Having a nightmare with simple txf wiring!!

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Spencerleehorton

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Joined
May 12, 2012
Messages
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Location
Felixstowe, Suffolk, UK
Hi All,

It seems it's always me that has horrendous problems with such simple things!!

I'm wiring up some power transformers (txf) into two 1n4007 half wave and 4 x 1n4007 for diode bridge.
I have 3 different txf, one brand new 2 x 18v toroidal and two other txf.
I cant understand what is happening as I've always been taught that dc voltage after diodes should be higher but I cant get anything higher the dc side?

The 2 x 15v 6va txt on one winding shows 19.98vac, same on the other winding, when put through two 1n4007 diodes it measures 9.56vdc?

Same txf put through diode bridge measures 7vdc and 10.14vac on bridge but measures 19.97vac when disconnected?

The 2 x 18vac toroidal is brand new put through the diode bridge it measures 42.4vac with both windings connected  but 37.7vdc?

In the off position of my power switch it also measured 1.160v dc or 0.554vac?

Help me please!!!!
 
It will only read the peak value (i.e. higher) if you have a reservoir capacitor connected across the output.

Without it your meter is struggling to measure a rectified waveform.

Cheers

Ian
 
Spencerleehorton said:
What capacitor to use? 100uf or 1000uf?
Is voltage nessicary as well as there is no load?
Best ask before I blow myself up!!

there is a nice piece of free software here: http://www.duncanamps.com/psud2/index.html
and some "light" theory here: https://www.electronics-tutorials.ws/diode/diode_6.html

Cheers
Jaco
 
ruffrecords said:
It will only read the peak value (i.e. higher) if you have a reservoir capacitor connected across the output.

Without it your meter is struggling to measure a rectified waveform.

Cheers

Ian

in fact, a rectifier with a damping capacitor can be considered a peak detector  ;D ;D ;D
 
Spencerleehorton said:
All good reading but doesnt say what capacitor to use if I'm I'm connecting up a 6v or a 600v txf?

I'm just trying to establish the dc output from a few txf.

here a pratical formula that you can verify with the software I talk about  in the previous post:
C (farad) = 0.7 * I(amperes) / V(ripple) * F(Hz)

(generally F(Hz) is 100/120 Hz for full-wave or 50/60 Hz for half-wave).

for 1A DC load current, 1V ripple and 100Hz freq the capacitor is 7000uF
 
Do I need a load formula as I'm just testing the txf without load and just what DC they put out not under load?

Also do I just need to make sure the voltage of the capacitor is rated above what voltage I'm putting into it?
 
Spencerleehorton said:
Do I need a load formula as I'm just testing the txf without load and just what DC they put out not under load?
Transformer loading down due to winding IxR losses is called "regulation" and may be specified in transformer data sheets if comprehensive.

Shorter term there is peak to trough voltage dip every charging cycle in reservoir capacitors from them discharging between charges.
Also do I just need to make sure the voltage of the capacitor is rated above what voltage I'm putting into it?
For conservative design it is good to use maybe 20% more voltage. I knew one amplifier engineer who cut corners to save money in a power amp design and ended up with a time bomb (big high voltage caps can get expensive). He ASSumed that the cap manufacturer was conservative with that voltage spec, the cap manufacturer ASSumed the engineer would be conservative with the design voltage headroom. Neither was conservative enough to avoid releasing electrolyte.

JR
 
Ok perhaps I need to be more specific to get my answer.

I have a txf which I have tested and can get 265v AC.

I've got a diode bridge but want to test what actual dc voltage I am getting.

I can work out 265 x 1.2 = 318v DC
But CJ has said use x 1.4 which gives me 371v DC.

My ideal voltage is around the 360v DC for this fender champ but there are many different schematics and I just want clarity on testing this txf so I can see what its putting out.

What resviour capacitor do I need?
 
OK< some basics.

Your transformer outputs a sine wave. For a sine wave the rms value is 0.707 times the peak.

Put the other way round, the peak is 1.414 times the rms.

When you rectify and add an unloaded capacitor to the output, the dc voltage will reach the peak value.

If your transformer is rated at 265VAC then the peak is 1.414 times this.

To test the transformer you should add a modest capacitor and a small load (to discharge the cap when you turn it off and help prevent you electrocuting yourself). A modest capacitor would be 100uF. The capacitor obviously needs to be able to withstand the peak voltage which you now know how to work out. A small load would be a 270K 1 watt resistor.

Now you can test transformers to your heart's content.

When it comes to using a transformer in a real amplifier there are a number of other factors to take into account.

1. The load. When the diodes are not conducting, the load partly discharges the capacitor which then recharges at the beginning of the next cycle. The power supply output voltage therefore goes up and down - this is ripple. The bigger the load, the bigger the ripple. The bigger the capacitor, the less the ripple. Others have given you the formula that links these two.

2. Losses. The transformer windings have resistance so they cause an output voltage drop. The diodes have a forward voltage which also causes an output voltage drop. The transformer also needs a magnetising current which is another loss.

All these factors mean:

The output voltage under load is less than the peak value.
The power output from the transformer/rectifier combination is less then the power input from the mains

Cheers

Ian
 
That's perfect ian thank you.

How did you arrive at 100uf and 270k?

I'll order a 100uf 450v and I'm pretty sure i have a bag of 270k 2w if not i have some 220k which i hope these will be ok?

If as you say i need a bit more grunt from the txf and i get 371v with the load resistor but then in the champ circuit it drops to around 360v that would be perfect.
 
For general bench testing purposes I found it convienient to have a bridge rectifier package and reservoir cap hard wired together and  mounted on a short lenght of timber  ,  the usual white electricians terminal blocks that you find in any hardware store will do the job just fine , wire it up then screw all the components down securely ,you can label it clearly with a marker . If you have the the components to hand it takes only minutes  to knock it together .

For valve gear and associated power transformers a bridge as well as  full wave with the centre tapped  secondary  connection would be needed , Just so as you never exceed the capacitor dc rating maybe run two  400 volt caps in series with a pair of voltage balancing resistors to ground , write off a watt or two in heat so 3 or 5 watt resistors for a good margin of error in applied volts .

Duncans Psud is very handy way to determine mains transformer characteristics  when all other circuit values such as thermionic/solidstate  rectification , caps, series resistance/ chokes and load are known , replicate your 'known' test set up in software ,re-jig the transformer values until it lines up with what your (trustworthy)mulitimeter tells you is happeneing in real world  Dc volts, its as good as it gets , you dont have to do any number crunching  ;)



 
Spencerleehorton said:
I'm just trying to establish the dc output from a few txf.

Well the transformer will not give a DC output, only AC.

So what you want to establish is what DC voltage you get from a specific transformer connected to a Full Wave Bridge Rectifier and Filter cap, and that is pretty easy like Ian said It's the AC voltage you measure from the transformer secondary multiplied by 1,414
 
Yeah I get it now was just missing the reservoir cap and load resistor to test txf.
Just hooking up to a bridge rectifier have me weird readings.

How does something like this look?
 

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Spencerleehorton said:
Yeah I get it now was just missing the reservoir cap and load resistor to test txf.
Just hooking up to a bridge rectifier have me weird readings.

How does something like this look?

The resistor should be 270K and not 270R
 
That 270 ohm will smoke very fast  ;D
More like 2x100 kohms tied into the centre point of the caps
 

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