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Pulsing volt meter

lesrollins

By lesrollins
in Boat Building & Maintenance

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Tony Brooks Veteran II

Tony BrooksDonate to Canal World

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At what sort of frequency? As long as all the charging & starting circuit terminals are clean and tight I would suggest that could be:

 

A fairly heavy electrical load turning on and off

A slipping drive belt.

A faulty alternator (could be an internal connection, brush/slip ring problems, regulator problems).

After several hours cruising and IF you have solar it might be fully charged batteries. Try running a fairly heavy load and see if that stops it, if so this may be the cause.

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FORTUNATA Experienced

FORTUNATA

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The DC alternator is a pule to be accurate. When an AC generated voltage is passed through diodes, only the positive voltage wave can pass. The current will also flow through the diode. With alternators the DC output will be a ripple. That's not an issue unless you plan to use the DC to power a radio. The ripple will have varied amplitude as it's derived from a sine wave. If you get the pulse with the engine off then that is another matter.

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Tony Brooks Veteran II

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The DC alternator is a pule to be accurate. When an AC generated voltage is passed through diodes, only the positive voltage wave can pass. The current will also flow through the diode. With alternators the DC output will be a ripple. That's not an issue unless you plan to use the DC to power a radio. The ripple will have varied amplitude as it's derived from a sine wave. If you get the pulse with the engine off then that is another matter.

 

Sorry to disagree but all but a few very early French alternators are full wave rectified so both the positive and negative halves of the AC wave are rectified.

 

They are also three phase machines but there are many poles (count the iron fingers on a rotor) with each phase consisting of many different cols around the stator but all in the single phase wire.

 

The upshot is that you en up with a nominal 12V DC current with a ripple on top. The only thing fast enough to measure the ripple is and oscilloscope.

 

Moving needle voltmeters have so much friction and inertia they can not register the ripple while the sampling rate of digital voltmeters is too low to register the ripple.

 

If two of the phases have failed then the pulses might be slow enough for meters to register.

 

This is one reason I asked about the frequency of the pulses. Its much more likely to be a poor connection or one of the things I mentioned.

 

 

 

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FORTUNATA Experienced

FORTUNATA

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It's not likeky to detect the ripple then. Full wave rectification should have no bearing on the ripple really. If such were the case capacitors wouldn't be used as a resevoir. Unless the French alternative includes a choke and large capacitor, which I guess is possible. You will find both full wave and half wave rectifiers still actually contain positive and negative amplitude which can eaasily be reconverted back to a sine wave. It's just that the negative halves are blocked. With full wave both positive waves at each end of the winding are passed through the diode. With half wave only one diode passes one positive cycle. And the centre tap is always the zero return.

Sorry I meant to say "diodes" for full wave the anodes of which are fed from opposite ends of the winding.

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WotEver Veteran II

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You will find both full wave and half wave rectifiers still actually contain positive and negative amplitude which can eaasily be reconverted back to a sine wave. It's just that the negative halves are blocked. With full wave both positive waves at each end of the winding are passed through the diode. With half wave only one diode passes one positive cycle. And the centre tap is always the zero return.

Sorry I meant to say "diodes" for full wave the anodes of which are fed from opposite ends of the winding.

 

Most of the above is entirely incorrect.

 

1. Half Wave Rectifiers (basically a single diode) have no negative component. They have a positive pulsating output.

2. A Full Wave rectified signal will have the same output as (1.) but with twice as many peaks, as the negative going part of the AC signal is also passed through as a positive peak.

3. It is not possible to easily reconvert a full wave rectified signal to a sine wave, and it is completely impossible to do so with a half wave rectified signal.

 

This nice image demonstrates it well:

Rectified_waves.png

  • Greenie 2
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You can pass negative cyles by diode bridges. Usually the rectifiers I mostly encounter myself work as follows: Each end of the transformer emits an AC sine wave. Each transformer wire is coupled to the diode of a rectifier. Usually the anodes were enveloped in one casing and the cathodes coupled but you could just as well use two diodes apart. Centre of the transformer would be a zero tap which is the same principle used in electric supply. The tap halves the original voltage (that would be without the tap). This runs to ground in power supply. Each positive AC cycle in this case can only pass through the anode and draw current. The negative cycle cannot pass via an anode. However, at the time of the negative cycle, a positive wave pulses from the opposite end of the transformer. As this is connected to the anode of the other diode, it can pass and draw current. No negative cycle will pass but be blocked. The current then flows but as a ripple. This is called rectified DC. By joining the cathodes of both diodes the full output of the transformer can be utilised. I have measured the DC ripple at the cathode of a diode under high voltage and without the smoothing circuit. With the correct capacitor (about 16mF for mains), the capacitor will charge via the positive pulses of the DC. And yes there is an amplitude in the ripple which is employed in TV engineering as AGC (gain control)

