected the wrong way around the FEt will not switch on and the internal protection diode will spring into action.<p>Colin<hr></blockquote>

Ron H
Posts: 360
Joined: Tue Dec 18, 2001 1:01 am
Location: Boise, ID
Contact:

Re: Reverse Polarity on 555 timer???

Post by Ron H » Tue Aug 26, 2003 3:39 pm

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by bodgy:
Instead of a Schottky you could use a MOSFET - with internal protection diode.<p>Gate to +V Source to -V Drain to circuit return<p> + G positive side of circuit<p> - S D- negative side of circuit.<p>The voltage drop will be negligable if a low FET with low RDS(on) is used - if the battery is connected the wrong way around the FEt will not switch on and the internal protection diode will spring into action.<p>Colin<hr></blockquote><p>Colin, if you mean a MOSFET with the protection diode from source to drain, the diode will conduct when the power is connected in reverse. This is bad. If you mean (which I doubt) a part such as IR's FETKY, which has an internal series Schottky diode, I see no advantage here over a simple Schottky diode.

User avatar
Edd
Posts: 885
Joined: Sun Jan 27, 2002 1:01 am
Location: Dallas Tx
Contact:

Re: Reverse Polarity on 555 timer???

Post by Edd » Tue Aug 26, 2003 6:35 pm

Todd:
Here is another simple circuit that you might consider to use in lieu of the 555 oriented circuitry.
This being, if you were wanting the ramping up or down of the LED driver xstr of the LED’s.
Its the 6th circuit down at this site. I also prefer the reverse blocking diode for circuit protection in case of a reversed batt snafu. Site is:
http://ourworld.compuserve.com/homepage ... m#eyes.gif<p>73's de Edd
[email protected].........(Interstellar~~~~Warp~~~Speed)
[email protected]........(Firewalled-Spam*Cookies*Crumbs)<p> ;)

Post Reply

Who is online

Users browsing this forum: No registered users and 14 guests