Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

555 50% precision.

Status
Not open for further replies.
That circuit is close, because of the >100:1 ratio of (R10 + RV1) :R9, but it is not considered to be 50% duty cycle. It also suffers from having R9 quite small. According to the datasheet:
View attachment 97272
NB:Ra in the datasheet = R9 in your schematic.
That quote from the datasheet doesn't make much sense when applied to the circuit provided above. The capacitor current will be very small due to the >100k (dis)charge resistor (datasheet for ref: **broken link removed**)
 
Why get stuck using an old original 555?
A Cmos 555 does not need an adjustment to produce 50-50. When its output charges and discharges the timing RC and its load resistance is high then it always produces exactly 50-50.
A simple CD4047 IC also produces exactly 50-50 from its divided output without adjusting anything.
 
I guess we will just have to accept your unsubstantiated word for it over Philips.

John

Hi,

That's one way of putting it :)

To understand why there are differences the individual has to take the time to look deep at what all the mechanisms are during the charge and discharge time. If the individual is not willing to do this they will miss some details, and by my definition that means they will miss part of the fun found in simple circuits like these. The best you can do then is try to prove how the circuit really works, down to the last detail. It does take time though to do this and sometimes you wonder if it is really worth it.
 
Some interesting ideas with 555's.
I sorta gave up on the 555 idea, but not completely, I used a 555 to double the supply to 10v for a tl494 to generate a dual phase square wave which drives a transformer in push pull, the primary of the trans uses the 5v rail, 10v from the 555 just feeds the '494.
I wondered if the '494 would operate at 5v, yes it has a 5v reference which needs the overhead voltage but I'm not using that I'm using it as a fixed square wave gen, I suspect some of the internal junk uses it though.
 
Last edited:
Hello again,

For what it is worth, i took a closer look at the charge and discharge mechanisms and found the following relationship. The upper transistor charges the cap through the resistor and with some voltage drop that changes things, but then i found that the lower transistor also has voltage drop and that changes things too. The final ratio between 'on' and 'off' times then came out more complicated but still easy to understand:

Ratio=log((3-(2*Vs)/VTH)/(3-Vs/VTH))/log((3-Vs/VTL)/(3-(2*Vs)/VTL))

where VTH is the drop in the upper transistor and VTL is the drop in the lower transistor.

This is a little hard to see in a one line text version of the equation, but by looking close we can see that the denominator log could be converted to -log if we invert the inside, and then we get a relationship with the same numerator and denominator if we just make the VTL=VTH or VTH=VTL, and that means we get a theoretically EXACT 50 percent duty cycle. The catch here though is that the upper transistor may drop more than the lower transistor and thus still throw the duty cycle off a little. If VTH=0.2 and VTL=0.1 then the ratio is still off by 2.2 percent.
Someone would have to be willing to take the time to measure both VTL and VTH so we can find out for sure what will happen.
This might mean that varying the series resistor value might bring these two closer together also, depending on how they have it arranged internally.
 
Here's a conventional way of getting an adjustable mark-space ratio, so that 50% can be set simply.
View attachment 97275

Setting a M/S ratio to anything you like is simple. It's being able to adjust the frequency and keep the MS ratio at what you set it that is the object. The use of diodes is problematic because of the drift with temperature. The first circuit I posted has a M/S ratio of 1:1 within 1%, and the second circuit does better than that and has 4 decades of frequency adjustment too.

If you select resistors to achieve a 1:1 MS ratio according to the data sheet your worst case MS ratio will be 1:1 +- 2* R tolerance and that is due solely to the resistors- there are other errors. So, for example if you used 1% resistors your MS ratio would be 1:1 +- 2%.

There is no such thing as absolute accuracy in the real world: everything has a tolerance, even an atomic clock :)
 
Hello again,

For what it is worth, i took a closer look at the charge and discharge mechanisms and found the following relationship. The upper transistor charges the cap through the resistor and with some voltage drop that changes things, but then i found that the lower transistor also has voltage drop and that changes things too. The final ratio between 'on' and 'off' times then came out more complicated but still easy to understand:

Ratio=log((3-(2*Vs)/VTH)/(3-Vs/VTH))/log((3-Vs/VTL)/(3-(2*Vs)/VTL))

where VTH is the drop in the upper transistor and VTL is the drop in the lower transistor.

