Hi,
I´m not shure what multimeters you guys are using but all the ones I´ve got (5) are perfectly capable of measuring ac on top of a dc voltage without a blocking cap.
Anders,
there should be no ac-voltage to speak of on your supply, especially without a load. Since the dc voltage you measure is allright something must be wrong with the way you measure the ac voltage cause (like Nigel said) 50V AC across your caps is not possible.
Have you tried measuring directly at the caps?
William
I´m not shure what multimeters you guys are using but all the ones I´ve got (5) are perfectly capable of measuring ac on top of a dc voltage without a blocking cap.
Anders,
there should be no ac-voltage to speak of on your supply, especially without a load. Since the dc voltage you measure is allright something must be wrong with the way you measure the ac voltage cause (like Nigel said) 50V AC across your caps is not possible.
Have you tried measuring directly at the caps?
William
Exactly, and you must know the instrument how it behaves. Some instruments can take much DC + some AC but not everyone.Nigel Goodwin said:
As Nigel Goodwin has tried to teach me to use a blocking capacitor I have to ask him. How do you connect it? In series between +PWR and GND if you want to measure AC?
I'm using this multimeter from http://www.biltema.se/ no. 15-133
if it helps. Can you measure AC with this without a blocking capacitor?
It would really suck if the caps where destroyed. I’m just measuring from the wire between +PWR and GND.
I’m using a resistor when unloading the caps.
I’ll try to post a few pictures within a day or so, as soon as I have learned how to post them. Hopefully it will be easier correct any mistake I have made. Hopefully it’s just a few simple wiring mistakes.
Also I’ll try to measure directly over the caps tomorrow, as it’s rather late right now.
I'm using this multimeter from http://www.biltema.se/ no. 15-133
if it helps. Can you measure AC with this without a blocking capacitor?
It would really suck if the caps where destroyed. I’m just measuring from the wire between +PWR and GND.
I’m using a resistor when unloading the caps.
I’ll try to post a few pictures within a day or so, as soon as I have learned how to post them. Hopefully it will be easier correct any mistake I have made. Hopefully it’s just a few simple wiring mistakes.
Also I’ll try to measure directly over the caps tomorrow, as it’s rather late right now.
You Biltema multimeter is probably one of the simplier kind (79 SEK = 10 USD!). If you just dig up a 100 V 100nF- 1 uF and put in in series with your DVM wires. You will get an measurement error this will not be a problem for you.
I think you can be satisfied if you meassure the voltage across + and - at your rectifier bridge. Put the instrument at DC. Read the value and see if it's about 1.4 times higher than the voltage at AC-AC at the rectifier. Meassure also the voltage from your PS, DC voltage. This is sufficient.
If the DC voltage is 1.4 times higher than the AC, then everything is alright... for now at least.
I think you can be satisfied if you meassure the voltage across + and - at your rectifier bridge. Put the instrument at DC. Read the value and see if it's about 1.4 times higher than the voltage at AC-AC at the rectifier. Meassure also the voltage from your PS, DC voltage. This is sufficient.
If the DC voltage is 1.4 times higher than the AC, then everything is alright... for now at least.
Yes, as 'peranders' says, simply add a capacitor in series with one of the meter leads, it makes no difference which one, but most people would probably choose the positive lead. A non-polarised capacitor would be preferable, as also suggested, anywhere 0.1uF to 1uF would be fine.
However, as I've already said a number of times, measuring the AC across a 47,000uF capacitor is a complete waste of time! - and I've never heard of anyone ever doing it before!. You should measure the DC across it - NOT the AC (which will be almost non-existant). If you are genuinely measuring 50V AC across a 47,000uF capacitor you wouldn't need to measure anything to know somethings wrong, the capacitor would already be blown all over your walls (perhaps taking pieces of you with it?).
However, as I've already said a number of times, measuring the AC across a 47,000uF capacitor is a complete waste of time! - and I've never heard of anyone ever doing it before!. You should measure the DC across it - NOT the AC (which will be almost non-existant). If you are genuinely measuring 50V AC across a 47,000uF capacitor you wouldn't need to measure anything to know somethings wrong, the capacitor would already be blown all over your walls (perhaps taking pieces of you with it?).
