555 driver and Transformer questions

[DanMann]

I apologize if this is the wrong section, I am having trouble searching for answers as I think I am unable use the correct words to explain for a search query..


Do the traditional transformers need to work at specific frequencies? A hypothetical, I have a radio shack transformer on my bench that is primary 120V 60Hz with an output of 12.6- 0 -12.6.
If I put a lower frequency the transformer gets noisy, which seems like trouble. However would running say 180Hz to about 1KHz have any ill effect on the transformer.?

In trying to make a 555 astable driver to a mosfet to cycle a (different)  transformer at 180Hz from a 6 volt battery.
In using online calculators  I have been playing with combinations of resistors and capacitor and see that I can get 180Hz in many ways, but the time high and time low vary.

Is there a ideal combination ratio of the high and low time?

[Andy Chee]

Transformers do have a hard limit at the lower end, going below will result in core saturation.

The upper limit is much softer and won’t damage the transformer. Basically you hit the transformer’s resonant frequency peak, then the response tails off like a low pass filter.

Ideal waveform is 50% duty cycle. This ensures there’s no net DC that will contribute to core saturation.

[Zero999]

It's the V*t which is important, i.e it's the voltage, as well as the frequency.

A 120VAC 60Hz transformer will be fine if the voltage is also reduced, as well as the frequency. It'll be able to work at 12VAC 6Hz with no problem. If the insulation can handle the extra voltage it'll also be fine with 240VAC at 120Hz, although there will be additional losses due to eddy and hysteresis, which will increase the off-load power consumption and heating.

Presumably you're using the transformer in fly-back mode, which is different to how it was designed to be used.

Which winding is connected to the 555 circuit?

Is it just single ended or push-pull?

Post a schematic.

[DanMann]

@Andy, I was unsure if high time and low time were the vertical or horizontal stage. By duty cycle I think I now understand.

@Zero999, the transformer is a typical laminated core with one primary winding and one secondary winding, no center tap. It was designed for use with a mechanical vibrator/oscillator that was supposed to operate at 180cps (Hz). Unfortunately these over time have gotten way off with them putting out voltage spikes and frequencies much higher up to 1.2KHz.
I am not understanding push pull, I am guessing single ended.

I am still learning how to use an oscilliscope and do not know all the terms or what what I am seeing actually means, so the following I will do my best of what I think I am seeing.

attached is the schematic (555 oscillator.jpg) of what I am building, sort of. I started with another configuration which would not work with the diodes, but the signal I am getting is showing a duty cycle of 97%. I believe this is due a possible skipped cycle, meaning the signal rises, sharp and quick drop followed by a slightly higher rise.
In changing the time base on the scope the duty and frequency changes, I suspect this is due to the trigger setting?

Attached are pictures. I am not seeing how to put them inline?

[xvr]

Transformer should not be used in this way. There is a strong DC offset in your schematic. Transformer will fall into saturation (that you can see on pictures)

[DanMann]

can you explain to me what to look for in the schematic that shows a strong DC offset? C1, R1, R2 I gave no values, but can very close to a 50/50 time.
I think I am starting to understand the scope a little more with some reading.
Please help me if I am wrong:
Top picture shows a 0 voltage on one box down from center line, rises up to 5 volts with some spiked up to the 7.45 volts?
the very narrow spikes are the -35 volts?
It shows a 57% duty cycle which I assume to be based on the 0 to+5 volts?
this actually drives the transformer but think it can be better.
With 6Vdc input, how is it generating the -35 volts? Could it be feedback voltage from the HV made by the transformer?

I have been playing and can get a fairly square wave. Once I introduce the MOSFET I can get a very different wave where the drop to minus is thin but curves for the remainder of the time up to zero.
Being that the transformer is not yet connected I am thinking wrong MOSFET or maybe I damaged it.

Thank you for your help.

[Andy Chee]

can you explain to me what to look for in the schematic that shows a strong DC offset?

Examine the output driver stage that is connected to the transformer winding.

Imagine the switching device/transistor/MOSFET/IGBT/relay is permanently on, or running at high duty cycle, say 99.99%

The voltage across the transformer in such a scenario, is effectively DC.  Not all transformers are tolerant of DC, and will result in saturation of the core.  Practically this means the core will heat up, and power will no longer transfer to the secondary winding (or less power anyway).  Also once the core saturates, your switching device will effectively be switching a dead short circuit, which will kill most devices, depending on power level.