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I was multi-tasking today so was trying to join in this discussion, in between testing my new Lucas alternator. I had to really rev it to get the warning lamp to "kick in". My apologies if I went a bit far off topic earlier. No, the meter isn't going to be picking up any ripple. I was just chipping in to say in most cases the D.C. Output isn't smooth. Another point that's worth noting for the sake of curiosity is the DC rectified output tends to be the peak of the AC generated output. I forget the sum but normally if you rectify 230 AC you may get about 280 odd DC through the diodes.

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WotEver Veteran II

WotEverDonate to Canal World

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230V is purely the RMS voltage. The peak value is 230 x 1.414 or around 325V, which is 650V peak to peak.

 

Full wave rectified mains will have peaks at 325V but the 'average' voltage to do any work (like heat a bar fire) is still the RMS of that voltage i.e. 325 x 0.707 = 230V.

 

Tony

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Tony Brooks Veteran II

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I was multi-tasking today so was trying to join in this discussion, in between testing my new Lucas alternator. I had to really rev it to get the warning lamp to "kick in". My apologies if I went a bit far off topic earlier. No, the meter isn't going to be picking up any ripple. I was just chipping in to say in most cases the D.C. Output isn't smooth. Another point that's worth noting for the sake of curiosity is the DC rectified output tends to be the peak of the AC generated output. I forget the sum but normally if you rectify 230 AC you may get about 280 odd DC through the diodes.

 

How can that be. The ripple is on top of whatever the charging voltage is and its the peaks of the ripple that show the peak AC generated voltage. Its perfectly clear when you look at the battery voltage while charging with an oscilloscope.

 

You are correct saying the DC output is not smooth but as far as a boat's electrical equipment is concerned it is. Unless you are using the ripple to diagnose blown diodes/faulty phases ripple is best ignored and the output thought of as pure DC

If it's a really old system (alternator type xx-AC instead of xx-ACR, with a relay-driven external regulator) pulsing of the ammeter is normal.

 

But not on every system on every boat. On the fleet we had boats with the batteries at the front and engine at the back (Brads design) and they all tended to pulse after (say) 15 minutes of charging. The same type of engine and batteries but all close to each other tended not to pule. I put it down to some weird interaction between the lead length and the regulator.

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It's been ages since I had to work out the DC output from an AC supply, but suffice it to say it will be high. It can be lowered using capacitors or a resistor or both. I would imagine as Tony states the batteries would probably "soak up" any ripple. By the application of 14 volts ripple on the battery + terminal, the battery + eventually equalises with the output + and electrons polarised in the plates. A simple system but reliable.

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jonathanA Experienced

jonathanA

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It's been ages since I had to work out the DC output from an AC supply, but suffice it to say it will be high. It can be lowered using capacitors or a resistor or both. I would imagine as Tony states the batteries would probably "soak up" any ripple. By the application of 14 volts ripple on the battery + terminal, the battery + eventually equalises with the output + and electrons polarised in the plates. A simple system but reliable.

I'll have pint of what's he's been drinking.....

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Keeping Up Veteran II

Keeping UpDonate to Canal World

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But not on every system on every boat. On the fleet we had boats with the batteries at the front and engine at the back (Brads design) and they all tended to pulse after (say) 15 minutes of charging. The same type of engine and batteries but all close to each other tended not to pule. I put it down to some weird interaction between the lead length and the regulator.

Agreed not all, but most. On our first boat the regulator was very crude, it was more like a dynamo regulator although it was controlling an alternator: a relay drove the alternator full on or completely off, and the on/off ratio controlled how much average current you got. The relay flapped about once a second.

 

It reminds me that my Adverc used to pulse in the same way for a while, and I put that down to an interaction between the lead length and the regulator. It started after I'd added an additional battery, and went away when I rearranged the wiring, and since then I've heard of two other similar examples. I wonder if the OP here has an external regulator?

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