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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 28th Dec 2019, 2:21 pm   #1
G6Tanuki
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Default Capacitor dropper... sort-of.

A friend has an air-compressor in his workshop; it's very handy for blowing dust out of things, inflating tyres etc.

It started playing-up: the motor would start when the air-pressure fell, but the electric 'unloader' solenoid (which vents the compressor-output to atmosphere while the motor spins-up) wouldn't close - so the compressor just sat there blowing into free-space.

The motor had a pair of thin wires leading from it into a small PCB-and-relay-containing box, whose output fed the unloader solenoid and also a 'blow-down' valve which briefly opens when the motor switches-off, to allow any accumulated water in the receiver to be exhausted.

The PCB had what was obviously a capacitor-dropper, feeding a 2-diode voltage-doubler into a surface-mount 7815 regulator which then fed a 14-pin DIL chip.

Aha! We all know about capacitor-droppers don't we? I supposed that spikes from the motor-control relay and the solenoids had eroded the capacitor-foils and so the doubler wouldn't be doing its thing satisfactorily. Waving a DVM at it showed the output from the doubler to be a measly 10V or so - hardly what you want as the input to a 7815-style regulator.

So I tested the capacitor (which was one of the 'motor run' types not a Class-X/Y) - it was just fine!

Turned out that one of the diodes in the doubler was doing a good impression of a resistor. It being a surface-mount one and me not having any surface-mount diodes to hand I carefully drilled small holes in the PCB alongside the diode's mounting-pads, and was then able to fit a 1N4007 on the *reverse* side of the PCB,

Reassemble, test - there's now a healthy 22V coming from the doubler, and the 7815 is doing its job. Press "Go" - the motor starts, spins up, the unloader-solenoid clicks and the pressure-gauge starts to rise.

My compressor-owning friend smiled, and uttered the immortal word "Pub?"
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Old 28th Dec 2019, 7:50 pm   #2
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Default Re: Capacitor dropper... sort-of.

That's a good one and a nice catch. The cap would definitely be my first port of call. Certainly a well earned trip to the pub!

I wonder how much expensive gear like this gets scrapped for the want of a small component worth a couple of pence? Doesn't bare thinking about...
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Old 28th Dec 2019, 8:38 pm   #3
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Default Re: Capacitor dropper... sort-of.

I also wonder how many compressor engineers would just change the pcb, the entire module or condemn the whole compressor unit?
I doubt many would have the knowledge to understand how the pcb operated and had basic skills to spot the obvious voltage issue into the regulator.
I used to come across that before I retired. No manual, then no repair could even be attempted. Or worse, manual available but not had the full training course, so cannot attempt any repair!
Years of experience is a valuable asset.
Worth a pint or two. Good un!
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Old 28th Dec 2019, 9:46 pm   #4
G6Tanuki
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Default Re: Capacitor dropper... sort-of.

This was only a cheap "Machine Mart" compressor - probably cost 200 in 2000.

if I'd been charging 'professional rates' - 50-an-hour - it wouldn't have made any kind of sense for my friend to have paid me 100+ +VAT to fix it.

I think I can get quite a few more double-Whiskies and front-panel-resprays out of him!
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Old 29th Dec 2019, 10:28 am   #5
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Default Re: Capacitor dropper... sort-of.

Similar thing happened to me some years ago when a pals central heating controller failed.
Initially he called an engineer who said 'Timers gone mate....haven't got one on me so will have to order one....' He then gave a price in excess of 100 for the timer including fitting to which my mate said to wait until he got paid.

I called that evening and he recited his tail of woe....long story short, I unclipped the timer (it was a plug-in type) and looked inside. It was difficult to work on it 'live' because of it's construction but it was easy to work out that there was a 4.7uF capacitor dropper (an 'X' type cap) feeding a 12V zener diode, a 470uF cap which then fed a controller chip. Fortunately I had a meter and soldering iron on the car so did a quick check of the cap dropper....it was virtually O/C. I nipped home to find a suitable X cap (with a higher voltage rating) and returned, fitted the new cap and then plugged the timer back in. It immediately started working (previously no displays or anything) and all original settings were retained. My pal was delighted and also said the immortal 'Pub?'

The timer operated for a further six or seven years until the boiler was upgraded.
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Old 29th Dec 2019, 10:27 pm   #6
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Default Re: Capacitor dropper... sort-of.

X-rated capacitors are now coming with a warning not to use them as voltage droppers, because the self-healing action which is required for the safety rating is actually detrimental in this application; but until the message gets through, or capacitive voltage droppers become unfashionable, there is going to be a lot of kit getting discarded for want of a capacitor. Noise on the mains is what kills them; and when you think how many times the motor in a washing machine gets started and stopped just in one cycle, and how many times a refrigerator cuts in or out, it's not surprising they fail so soon. A noisy appliance will wear out its own X capacitor, and then leak noise onto the mains that will wreck every other X capacitor in every other appliance within short enough of a cable's length to get through its inductance!

When I was working on appliance controls with capacitive droppers, we tested new production samples for noise immunity (which they passed easily; a single-probe gas ignition control is already a pretty serious noise source, and you just have to hope for the sake of radio and TV audiences in the neighbourhood that the gas people designed the burner to light on the first spark), and we ran endurance tests on a normal mains supply (neither especially clean nor especially noisy) but never performed any long-term tests with a noisy supply.

A motor run capacitor sounds slightly overkill for a voltage dropper, but is certainly a "fit once" solution! What's the betting the diode was a victim of noise on the supply lines, that would have killed a lesser capacitor?
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Old 30th Dec 2019, 6:47 pm   #7
G6Tanuki
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Default Re: Capacitor dropper... sort-of.

Quote:
Originally Posted by julie_m View Post
A motor run capacitor sounds slightly overkill for a voltage dropper, but is certainly a "fit once" solution! What's the betting the diode was a victim of noise on the supply lines, that would have killed a lesser capacitor?
Given that there was already a motor-capacitor in this compressor I suspect that the (Italian) manufacturer could well have made his supplier a Sopranos-style ~~offer they couldn't refuse~~ something like "We're buying 50,000 of your big capacitors. My friend, my family-friend - you're going to give us 90% discount on these tiddly-little ones, aren't you?"

The main motor does have 'proper' suppression on its contactor [a capacitor across the contacts, and a VDR not so far away] which should have made life easier for the diode - despite which it'd failed after 'only' a couple of decades.

Must admit though, I'd have liked to see a nF or two of capacitance across each of the diodes in the doubler, to squib away any seriously-high-frequency high-energy transients.
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