Reader suggestions, alternatives Vol 1

Following are suggestions, techniques, research, alternative approaches and information submitted by AAC members (to be linked in tutorials).

Authors of content quoted here are urged to review their contributions such that any desired 'adjustments' may be affected in a timely fashion -- Please be advised that any and all alteration of content must be by its author only (thence its 'amendment' here via 're-quoting' of the author-edited content at its original location)...

Note also that author submitted requests for alteration or deletion of content from these entries (and, hence, reference from our tutorials) will be honored at any time) -- Content is 'anchored' here merely as a convenience - Authors retain full ownership of their content!

---Begin member feedback (Ordered alphabetically by user name)---


I got myself one of these. At $119.00 dlls, I consider it a bargain.


While browsing for some other stuff, I stumbled upon this cheap-o brand inductance meter.

It doesn't look as sophisticated as the one I previously recommended, but it's so cheap that it might be worth considering as a poor-man's alternative.

Re: Fabrication of pot-core reluctance gap spacers:

A small suggestion, HP. Instead of using the soft-material standoffs as patterns for performing the cutting of the gap spacer, I'd suggest you use a pair of ordinary steel flat washers. It wouldn't matter if the washers were a bit larger or smaller than the standoffs.

With a bolt and a nut sandwiching the two metal washer together, it's a lot easier to firmly tighten the assembly and then use an x-acto knife or box-cutter to trim the excess material in a more precise way. A much more even cut finish would be obtained this way. It would also have the added advantage of not risking the integrity of the standoffs in the process. One could also add more than one layer of material so as to obtain several spacers at a time instead of just one.

Here's an addition to my previous suggestion ... so as to "gild the lily" ... :)

Instead of drilling the sheet stock one by one, and using one of the standoffs as a template, a better idea would be to sandwich several layers (up to ten, for starts) of sheet material for the spacers between two pieces of wood, and tightly hold it together using a couple of c-clamps (or screws, or whatever you can find available for this purpose). The wood would be sacrificial, as you'd be drilling through the whole thing. But the wood, and the pressure applied, would prevent the sheet material from moving, and more importantly, it would produce a much cleaner, sharper, and rounder hole, since the wood's fibers will be pressing tightly against the edges of the hole as it is being drilled. Afterwards, you may remove the drilled material and clamp it between the two washers that I had previously suggested to trim its outside diameter.

I've used this sort of technique many times before, and it always works like a charm.

Re: Agents effective against 'Epoxy' adhesives:

Cmartinez tnx but since I don't have time to properly read 6 page thread right now what I got from skimming is ur suggesting nitrogen hydrides (like NH3 and N2H4)?

Nope, I'm suggesting you try NMP ...

Ok ... after some thought regarding the PTFE spacers, the obvious answer has come to me.

I've owned a signage business for more than 20 years now, and have extensive experience regarding the cutting and machining of plastics. But I have very seldom (maybe once or twice) had the opportunity to work with PTFE. But now I remember.

Like all materials, PTFE is elastic and can be plastically deformed, but in this case it is especially so. When you cut through any material, the force exerted on its surface while the cut is taking place distorts the material in the inner layers. Think of it this way, how can you cut a perfectly orthogonal cube of jell-o using just a knife? The answer is that it's close to impossible.

What's happening is that the ordinary two-flute, spiral drill bit being used to drill through it is tearing the material upwards as the bit penetrates it. Now, drill bits are only really sharp at their lips. So once the material is drilled through, the circle's edges are not really being cut through by the lips, but are rater being teared off by the flute's heel's. That's why the finished hole comes out distorted.

See this diagram so you may have a clearer picture of what I'm talking about.

What we need to do is make sure that the hole's edges are effectively being cut instead of teared off.

The answer to that is a straight-flute cutting tool that has a chiseled tip. And there are many types of that sort of tool out there, but some of them are not easy to find. Here are the two types that I'd recommend:

And another option would be a step drill bit, which not only complies with our requirement of a chiseled-tip and straight cutting edge, but has the added advantage of having multiple diameters to choose from when cutting sheet material:

Again, it is important to sandwich the sheet stock between two layers of sacrificial material to make sure that it remains firmly in place while the drilling operation is taking place. In this case I would try two layers of 1/4" hard marine plywood, or two 1/8" thick plates of aluminum. You'd have to test and see what works best for you. So buy yourself a couple of these tools, and test test test until you get what you want.