This is a little hard to see in a one line text version of the equation, but by looking close we can see that the denominator log could be converted to -log if we invert the inside, and then we get a relationship with the same numerator and denominator if we just make the VTL=VTH or VTH=VTL, and that means we get a theoretically EXACT 50 percent duty cycle. The catch here though is that the upper transistor may drop more than the lower transistor and thus still throw the duty cycle off a little. If VTH=0.2 and VTL=0.1 then the ratio is still off by 2.2 percent.
Someone would have to be willing to take the time to measure both VTL and VTH so we can find out for sure what will happen.
This might mean that varying the series resistor value might bring these two closer together also, depending on how they have it arranged internally.
If you use a voltage drive you can cut across a lot of that, because the up and down capacitor charges are just CR. Why they didn't design the 555 to have a voltage drive, I can't understand.

The best way to get a 1:1 M/S ratio and simple frequency adjustment is the use a 556 chip with the first 555 as the integrating oscillator and the second 555 as a divide by 2.
 
Hello again,

Add to that the duty cycle is not the only factor that influences the current offset in the primary (and thus the flux ratchet effect) but the transistor drive matching also plays a big part in it, and that is subject to second to second change in offset. When i worked in the industry the only solution we found was to create a dynamic flux adjustment circuit to keep the flux balanced. There was no constant duty cycle of any kind that could do this as well. Flux feedback was the only way that could keep up with the constantly changing parameters due to environment.

If you use a voltage drive you can cut across a lot of that, because the up and down capacitor charges are just CR. Why they didn't design the 555 to have a voltage drive, I can't understand.

The best way to get a 1:1 M/S ratio and simple frequency adjustment is the use a 556 chip with the first 555 as the integrating oscillator and the second 555 as a divide by 2.

Yes that sounds like an interesting idea. Luckily 5 watts isnt usually too much of a problem. 5kW is another story :)
 
No you don't, you can work it out!

Hi,

I guess i missed what you were trying to say then too. Maybe you could restate it.
 
Hi MrAl,

What I am saying is that instead of a load of innuendo the function of my circuit can be analyzed to determine it's performance.

spec
 
Why get stuck using an old original 555?
A Cmos 555 does not need an adjustment to produce 50-50. When its output charges and discharges the timing RC and its load resistance is high then it always produces exactly 50-50.
A simple CD4047 IC also produces exactly 50-50 from its divided output without adjusting anything.

I assume you are talking about using the Q output of the LMC555 to drive the integrator. This is attractive but the up capability of that output is not brilliant according to the data sheet.

The CD4047 is good but it does not have the high source and sink currents of the LM555 and to a lesser extent the LMC555.

If you want to do some fancy stuff, the ICL8038 (NTE864, XR8038) function generator is worth exploring. It pees all over the LM555/LMC555 except for current drive. Although the ICL 8038 is obsolete, there are tons on Ebay- has anyone tried the Ebay versions? I would be very interested to know if they are any good.

I believe the Exar XR2206 is sill in production, but that is limited compared to the ICL8038

Another way to get a 1:1 M/S ratio and high frequency is to roll your own integrating oscillator using a high-speed comparator, four resistors, and a capacitor.
 
Last edited:
I used the now obsolete ICL8038 in the '70ies. The XR2206 is also not made anymore.
 
ETO_2016_02_06_Iss01-00_MULTIVIBRATOR_COMPARATOR_OUTLINE_ONLY.png

ERRATA
(1) Note, the comparator is, low VIPOS, rail to rail output, FET input, and high speed.
(2) Note, timing up and down is proportional to R4/C11
(3) Note, LF and HF decoupling are not shown.
(4) There should be a joining dot at the start of the output line.
 
Last edited:
I used the now obsolete ICL8038 in the '70ies. The XR2206 is also not made anymore.

Me too. You can make quite a nice bench signal generator with an ICL8038 and, of course, they are used extensively in music effects circuits. :)

Oh, the XR2206 is gone too. :mad:
 
Me too. You can make quite a nice bench signal generator with an ICL8038 and, of course, they are used extensively in music effects circuits. :)

Oh, the XR2206 is gone too. :mad:

Hi,

You think using a rail to rail op amp is really practical just for a 50 percent duty cycle square wave oscillator though?

On to the sine wave generator chips and like...

I loved my 8038 generator as at the time i was just fooling around with audio. It was great for me at the time. I have a 2206 chip around but never used it because the bandwidth bites. These days a dedicated DSP based wave generator is just phenomenal with an output frequency of 0.01Hz to 40MHz !