If you change the duty cycle to 50%, your circuit will still pump DC into the transformer winding, but at 50% voltage.  This may still be plenty enough to saturate the core.

In summary, you need an alternative circuit topology.

The traditional alternate topology is a centre-tapped transformer with each winding being driven push-pull.  Given you lack a centre-tap, you'll have to go with a H-bridge configuration.  Or if you have a bipolar supply, you can push-pull using that.

Finally, you could try a large 1000uF non-polarised capacitor* in series with your transformer winding.  This circuit topology is simple, but generally low power.

(*they're hard to find, so use 2x 2000uF polarised capacitors in series, with polarities reversed back-to-back)

[Zero999]

can you explain to me what to look for in the schematic that shows a strong DC offset?

The current direction through the transformer is always the same, thus the net DC flux in the transformer's core.

[DanMann]

I am starting to understand why I have luck with other similar power supplies, but this type has been problematic.
A little more background on this unit, The transformer, capacitors, resistors needed to accomplish what is needed are all potted and not serviceable. The unit is meant to be portable and operates off of a 6 volt battery. So a bipolar bench supply is out of the equation.
Attached is a schematic that has been cleaned up removing the voltage divider, which should not effect this discussion.
The blue box is the potting and you can see there is no way to separate the ground of the primary wingding from the ground. The two capacitors in series I believe are just to filter.
The purple box is an external mechanical vibrator that works a little different than others I have played with on old car radios, portable two way radios. These units have center tap transformers.
There are electronic replacements out there, and I have bough or build many of them, and they work in other power supplies, but not this unit.
Trying various other mechanical vibrators I have found they get hot quickly. These are all 80 plus years old, inefficient, and many just no longer work. As mentioned the correct vibrators can be too slow or two fast and create voltage spikes.
this vibrator when energized takes the path of least resistance to the internal coil which pulls the read from the lower connection to the upper and puts power to the primary side of the coil. As this happens the current is limited by the internal resistor which releases the read where it begins the cycle all over again.
As I understand it the vibrator is doing nothing more than pulsing the DC to the primary.
This is where I got the idea of a 555 timer.

There may be high voltage feedback involved.

I am unclear what the .1uf capacitor is doing. is boosting the primary or vibrator coil? And to what end? or is it as Andy said "you could try a large 1000uF non-polarised capacitor* in series with your transformer winding.  This circuit topology is simple, but generally low power." and this helps with the saturation.
Or maybe saturation was part of the design. I say this as the  HV output is only a few micro-amps but it can zing you at the battery.
 

Looking into the H-bridge, would this even be possible as it would be reversing through the ground which might upset the voltage dividers (not shown)

[Andy Chee]

This context is most helpful and would’ve been appreciated in your first post!

So just to confirm, you’re seeking to build a replacement HT supply for a car valve radio operating from 6V system?

In other words, you’re not attempting to use this power supply to run a different HV device like a Tesla coil, taser gun, electric fence, or other?

So extra questions, are you just trying to replace the vibrator unit? Or the entire HV power supply?

[DanMann]

to confirm, I attempting to use this power supply for its original intended purpose.

I do not want to replace the power supply as it passes all checks I  am looking to maintain as much of the original parts as possible, such as the vacuum tube rectifier.

I am trying to replace the mechanical vibrator (interrupter) which is the weak link and is shown in the purple box in the diagram I posted in my previous post. 










 

[Andy Chee]

I have a few thoughts....

Is your current mechanical vibrator system working adequately at the moment?  Can you connect your oscilloscope to the working vibrator to see what kind of duty cycle it generates?

Another possibility is that the working vibrator can be used to drive the gate of the MOSFET (instead of being driven by the 555 timer), the MOSFET (or transistor or SCR) can handle the heavy current switching of the step-up HT transformer module.  This approach would eliminate heat from vibrator contact arcing, but you would still be susceptible to mechanical failure.

[xavier60]

The 0.1uF capacitor could be there to resonate with the primary inductance causing damped oscillations of a more controlled voltage rather than a massive narrow spike.
There could be more going on than meets the eye. Although the vibrator is mechanically tuned, it might possible that the duty-cycle is affected by battery voltage resulting in some crude regulation.