And yes, punching the material with a round sharpened tool would be an excellent way of doing what you want. All you'd have to do is apply pressure to the material (using a flat piece of wood as backup), using a bench press, and voilá! ... instant excellent results. But although fabricating said tool (which would also punch through the piece's inner diameter) would be a piece of cake for someone like me, for the amateur building this thing it would be not just hard but also a waste of time and money if other, easier to implement options are available.

halogen bulbs such as these have an impedance of between 3.8 and 4.0 ohms (I know, because I just measured it) and 5 of them (connected in series, of course) will cost you a bit short of $10.00 dlls in Amazon:

Of course, the impedance will change once they heat up a bit. But it might not matter, because you want them only for startup purposes, if I'm not mistaken.

This link describes a flame test to tell acrylic plastics from polycarbonate:

As primitive as that test may be, I have used it for that identification as well as to tell other plastics apart. Test your chips from drilling. Probably good to use known materials for your comparison.

Although acrylic does tend to crack more -- particularly with standard 118° drill bits -- smaller bits (<0.125") may do just fine, particularly if they are a little dull. In fact, a purposely dull bit ("dubbed" off) works well and over 1/2" holes can be drilled easily with acrylics. The purpose of dubbing is to create shavings and prevent "hogging in" (where the bit pulls itself into the material). Dubbing means to grind the cutting edge of the bit so it is not an acute angle, but rather meets the drilled surface perpendicularly and gives a scraping action.

Also, PC is more chemically resistant. Chloroform melts acrylic easily and less so with PC.

Just a few notes about acrylic:
1) The bubbling is characteristic. I believe it has to do with acrylic sheets being cast. If vacuum forming, it is important to let it degas first. That step is not needed with PC, and I have not seen that caution mentioned when just bending with heat.

2) Acrylic glues that are based on chloroform (and probably similar) work well, but eventually cause crazing. Similarly, exposure to oils as in some types of caulking or even prolonged stress will cause crazing. That may take years to develop. PC does not seem so prone to crazing. When I needed a safe caulking for an acrylic windshield, I got a two-part, polysulfide adhesive/caulk that had mil approval. No problems with crazing over several years. Unfortunately, it is black.

3) Finally, very large holes (e.g., 100 mm) can be done with a fly cutter and a rigid drill press or mill.

there are different grades, which probably has to do with the way it is processed, surface coatings, and/or heat treated. One source mentioned "extruded" properties versus the usual cast properties. For example, if a plastic were reheated after casting and stretched or extruded, I would expect changes in physical properties. If plasticizers are used, they were not mentioned and are often trade secrets.

Here are a few of the interesting hits I came across:

1) This link describes various aerospace grades and MIL specifications for them. The terms, "cross-linked" and "semi-crosslinked" are used, so there must be some differences:

2) These two links are nice "how to" resources: with Acrylic.pdf Sheet Fabrication Manual--Plexiglas.pdf

3) Finally, Boedeker offers two grades, a bendable version (AC-350) and non-bendable version (AC-300). The specifications provided for each are identical, except for one. The heat deflection temperature (ASTM D648) for the bendable version is 98°C and the non-bendable is 96°C. Since you seem to like the flame test, Boedeker presents an extended table here:

Max Headroom:

I use Antek, he manuf. and sells Toroidal Txr's, also on Ebay under Antec-inc.

I also have bought from him over the years. Can't get a better transformer for the price. Not sure but think he will wind what is needed and the price drops with the number ordered. The set up is what costs, like in any mechanical/machine operation.

@The Electrician

Am I to understand that --in your considered opinions-- you feel Antek is in PowerVar's 'league' as regards the 'balance' of quality, cost and capacity?

The price is unbeatable and you can get the turns ratio you need.

The manufacturer says in the description: "These transformers have heavier gauge wires then the normal requirement to avoid the copper lost during the full power output. The dielectric test is more than 3500V in between primary and secondary coils. Please see the test data for short circuit and open circuit. In most of the cases, this transformer can be output 20% more power from its rating at 60Hz power source without any problem."

Never had any problems, even stuck a small overwind on for a low voltage low current supply in a pinch.

I just used them as power for stepper motors. So can't really comment past that. But will say they look as good or better than the ones HP showed in those units. They are a very professional company.


As promised a simplified sketch of the mounting plan. The dimensions are based on a 4 inch meter, since I don't know the size, I guessed.


Now for the core disassembly part.

The first picture shows how to measure for the hole positions in the parts to be made.

The second shows the individual parts to be made. While I show one way of making the screws, out of allthread rod, wing nuts, hex nuts, and lock washers, that is not the only way of doing it. An eye bolt that is long enough and with threads the full length of it's shank would also work. I show the "U" type clip nut because most people won't be able to tap the holes for the screws. It has to be the "U" type clip nut shown, there is another type that is just sheet metal without the thread tower that won't give enough strength or threads for this purpose. All of the things shown in this picture are available at McMaster-Carr, most Ace hardware stores will also carry them, or the ones in my area do.

