Parameters for using 555 Timer

[Cincinnati10]

Q1: — What is the frequency range I can get out of a 555 timer?

Q2: — Can I use a 555 with a positive ground system? IE: will it work as long as V at pin 1 is lower than V at pin 8? (If I connect pin 8 at 0 VDC and pin 1 at -12 VDC, will the waveform output be a negative voltage instead of a positive voltage output when I connected pin 8 to +12 VDC and pin 1 to ground?)

 

[GonzaloEE]

No problem Cincinnati
Wikipedia says the 555 was born in 1971 (even older than Iron Maiden and myself!). Here some few alternatives for this faithful chip:

ICM7555 : low-power 555 (3-18V VCC, 1/3 mA quiescent)

566: VCO-IC with built-in 50% duty, buffered square/sawtooth output, less external parts.

U2102B: Integrated timer from Atmel. Useful for dimmers and phase control for IGBT/MOSFET drivers.

8253: Rather old, but somewhat interesting for digital apps.

Regards,
Gonzalo

 

[felixc]

I concur with Mike, the 555 may very well do the job for you. Simple and well documented chip. We know its bugs and its remedies. But let's take the other route. What frequency range do you have to work with? What do you intend to do with this part? Perhaps we can give you better inputs if we know better about what you are trying to achieve.

 

[MikeHalloran]

As for the other chips mentioned and not mentioned:

The 556 is a dual 555 that you may be able to find.

The 566 is a VCO, not a timer.

The 8253 is a triple timer with a lot of interesting behaviors, but it starts up stupid, and requires programming over a host bus before it does anything useful.



Mike Halloran
Pembroke Pines, FL, USA

 

[Cincinnati10]

Clarification on Signal Requirement:

I need an electrical signal that resembles the shape of a heartbeat pattern. I need it to carry 50-100W. I want to operate at 0V baseline. At t=0, I want to create a +5000V spike, and immediately drop it back to 0V, followed immediately by a -5000V spike, immediately returning to 0V. I want approximately 10% on-time. I want capability to repeat this cycle, varying the rest period between voltage spike pairs to generate a frequency between 40KHz and 1.5MHz.

I originally thought I would start with a +/- 12V signal and boost it to 5000V; I picked 12V because I believe I could use an ordinary automotive coil to transform voltages from +/-12V to +/- 5000V. However, since coils operate today in the magnitude of 25kV output, perhaps I could use a smaller input voltage (1-2 V) to get the Vout in the 5000V range.

Although an ME, I have a little experience with Process Control. I seem to recall DACs can output 1-5V. I could foresee using either a timing circuit or a computer and a lookup table, to create an output waveform.

I hope this clarification helps you visualize what I am attempting to accomplish. Can you give me additional assistance?

 

[MikeHalloran]

If you can buy the device you want, do so. At any price, it will be cheaper than building one.

The 555s that I've let the smoke out of wouldn't go that fast. Neither would the DACs.

I'm sure there's been progress, but I'm also sure that Moore's Law doesn't apply to analog circuits.




Mike Halloran
Pembroke Pines, FL, USA

 

[Cincinnati10]

As far as I know, I cannot buy one.

If it will be difficult getting a chip or DAC to run this fast, how feasible is it to use maybe 4 channels each at 1/4 the frequency and doing a delay between them to get to the desired frequency?

In this scenario:

Channel No 1 would be creating pulse no. 1, 5, 9, ...
Channel No 2 would be creating pulse no. 2, 6, 10, ...
Channel No 3 would be creating pulse no. 3, 7, 11, ...
Channel No 4 would be creating pulse no. 4, 8, 12, ...

How many channels would I need to reach 40KHZ to 1.5MHz using a DAC?

What is your recommendation on this type of design? Is there a better way to go?

 

[MikeHalloran]

If you always want the same wave shape and amplitude, just a scalable rep rate, then you can use an astable circuit, and trigger it when you want a pulse pair (probably not with a 555). But the wave shape you get will depend on the load, and you haven't told us much about that.

At this point, it would probably be best to befriend (or pay) an electronics engineer with linear high power RF experience. The ones you want are probably old, cranky, and not into computers, so you won't find them on the Web.



Mike Halloran
Pembroke Pines, FL, USA

 

[felixc]

A heartbeat frequency of 1.5MHz. Well, if you want a total control over the shape of the waveform, one way to go is to use a video DAC, and memory system to feed say 128 samples per waveform period. Video DACs can clock at more than 200Mhz. That puts us far from a 555 timer though... but you would have a complete control over your waveform.

I have never used them but, but aren't there on the market the so-called "arbitrary waveform generators" to perform such a function? If so, it could avoid a design for this part of your project.

 

[GonzaloEE]

5KV wideband spikes...I guess building digital signals at those frequencies won't be very cheap.

Instead of using the full output from a 555, You can detect the pulse edges, thus getting a signal with twice the frequency.
I guess an opamp differenciator could do the job, with +/- power rails and rectified output.

I'm still figuring out the 10% duty issue.

Good luck with it!

Gonzalo

 

[pauldude000]

Ok, first let me recap your desire, to make sure we are one the same page. You want a variable A.C. between 40Khz and 1.5 Mhz using a 555 in astable mode. You want a short duration spiked waveform, to drive a HV transformer.

This, at first glance, does not seem to be much of a prob. First, there are many 555 timer brands/types. They do not all share the exact same ratings. These break down into two major categories, Bipolar and CMOS.

The best specs I have found for a bipolar (NE555) have a maximum stable frequency around 500Khz in astable mode. However, the CMOS LM555C is rated as stable to around 3.0 MHZ. The CMOS cant source directly what you need current wise, but has the frequency capability. The bipolar can source the current you need, but lacks the frequency.

SOLUTION 1:

Use the CMOS 555 to drive a fast switching NPN, which sources enough current to drive a;

1. NPN power MOSFET. This will give you square wave 0v baseline DC.
2. A Triac with a separate A.C. supplied source, for A.C to the load.

SOLUTION 2:

Use the CMOS (or maybe a CMOS 556) split the output to drive a JK FlipFlop to invert the signal on one branch, and then join the two signals back together to form a square wave A.C. signal. (If I remember correctly.) Using paired NPN and PNP transistors and mosfets should do the trick.

Don't get me wrong, I am not an engineer, but a hobbyist who has built a square wave function generator using a 555, that puts out a better wave form on my B&K scope than my factory built B&K Function Generator does, and worlds better of a waveform than my el cheapo victor!

555's are anything but a "has been", though you have to consider their shortcomings, as well as their strengths. However, tell me of ANY component that isn't true of.

pauldude000

 

[Warpspeed]

I believe we may need to start at the load and work backwards.

Plus and minus 5KV spikes, fair enough, but into what sort of load ? What is the required half amplitude duration of these spikes ?

From that, we may need to design some type of resonant pulse transformer system to do the job. Then perhaps some sort of capacitor discharge system to fire it off, and a damping or energy recovery system to stop it after one full sinusoid.

If the pulse repetition frequency is 1.5 Mhz, these pulses are not going to be that wide, if they still look like narrow pulses at that frquency.

 

[VEBill]

On the previous thread, we were left the initial impression that the issue was creating an arbitrary waveform, precisely controlling the waveform using PROMs etc. Now the requirement seem to be just a 'spike'.

WS: "I believe we may need to start at the load and work backwards."

An excellent point. Making an arbitrary waveform (if still required) is relatively trivial in comparison to making it +/- 5000 volts and 50-100 watts.