EXTRA: I see a possible problem, the primary wants to resonate but the 2nd half-cycle is being clamped by the MOSFET's body diode, returning the energy back to the supply.

[Xena E]

The 0.1uF capacitor could be there to resonate with the primary inductance causing damped oscillations of a more controlled voltage rather than a massive narrow spike.

This. (A very astutely made comment).

The tuning of these old vibrator supplies is a very subtle science and was key to their success or failure.

The buffer capacitor as they were known could be fitted to either the primary or secondary side of the transformer and were not there to resonate the winding to the operating frequency as some believe, but to resonate the dead time to synchronise the free ringing part of the cycle to allow the contacts to close with as little voltage across them as is practically possible.

The foregoing is compromised if the capacitors change value (due to age for instance, if this is auto related, this unit is likely to be 70 years old, and often paper dielectric caps were used, which are crap). This leads to sparking of the vibrator points and is the usual cause of unreliability in that component.

In the normal push pull topology replacement of the vibrator interruptor with a pair of MOSFETs driven out of phase with each other and at perhaps 90% duty cycle is a common replacement practice.

This circuit seems unusual as it is effectively a high side fed flyback converter, that is in itself going to be problematic with the drive transistor on the 555 circuit, and has been mentioned may have implications due to the body diode of the MOSFET.

Perhaps using a pnp bipolar device and protect it by bounding the backswing of the transformer primary with a Zener snubber circuit?

The circuit is OK, however do use a resistor of 47-100 ohms from pin 3 of the 555 to prevent backward propagation of any fault conditions that may occur.

I would also be inclined to provide a means to alter the frequency of the circuit so that fine tuning of the off load current consumption can be minimised which will be around the original operating frequency if all other components are in good order.

Regards,
Xena.

[DanMann]

Xena, you are close, 80 year old technology.
I did incorporate potentiometers in parallel to the two. I also used one to both resistors with the center to pin 7 in order to try balancing both sides in one shot.

Even though potted I was able to check all components from various taps. Some the resistors have wandered off. Of all the units I played with only one had a bad capacitor.

I will try the resistor from pin 3 and look into the bipolar with zener, unless anyone has a suggestion where to start with a circuit, or for me to read up on it.

@Andy, the problem is the vibrators have a limited life span and have not been available in a very long time. This is due to better vibrator/transformer configurations.
I did run one with a vibrator and got two pictures of settings at different time intervals. Notice how the frequency changes. I think this may be due to how this cheap oscilloscope takes the readings. I did read the frequency on my multimeter and in the range of 200hz even though the scope said otherwise.

Thanks fro all the help, it is helping me to better understand what is going on as well as learning more about circuits.

I did pull out an old bench scope that I just acquired and as it happens, the vibrator started buzzing and failed before I could tune the scope. It was vibrating but no output. Putting in another vibrator it seems this one is also vibrating, but no output as well.
This is due to the contacts fouling up.
This is why I am looking for a solution to oscillate the power supply

[xavier60]

Because the 0.1uF isn't wired directly across the primary, it can be easily checked for leakage with an insulation tester set to a suitably low voltage. The output capacitors should also be tested.
The damped oscillations appear to be too damped. It could be just due to the filament loading.
I see no easy solution to the possible body diode problem.

[Circlotron]

The 0.1uF capacitor could be there to resonate with the primary inductance causing damped oscillations of a more controlled voltage rather than a massive narrow spike.

This. (A very astutely made comment).

The tuning of these old vibrator supplies is a very subtle science and was key to their success or failure.

The buffer capacitor as they were known could be fitted to either the primary or secondary side of the transformer and were not there to resonate the winding to the operating frequency as some believe, but to resonate the dead time to synchronise the free ringing part of the cycle to allow the contacts to close with as little voltage across them as is practically possible.


Regards,
Xena.

Yep. Old timey version of zero voltage switching. Glad someone else recognises this. I did a bit of fiddling with valve car radio vibrator circuits a while ago. Looks simple, but took someone quite a bit of engineering to make it go just right. Hats off to the guys who made it happen all those years ago with so little resources.

Doesn't look like a car radio vibrator cct to me. Every one I have seen was push-pull with a dual diode full wave rectifier. I'm guessing it's part of a WW2 vintage Geiger counter.