The third and fourth pictures show the assembly of the whole. In use the angle iron pieces would be clamped to the core as shown. Then the screws would be snugged up against the bottom angle irons and a slight pressure put on the assembly with them. This will then start to pull the two part of the core apart. The angle iron keeps the core from twisting and supports it through the process. After the assembly of this "puller" it's into the oven to bring the whole up to heat. After letting it heat for a while it would be removed from the oven and both puller screws tightened a little more, it may take a few times of the heat and tighten cycle to get the job done. The big thing is making sure both screws are turned to keep the pressure equal on both sides.

This 'puller' is similar to how a gear puller works, it's just a glorified gear puller made to fit the job at hand.

Re: Ballast resistor selection
is there as reason that you need a Dale type resistor? Wouldn't a regular wire wound resistor ,like the more common 'corrib' type work? They are much easier to find and cheaper in the surplus market. And they are both wire wound.

The Electrician:

Research/info in regards to inductor inrush mitigation and ripple filter considerations:

I received the AN-10435 transformer from Antek today: and

My first measurement is of the turn-on surge. Using a standard shunt to sense primary current and displaying applied voltage and resulting current on an oscilloscope, here's the result. The secondary is open circuited and 120 VAC is repeatedly applied to the primary until it happens that the voltage is applied just as the sine wave is crossing zero in the positive direction.

The yellow trace is applied voltage and the blue trace is resulting current. Notice the scale on the blue trace--50 amps per division! A peak current of 300 amps results. The lights in the room flicker! The applied voltage (yellow) never reaches its normal peak; the high surge current drops a considerable voltage in the resistance of the house wiring! However, subsequent current pulses are much smaller; the second is only about 20 amps peak.

Next, the same test but with a 10 ohm resistor in series with the primary of the transformer. Notice that the current scale has changed to 10 amps per division.

Finally, with a 5 ohm resistor in series with the primary. Here the maximum surge current reaches 30 amps peak. This would be perfectly safe, without risk of damage to the on-off switch.

Continuing the saga. Getting involved in all this has now given me an excuse to buy a bigger variac which I've been wanting for a while.

I got one of these:

First thing I did was to measure the maximum turn-on surge as I did with the transformer; here's the result:

The peak current is about 275 amps; this is only a little less than the 300 amps peak for the Antek transformer. I don't show a scope capture, but adding a 5 to 10 ohm ballast resistor decreases the surge to a safe value just as with the Antek transformer.

For the benefit of non-EE readers: regarding the value of these peak currents I should mention that the DC resistance of the primary of the Antek AN-10435 transformer is .218 ohms. Since the peak voltage of the 120 VAC line is 170 volts, one might expect that the peak current ought to be 170/.218 = 780 amps. So why is it only 300 amps? Because the resistance of the house wiring is added to the resistance of the primary in determining the value of the peak current. When the core of the transformer (or variac) saturates, the inductance of the primary drops to a negligible value, and it's only the total resistance of the circuit that determines the peak currrent.

The surge that occurs when the AN-10435 is connected to the output of the variac, and the variac wiper is set to the position on the winding just where the line input is connect is essentially not much different than the transformer alone.

The next thing I wanted to measure is the surge due to the charging of the big electrolytic capacitors following the bridge rectifier. I obtained a couple of 34,000 uF 50 volt capacitors and a heavy duty bridge rectifier. I connected everything in the manner of this post: except for some of the details such as the line filter and the over voltage protection devices.

There is no ballast resistor in series with the on-off switch for the following captures.

The line is connected to the variac at a point most of the way up the winding. When the wiper of the variac is set to that same place, the insertion impedance of the variac is at a minimum, and the electrolytic charging surge would be expected to be a maximum there.

When the turn-on surge of the toroidal transformer (AN-10435) by itself is measured, what determines the peak current is the resistance of the house wiring and the resistance of the primary of the transformer; note that the resistance of the secondary is not involved. But for the case of the charging surge of the filter capacitors, the resistance of the variac and the secondary of the transformer come into play, and we shouldn't expect such a large surge.

