rule of thumb multipliers for PS rectifiers and filters to achieve required voltage - diyAudio
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Old 21st August 2014, 03:42 AM   #1
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Default rule of thumb multipliers for PS rectifiers and filters to achieve required voltage

Forgive me for how many years it's been since my electronics classes before all these years in software and repairing what I break...my brain only works .707 as well as it used to, and if I try to reason or derive these simple things I'll make mistakes. But...now I've got my VTVMs with insulated clip leads and pointy probes, scope with X1/X10/X100 probes, an L/R/C meter, Variac, amp-clamp, line voltage monitor, series lightbulb, dummy load, and some chopsticks to poke around with, so I'm about ready to fool around without injuring myself!

I'm looking at my transformers and chokes and caps, and tubes, starting to come up with a single-ended amp. So, long before I get to simulation of the power supply using PSUD2, I need a quick review of simple approximation rules of thumb for estimating the output voltages from various common rectifier circuits and common filter circuits. Someone recently actually posted these but they don't come easily to search results for me.

1- Starting with the transformer mfgr's volts rating (for instance, 500 Volts center tapped) , what's the multiplier to estimate the DC output voltage from a 4-diode fullwave bridge?

2- Starting with the transformer mfgr's volts rating (for instance, 500 Volts center tapped), what's the multiplier to estimate the DC output voltage from a 2-diode rectifier using the centertap?

3- Starting with the DC voltage from the rectifier, what's the multiplier to estimate the filtered DC voltage from an L-C network (rectifier right into a large choke in series w/ one leg, then into cap in parallel with the load)?

4- Starting with the DC voltage from the rectifier, what's the multiplier to estimate the filtered DC voltage from a CLC network or CRC network where the rectifier output dumps right into a cap in parallel with the output and load?

Thanks in advance for even reading this.

Last edited by cyclecamper; 21st August 2014 at 04:11 AM.
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Old 21st August 2014, 04:17 AM   #2
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Did some research.
1- For 2 silicon diodes using the center tap, 1.2 times the RMS from the trans. Starting with a trans that's 500v center tapped, I assume that's 250 on each half, (doc was clear as mud) so I'd get about 300 volts?

2- For 4 silicon diodes in a fullwave bridge, 1.2 times the RMS from the trans. So for the same transformer, I'd get about 600 volts?
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Old 21st August 2014, 04:33 AM   #3
Einric is offline Einric  United States
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I think Full Bridge = (Secondary V * 1.414) - (2*Single Diode Forward Drop)
Half bridge is half that I think = (Secondary V * .707) - Single Diode Forward Drop.
Half Bridge w/ CT = Full Bridge
CLC Stuff I can't help, sorry
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Old 21st August 2014, 05:24 AM   #4
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Yes, my 1.2 figure is a realistic "something less than 1.414 theoretical"

I'm hearing that a "choke input" drops the voltage by a factor of about .9

This gives me a place to start dreaming...
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Old 21st August 2014, 09:46 AM   #5
DF96 is offline DF96  England
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Quote:
Originally Posted by cyclecamper
1- Starting with the transformer mfgr's volts rating (for instance, 500 Volts center tapped) , what's the multiplier to estimate the DC output voltage from a 4-diode fullwave bridge?
Assuming the bridge feeds an unloaded or lightly-loaded reservoir capacitor, then 1.414 times the secondary RMS voltage, minus two diode drops.

Quote:
2- Starting with the transformer mfgr's volts rating (for instance, 500 Volts center tapped), what's the multiplier to estimate the DC output voltage from a 2-diode rectifier using the centertap?
Half the above. However, a 500V CT secondsry will often be specified as 250-0-250V so then it is 1.414, minus one diode drop.

Quote:
3- Starting with the DC voltage from the rectifier, what's the multiplier to estimate the filtered DC voltage from an L-C network (rectifier right into a large choke in series w/ one leg, then into cap in parallel with the load)?
Choke input supply gives 0.9 times secondary RMS, minus a diode drop.

Quote:
4- Starting with the DC voltage from the rectifier, what's the multiplier to estimate the filtered DC voltage from a CLC network or CRC network where the rectifier output dumps right into a cap in parallel with the output and load?
See above, then use Ohm's Law. You need to know the current drawn by the load.

Note that when the load becomes heavy then the average DC level from a capacitor inout supply (1 and 2 above) will droop during the mains cycle. To estimate the droop you need to use V=Q/C, and Q=It, where I is DC current and t is 1/2f, where f is mains frequency.

Use PSUD2.
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Old 21st August 2014, 10:00 AM   #6
AndrewT is offline AndrewT  Scotland
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as stated by Einric and DF, the 1.2 factor is wrong. As a result your "model" massively understates the DC voltage after the rectifiers.

The output voltage of a transformer is given by
Vout = Vmainsinput / VratedPrimary * Vrated secondary * Transformer regulation
Convert to DC using the sinewave peak of Vpk = sqrt(2)*Vac
From this maximum you subtract the diodes' Vf.

If you know your transformer is a 230:250+250Vac 10% regulation and the mains input is 238Vac then Voutdc = 238/230*(250+250)*(1+0.1)*1.414 minus 1Vdrop ~805-1 =804Vdc
If that is split into dual polarity using the centre tap you get ~+-402Vdc
NOT ~300Vdc.

That is not the worst case voltage. You should allow for the highest voltage that your supplier specifies as part of their supply contract. This maximum supply voltage could be anywhere from 110Vac (Japan) to 254Vac (UK and others).

Now that you know the normal maximum voltage when no current is drawn from the smoothing capacitors, you can work towards the operating voltage when current is drawn.

This gets quite complicated and requires a lot of data. Easier to use PSUD2.
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Last edited by AndrewT; 21st August 2014 at 10:04 AM.
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Old 21st August 2014, 10:30 PM   #7
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I thought any 'rule of thumb' in PS design was to not underestimate the voltage rating the PS caps need to have. Thus, measure transformer no-load voltage and multiply by 1.414, as that is the highest theoretical DC voltage (without any real losses). Don't over-analyze...
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Old 21st August 2014, 10:47 PM   #8
sreten is offline sreten  United Kingdom
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Hi,

A choke input is archaic (LC), CLC is much more common.
As you say a choke input averages the voltage, CLC doesn't.

rgds, sreten.

Back in the day they used the choke as the speaker field coil .....
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Old 23rd August 2014, 01:49 PM   #9
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Quote:
Originally Posted by sreten View Post
...........................
Back in the day they used the choke as the speaker field coil .....
Only a few of us still remember that day.
My parents bought a console with an AM/FM tuner & turntable that used a speaker field coil. Then 33 & 45 RPM records became popular and the US FM frequency band was changed. So much for that console.
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