[Circlotron]

Put a diode in series with the mosfet to allow the primary voltage to ring without the body diode clamping the voltage.

[Xena E]

Xena, you are close, 80 year old technology.
I did incorporate potentiometers in parallel to the two. I also used one to both resistors with the center to pin 7 in order to try balancing both sides in one shot.

Even though potted I was able to check all components from various taps. Some the resistors have wandered off. Of all the units I played with only one had a bad capacitor.

I will try the resistor from pin 3 and look into the bipolar with zener, unless anyone has a suggestion where to start with a circuit, or for me to read up on it.

So, I've drawn up  a quick sketch of something that you may be able to develop: principally sound but it may need tinkering to work.

I'm not saying the use of a MOSFET isn't possible it could work OK, but its trying to work in follower mode, series diode as suggested by Circlotron is a first option. This is just something else to try.



PNP transistor to replace the contacts, that being protected by the Zener snubber, the transistor is WHY? But of course there will be an Ic consideration and the induced voltage should be limited to around ×10 of the supply, so choose the transistor and Zener appropriately, with the transistor Vce higher than the Zener voltage of course. The drive to the transistor must also be able to swing near enough to the + supply to switch it off.

I built an ignition unit for a friend who has an old Fordson gas/kerosene tractor that used a similar arrangement. the transistor was a TIP 3055 (6volt + ground, so circuit inverted, and a Kettering ignition coil as load, it survived for over two years with a 56 Volt 1W Zener for protection)

It may also be possible to just use a series capacitor/resistor combo across the primary but without the actual coil in front of me the component values would be pure guesstimate.

I'd be inclined to start with the transistor drive "low" (on time) short, and increase from there watching for activity from the tube heater.

Normally setting this kind of circuit up is a bit tricky if you don't know what HVDC voltage you're aiming for, though if all else fails the heater power of the tube can be determined using a lamp/LDR combination.

Regards,
Xena.

[Circlotron]

Xena, if you use a BJT for switching, it could be useful to put a diode from base to emitter. Cathode end to emitter. If the CE gets reverse biased from a bit of oscillation it will forward bias the CB junction (no big deal) and that will reverse bias the BE junction and risk popping the transistor.

[Xena E]

Xena, if you use a BJT for switching, it could be useful to put a diode from base to emitter. Cathode end to emitter. If the CE gets reverse biased from a bit of oscillation it will forward bias the CB junction (no big deal) and that will reverse bias the BE junction and risk popping the transistor.

Good call!

I assumed the OP would be familiar with the drive requirements.

The most likely thing I'd do would be also add a parallel resistor across the be junction to hold off the transistor and limit any leakage due to charge carriers (≈10k), and a series one as mentioned before in this case to limit base current (≈120 Ohms)

I was also assuming the use of a standard bipolar 555 device as well... the drive ability of the cmos ones are a bit weedy when sourcing current.

X

[xavier60]

But we possibly still don't want to clamp the positive swings.
A series diode will work. I was overthinking it, with the idea of needing back-to-back MOSFETs.

[Andy Chee]

@Andy, the problem is the vibrators have a limited life span and have not been available in a very long time. This is due to better vibrator/transformer configurations.
I did run one with a vibrator and got two pictures of settings at different time intervals. Notice how the frequency changes. I think this may be due to how this cheap oscilloscope takes the readings. I did read the frequency on my multimeter and in the range of 200hz even though the scope said otherwise.

Ok, so just to confirm.  Everything completely original as possible, but you're more than willing to ditch the vibrator unit?

My suggestion was to use the vibrator (with appropriately serviced and cleaned contact points) to drive the MOSFET/transistor/IGBT/whatever. 

Due to the MUCH lower current involved and no voltage spikes, the contact points would last infinitely longer if used solely for driving a MOSFET/transistor/IGBT.

From what little knowledge I have about these things, vibrators predominantly fail from contact arc burning, so this approach of driving an external semiconductor with the vibrator should be a viable one.

But if you're willing to sacrifice originality and ditch the vibrator, that's ok too! 

Are you able to at least get your existing vibrator working electrically (i.e. clean the contacts), and hook it up to the oscilloscope disconnected from the HV module?  This should give you a starting point for duty cycle, then you can adjust the 555 to match duty cycle.