Setting the variac wiper to the same point where the line is connected and applying line voltage the the input of the variac repeatedly, I captured the maximum peak current into the bridge rectifier. This is the same as the current out of the secondary of the AN-10435 transformer. The first image here is with a single 34,000 uF capacitor on the output of the bridge. The green trace is the current into the bridge rectifier, and the purple trace if the voltage across the filter capacitor:

The peak surge current is nearly 200 amps and you can see the voltage across the filter cap increase rapidly during that first current pulse. Subsequent current pulses gradually decrease and the voltage also gradually approaches its final value. Since this current is the current out of the secondary of the transformer, we could expect that the primary current surge would be less by a factor equal to the turns ratio of the transformer, which is 120/35 = 3.43. This would give a primary peak surge current of 58 amps. Of course, this is separate from the transformer turn-on surge due to saturation of the core.

Next I connected another 34,000 uF capacitor in parallel with the existing one. Again line voltage was applied repeatedly to the input of the variac until I captured the maximum charging surge. Here's the scope capture:

Notice that the peak current of the first pulse is the same as with only one capacitor. This shows that it's the resistance in the variac and the primary and secondary of the AN-10435 that limits the current, not the capacitance (total microfarads) of the filter. However, one difference is that the capacitor voltage (purple trace) increases more slowly.

Now for the dirty little secret. The "heavy duty" bridge rectifier I used for these measurements was this:

Having a look at the spec sheet: we see in figure 2 that the non-repetitive surge rating for the 35 amp bridge is 400 amps. Since HP wants the power supply to supply 30 amps, for some extra margin of safety I used two such bridges in parallel. Since the peak surge current I measured is 200 amps, and since I have two bridges in parallel (I'll discuss sharing in a moment), the actual peak surge seen by each bridge is only 100 amps, well below the allowable 400 amps.

Adequate sharing of current in the two bridges is accomplished by using longer than needed (12 inches) hookup wire to each bridge from the secondary of the AN-10435 transformer, and making the lengths of the "longer than needed (12 inches)" identical. Measuring the actual currents into each bridge showed that sharing is so good one can hardly tell any difference.

And finally, all this so far has been without any ballast resistor in series with the line at turn-on. With a 10 ohm ballast resistor the initial charging current surge from the secondary of the transformer into the bridges is greatly reduced. Here's a capture of the turn-on current onto the filter caps with a 10 ohm ballast resistor. Notice that the scale for the purple current trace is 10 amps/division rather than the earlier 100 amps/division:

Using a ballast resistor and a switch with make-before-break contacts tames the surge due to saturation of the variac and transformer, and also the filter cap charging surge.

The setup (kluge?) so far:

By the way, the little clamp-on meter seen in the photo would be a good thing for a builder to have. It's a UT210E and can be had on eBay for less than $50. It's a voltmeter and a clamp-on ammeter. The clamp-on ammeter feature not only measures AC current as clamp-ons have done for years, but it can also measure DC current. This is a feature that is fairly new in a low-cost clamp-on.

In an earlier post:

I discussed the effect in capacitor input power supplies like this where the secondary RMS current is larger than the DC output current due to the very peaky current pulses drawn from the transformer.

It's difficult to model and calculate the ratio of secondary RMS current to DC output current without knowing the many parameters involved, such as DC resistance of windings, ESR of the filter caps, apparent impedance of the line at the service entrance, resistance of the house wiring, etc. So we resort to measurement.

For the following captures, the variac was set to output a voltage equal to the grid voltage of 120 VAC which is applied to the transformer primary. The filter capacitance was 68,000 uF (two 34,000 uF in parallel) and for a load I used a 4 ohm, 120 watt rated power resistor (the green thing in the picture above) and severely overloaded it to 400 watts for 5 seconds at a time. The variac was turned up until the DC output voltage was 40 volts and the current in the resistor was 10 amps.

The measured ratio of secondary RMS current to DC output current is 18.6/10 = 1.86. This is not as high as 2 or more as I speculated it might be for a large toroidal transformer, but a value of 1.86 means we have to significantly derate the transformer for continuous duty.

A linear extrapolation tells us that if we want 30 amps DC out of this supply, the secondary RMS current would be 55.8 amp, almost double the rated 30.8 amp output for the AN-10435 transformer. Antek says that their transformers can be overloaded 20% without any problems which helps a little.

Another matter of concern is the ripple current in the filter caps. With 30 amps DC out, the total filter cap ripple current would be 48.6 amps, a value not to be ignored. :(

This capture shows the current in the secondary of the transformer (green trace) and the ripple voltage riding on the 40 volt DC output. The ripple voltage was about .85 volts peak-to-peak. The secondary RMS current was 18.6 amps:

This next image shows the ripple current in the parallel combination of the two 34,000 uF filter capacitors:

And, finally, the current in the primary of the transformer:


Re: Reluctance-gap spacer fabrication:


Blog entry information

Hypatia's Protege
Last update

More entries in General

More entries from Hypatia's Protege

Share this entry