Cap value for timing circuit

On the face of it anything that'll get up to 330mfd and withstand 150V should do. That's a honking big cap, and anything that's not a 'lytic will probably be bigger.

--
www.wescottdesign.com

DaveC wrote in news: snipped-for-privacy@news.eternal-september.org:

A motor start cap should be ok. They're meant for intermittent use under heavy load so they don't accept heavy ripple currents for lomg, but this one won't have to, with a few tens of kilohms between it and the mains.

You can potentially vary the values of the resistor and capacitor greatly depending on how much drive is needed. As long as the time constant R*C is the approximately the same it should approximately work. Obviously if it's driving a relay then it needs enough current to turn it on. So your probably not going to be able to vary R much unless it wasn't chosen well in the first place.

The electrolytic caps were used because of the high voltage and high capacitance. I doubt you can find anything but an electrolytic to replace it with. Remember you can put them in parallel to increase the capacitance. Just get you two, three, or how ever many you need to get approximately

300uF at 150V. I woudln't get any lower voltage and try to use them in series because it is usually more trouble than it's worth.

Note, you can replace this simple delay circuit a mosfet who's gate is charged up by a lot smaller capacitor and larger resistor.

Basically the gate is turned on in a similar time frame as the original but requires virtually no current to do so. The mosfet then turns on which turns on the relay.

All you need is a resistor(actually two), a cap, a suitable mosfet that can handle the current and voltage(relatively easy to find for a few dollars), and a diode for a snubber.

Since your input is 150V or so you'll have to reduce it for the mosfet's gate voltage. I'll want to use a P ch but could probably put an Nch on the low side of the coil.

R1 |

+-----+---mosfet Gate | | | C R2 gnd | gnd

(use a fixed width font to view)

The R1 and R2 form a voltage divider. You'll want something about 12 to 1 so that the mosfets gate only sees at most about 15V. A large voltage spike could ruin the mosfet gate so technically it would be best to proect it. You can do this by adding a 15-20V zener from the gate to ground.

Add the diode across the mosfet drain to source(look up diode snubber). Most power mosfets actually have a diode built in to do this though.

In any case if you want to go this route then I'm sure someone can whip you up a circuit. It's relatively simple and should do the trick better than the large cap(And probably cheaper). Theres just a few small issues such as getting hte polarity right and getting the voltage divider right(pch on the high side would reverse the resistors and the cap would be to vcc rather than to ground).

DaveC wrote in news: snipped-for-privacy@news.eternal-september.org:

What was the existing cap? Any clear indications as to why or how it failed?

BTW, if ac is on the cap and not rectified you'll either want to add rectification or go with a triac/scr version instead. Mosfets only work well one way. I'm assuming the relay is DC since you mentioned an electrolytic cap.

DaveC wrote in news: snipped-for-privacy@news.eternal-september.org:

Another thought, if you ever need a high quality cap with high voltage and a few hundred µF at short notice and can't buy one, look for disposable or otherwise surplus camera flashes. they're usually good for electrolytics at 350V at 200µF or more.

No. Guesses: Old age? Heat?

A single rectifier in series with 50 ohms feeds the charge current to the cap. Seems that 150v is a bit overkill, no? (The 25K pot is on the discharge side of the circuit which is switched in at the appropriate time.)

I think I'm going to go with the 270mfd / 125v start cap. It's sourceable locally, fits the space, and although a bit low on the capacitance it will fit the bill better than what's there.

DaveC wrote in news: snipped-for-privacy@news.eternal-september.org:

Don't worry if the cap is specced for a higher voltage, if it's cheap, have at it, the margin of safety is good, the wider, the better.

DaveC wrote in news: snipped-for-privacy@news.eternal-september.org:

Old age isn't a mechanism. :) Heat could well be right. Also accelerated loss of electrolyte. Did the cap look like it had burst from inside earlier than any final appearance of internal gubbins? Another possibility is the voltage it saw. When you say 110 volts, you mean the mains, right? If so it will be rectified and that cap will be seeing the peak value, not the RMS, so around

125V when under no load, so you really do need it to have a higher voltage rating than that.

Lostgallifreyan wrote in news:Xns9C9E2D5A92AE2zoodlewurdle@216.196.109.145:

Btw, I know that that cap ought not to be unloaded to the point where the voltage gets to full height in that system, but best assume it will, so if the load fails the cap remains safe and you get no bad future comeback. I've seen an amplifier that had an internal fire resulting from a cap failure. It shocked me. Very very messy scene. I wasn't given to me for repair, I didn't even get much salvage out of it.

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    peak = RMS * sqrt(2) = 110 * 1.414 ~ 155V

JF

John Fields wrote in news: snipped-for-privacy@4ax.com:

Agreed. I even knew the formula and entered exactly 1.414, no less, so I'm still wondering what the hell else I wrote into the Windows calculator this morning to get an answer of fractionally over 125V. I'll put this down as yet another reason to aim for safe margins if the cost doesn't prevent it.

(Actually I found it. I typoed as 1.141...)

This is for half-wave rect.?

Thanks.

DaveC wrote in news: snipped-for-privacy@news.eternal-september.org:

Yes, under no load either peak of a sine wave defines the maximum (absolute) value for voltage across a capacitor. Full wave rectification just means more energy and less ripple present for a given load.

Half-wave and full-wave rectification result in the same peak voltage. It's just that for full-wave, the peaks occur twice as often.

My line voltage runs well above 110.

PG&E says they target for a min of 114 and a max of 126. That's at the handoff point. I think the idea is that you get a min of 110 if you allocate a few volts for drop on the wiring within your house.

Here is PG&E's blurb:

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These are my opinions, not necessarily my employer\'s.  I hate spam.

Yep, yep.. half-wave is a single rectifier, full-wave is 2 rectifiers often with a center-tapped AC source, and full-wave-bridge rectification is 4 rectifiers.

-- Cheers, WB .............

No, it's for AC and for a full-wave rectified sine wave.

What the deal is is that if 120VDC is placed across, say, a 120 ohm resistor then the resistor will dissipate 120 watts and generate heat.

In order to get the same amount of heat generated by an AC voltage connected across the resistor, (called the root-mean-square, or 'RMS' voltage) it'll have to go above the steady DC value, on its peaks, because that's the only way it can make up for the valleys which fall below the steady DC voltage.

For AC and full-wave rectified AC, the number of peaks and valleys are the same, and to get either of those voltages to heat up the resistor the same amount requires that the peaks rise to the steady DC voltage multiplied by the square root of two, and that voltage is called the 'peak' voltage.

For half-wave rectification it's an entirely different story because half of the half sine waves are missing.