Hi all,
I have been following this thread for a while.
My contribution:
I have measured the DC on my smoothing caps +-70.1Vdc. A bit higher than usual this morning
I have measured the ripple using an oscilloscope. About 130mV peak to peak.
I decided to try reading the caps again using the AC range set to
200Vac =>0.0Vac.
20Vac scale => 0.04Vac
2Vac scale =>0.042Vac
200mVac scale =>42.2mVac
The meter did not blow up. No sign of a blocking cap on my test leads.
If the ripple was a sinewave, which it is not, then 42.5mV converted to peak to peak is about 120mV cf. 130mV off the oscilloscope.
I begin to wonder if many of you have tried to measure what you are talking about.
I have been following this thread for a while.
My contribution:
I have measured the DC on my smoothing caps +-70.1Vdc. A bit higher than usual this morning
I have measured the ripple using an oscilloscope. About 130mV peak to peak.
I decided to try reading the caps again using the AC range set to
200Vac =>0.0Vac.
20Vac scale => 0.04Vac
2Vac scale =>0.042Vac
200mVac scale =>42.2mVac
The meter did not blow up. No sign of a blocking cap on my test leads.
If the ripple was a sinewave, which it is not, then 42.5mV converted to peak to peak is about 120mV cf. 130mV off the oscilloscope.
I begin to wonder if many of you have tried to measure what you are talking about.
AndrewT said:
I don't believe anyone has ever suggested the meter might 'blow up'?, and while you may not have a capacitor in the lead, it's quite possible (in fact probable) that your particular meter has an internal blocking capacitor for the AC ranges. However, this certainly isn't universal!.
To go back to basics, a conventional analogue meter consists of a moving coil meter, with resistors switched in parallel (for current ranges) or series (for voltage ranges). For the AC ranges a rectifier is switched in to circuit, and the resistors values changed to make it read correctly for sinewave inputs.
This type of meter will read DC inputs (with the wrong values) when set to AC - if the rectifier is halfwave, then it will only read one way round, if it's fullwave, then it will read DC either way round.
Exactly the same principle applies to some digital meters, although some do include blocking capacitors, others include precision opamp rectifiers (which include blocking capacitors), and some even include true-RMS AC readings (which may, or may not, read the DC level as well as the AC, depending on design).
So it's important to know how your specific meter reads, you've now tested your meter, and know that it doesn't read DC on the AC ranges (I presume you tried it both ways round?).
You mentioned checking with a scope, which is obviously by far the best method - but you will (also obviously) know that the front of the scope has a switch labeled AC-DC, this switches a DC blocking capacitor IN and OUT of circuit for exactly the reasons we're discussing!.
anders.a said:
You wouldn't, they can't be any!.
Rather messy wiring, and with leads longer than required - but what's with the colouring scheme?, are you using brown for negative and blue for positive?.
About the wiring.
As this a project only for me I don’t care much for colouring.
Also I don’t know what you mean messy. They are as long as they should be.
As this a project only for me I don’t care much for colouring.
Also I don’t know what you mean messy. They are as long as they should be.
Yes there are lot things to do, but this is just for testing. The bridges should be mounted on the heat sink for instance.
Actually,there WILL be the AC ripple of the pulsating DC coming from the rectifiers.There will *always* be SOME ripple,with the exception of well regulated DC circuits. Granted it's usually at a low level in most well designed supplies.
CAPACITOR DISCHARGE
I think i would be correct in saying that if you discharge a cap to quickly, it will charge up again !! as it discharges there is a potential difference and this induces a voltage, so, be carefull as you might get a wee fright if you think it's discharged when in fact, it has charged up again (although not to the same level) I prefer to use a resistor and an LED as this discharges slowly, and no surprises when it touches something metal !!.
wildbill
I think i would be correct in saying that if you discharge a cap to quickly, it will charge up again !! as it discharges there is a potential difference and this induces a voltage, so, be carefull as you might get a wee fright if you think it's discharged when in fact, it has charged up again (although not to the same level) I prefer to use a resistor and an LED as this discharges slowly, and no surprises when it touches something metal !!.
wildbill
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