There was another chip, similar to the XR2206, that was put out some years ago (maybe 10 or 15) and i was after one back then, but found that they had discontinued them too. They actually went up to 20MHz sine/sqr/triangle which aint bad. I wanted one but got to looking for one a little too late so i could not get one. This chip was made in a similar fashion, with the symmetrical non linear multi transistor wave shape circuit seen in the other chips, but faster.

I also wanted to build one from scratch but never got around to it. A handful of transistors would do the wave shaping, and the circuit could be tweek'd for optimal wave shape. One of these days maybe i'll tackle it again. BW should to up to at least 10MHz with common parts.

For now i intend to use the DSP chip, which goes up to 40MHz sine, but have yet to get that into a box with all the knobs and stuff :)

It is very interesting to hear other people talking about these chips too. It is hard to believe that all three of these chips are not made anymore. I would think there would be some use for them. Maybe the problem was that the sine was never that good. Not bad, but not good enough for real audio work and after all that's what a lot of the sub 1MHz chips would be used for i would think. That puts them in a niche that just isnt there. Who wants a chip that puts out a rough sine wave. That's my guess as to why they disappeared anyway. Still, the discrete version would be very interesting to build from an analog hobby viewpoint. I'd love to see how good we could get it. These days you can get multi transistors on a chip too, so the parts count might not even be that high, except for the multitude of resistors required.
 
You think using a rail to rail op amp is really practical just for a 50 percent duty cycle square wave oscillator though?

A rail to rail op amp would be OK for low frequencies, low Ct current applications, but in my schematic above I recommend a high speed comparator which makes a much better integrating oscillator.

I loved my 8038 generator as at the time i was just fooling around with audio. It was great for me at the time. I have a 2206 chip around but never used it because the bandwidth bites. These days a dedicated DSP based wave generator is just phenomenal with an output frequency of 0.01Hz to 40MHz !

There was another chip, similar to the XR2206, that was put out some years ago (maybe 10 or 15) and i was after one back then, but found that they had discontinued them too. They actually went up to 20MHz sine/sqr/triangle which aint bad. I wanted one but got to looking for one a little too late so i could not get one. This chip was made in a similar fashion, with the symmetrical non linear multi transistor wave shape circuit seen in the other chips, but faster.

I also wanted to build one from scratch but never got around to it. A handful of transistors would do the wave shaping, and the circuit could be tweek'd for optimal wave shape. One of these days maybe i'll tackle it again. BW should to up to at least 10MHz with common parts.

For now i intend to use the DSP chip, which goes up to 40MHz sine, but have yet to get that into a box with all the knobs and stuff :)

It is very interesting to hear other people talking about these chips too. It is hard to believe that all three of these chips are not made anymore. I would think there would be some use for them. Maybe the problem was that the sine was never that good. Not bad, but not good enough for real audio work and after all that's what a lot of the sub 1MHz chips would be used for i would think. That puts them in a niche that just isnt there. Who wants a chip that puts out a rough sine wave. That's my guess as to why they disappeared anyway. Still, the discrete version would be very interesting to build from an analog hobby viewpoint. I'd love to see how good we could get it. These days you can get multi transistors on a chip too, so the parts count might not even be that high, except for the multitude of resistors required.

Interesting stuff- let us know how you get on with that project. :cool:
 
I want to put togther a cheap nasty smps to power a project.
A 555 is used as the circuit is usb powered, the circuit works well as a 2 switch forward converter but I want a dead simple single switch so I need a 50% duty o/p from the 555 with good accuracy.
I saw somewhere a diode from discharge to trigger, is this going to work?

Edit: I shouldnt be so lazy, looking at the schem its clear to see that the timing cap charges up through the resistor to + and the forward biased diode, then the cap discharges through the resistor between trigger and discharge with the diode reversed, if if the 2 resistors are the same duty should be 50%, I must have poxy resistors or something as I get 60%.

Hi doc,

A Royer inverter may be worth considering for your application. They need the minimum of components and inherently use the transformer core to the maximum thus keeping the transformer as small and light as possible. Royer inverters are widely used to generate the high voltage for CCFL back lights in laptop LCD displays and also for generating the power lines, including EHT, for scopes. You would probably need to wind your own little transformer, but that would not be difficult. https://en.wikipedia.org/wiki/Royer_oscillator
 
Yes I was thinking on those lines too.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top