Film-Tech Forum ARCHIVE: Rectifier Smoothing Check

This is topic Rectifier Smoothing Check in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Christopher Woollard (Member # 2455) on 09-14-2019, 08:01 AM:

Today I tried a simple smoothing test on our rectifier and was wondering if this is a satisfactory method.
Placing a small solar cell in the lamp house, a DC and AC reading was taken using a scope. The AC ripple peak to peak voltage was 2.1% of the DC offset. When the smoothing was installed some 16 years ago, this reading was 1.7%.

It is my understanding that 3% is the upper limit if you wish for long lamp life. Obviously the electrolytic capacitors are aging, still "acceptable" though ideally 0% would be found. There is 100,000uf smoothing on a single phase rectifier.

Posted by Mark Gulbrandsen (Member # 72) on 09-16-2019, 07:29 AM:

Normally it is read across the DC lines with a scope or via a shunt in series with the positive lead of the lamp house. I use an ORC Inrush/Ripple meter to do it. But those are pretty rare pieces of equipment... Inrush current is usually the more detrimental aspect to a lamp. You need an accurate shunt and a storage sacope to measure that.

Mark

Posted by Dave Macaulay (Member # 813) on 09-16-2019, 08:51 AM:

Assuming this is a "brute force" rectifier with a big heavy transformer and diode bridge - switching rectifiers are very different:
The big capacitor provides some smoothing but also the starting "inrush" current to initiate a stable arc.
These also have a large inductor to provide most of the power smoothing.
A single phase "brute force" rectifier inherently has more ripple than a similar power 3-phase type.
You can measure AC ripple directly on the lamp power, or across the meter shunt in the lamphouse (this is a rather low voltage). Be careful at lamp start, the lamp off DC voltage is dangerously high and the starting voltage quite deadly. A meter or scope will be fried if it's connected where the starting voltage is present.
I would look for lamp electrode damage first - if long hour lamps still look reasonable, ripple is not a problem for you. If the anode grows horns and you have flickering light before the lamp is close to warranty hours, ripple may be excessive.
The capacitor(s) can be replaced easily, Digikey or Mouser or whoever you use for electronic parts should have replacements. Be careful on first powerup with new caps: they can fail (dramatically!) then, so have covers on and stand back when you push the go button. If they are OK for this "forming" powerup they seem to be good for years.
The inductor is not serviceable, it looks like a big transformer but with only two connections to a single large wire winding.

Posted by Christopher Woollard (Member # 2455) on 09-16-2019, 12:28 PM:

I will be on the lookout for a modern switched mode rectifier and replace the old unit.

Posted by Mark Gulbrandsen (Member # 72) on 09-16-2019, 05:52 PM:

quote: Christopher Woollard
I will be on the lookout for a modern switched mode rectifier and replace the old unit.

Beofre you run out and do that, what type of rectifier is it? Switch mode rectifiers carry their own, and completely different types of problems over brute force units. There are still some very good brute force units in service.

Mark

Posted by Christopher Woollard (Member # 2455) on 09-17-2019, 08:29 AM:

It is an old T&R rectifier with new electronics (silicon diodes + electrolytic smoothing capacitors) which is large, heavy but seems to work well. No problems detected with it in fact but a lot of modern equipment is available for free these days.

Posted by Tony Bandiera Jr (Member # 2365) on 09-19-2019, 11:35 PM:

RE the technique used to detect ripple:

The solar cell idea is a good one, with one small flaw: any arc instability caused by the lamp itself or other factors (bad electrodes, poor gas fill, loose connections, bad arc stabilizing magnet) would influence the readings, so it would not necessarily be an accurate indication of the rectifier's output.

That said, it IS a very smart and clever way to check for lamp instability that may not be visible to the eye, so it has merit.

I have used the DC lines and scope method in the heyday of film as a tech. I felt it gave the best and most accurate results.

Posted by Gordon McLeod (Member # 33) on 09-20-2019, 07:05 AM:

One way is to measure the AC voltage across the lamphouse meter shunt as it will be close to non inductive

Posted by Christopher Woollard (Member # 2455) on 09-24-2019, 12:29 PM:

Interestingly the tiny ripple visible is a pure sine wave, indicating ripple. There is no arc instability visible on the trace (or the screen) which is very good news. The projector has a horizontal rake. My only objection to the rectifier is that it is only 68% electrically efficient, ac power in to dc powe through the lamp.

Posted by Mark Gulbrandsen (Member # 72) on 09-24-2019, 05:33 PM:

Thats actually a pretty good efficiency for a boat anchor rectifier. They are usually in the 35 to 40% region, with a lot of the loss being heat in the transformer, choke and rectification devices. You need to figure the ripple as a percentage so you know if you are within tolerance for the lamp. Osram used to specify <7% for 3kw and up size lamps. And <10% for 2.5kw and smaller.

Mark

Posted by Christopher Woollard (Member # 2455) on 09-25-2019, 01:34 PM:

Interestingly the tiny ripple visible is a pure sine wave, indicating ripple. There is no arc instability visible on the trace (or the screen) which is very good news. The projector has a horizontal rake. My only objection to the rectifier is that it is only 68% electrically efficient, ac power in to dc powe through the lamp.

Posted by Mark Gulbrandsen (Member # 72) on 09-26-2019, 09:01 AM:

Even the best switchers only hit about 80% efficiency. So again you are not doing too badly in that department. The sine wave indicateds that the choke is doing its job.

Mark

Posted by Christopher Woollard (Member # 2455) on 09-30-2019, 01:12 AM:

Interestingly the tiny ripple visible is a pure sine wave, indicating ripple. There is no arc instability visible on the trace (or the screen) which is very good news. The projector has a horizontal rake. My only objection to the rectifier is that it is only 68% electrically efficient, ac power in to dc powe through the lamp.