full wave bridge rectifier volts change question

Thread Starter

minkey01

Joined Jul 23, 2014
185
Good morning. Could someone help explain what is going on here?

I have a full wave bridge rectifier that changes volt amounts outputted when I hit a button in the circuit. Maybe I don't understand them properly. I thought their output stays at a fixed DC volt like a battery. Here is what I'm testing.

Measuring volts with voltmeter at point 13 (+) and point 2 (-). Point 2 is ground. Point 13 is the + output of the full wave bridge rectifier coming from that transformer. I highlighted these points in yellow. Initial reading is around +30V. When I press PLAY (highlighted green button) the volts drop to about +27V. Any ideas? Please enlighten me.

Thanks for the help!!

recitifier volts question.jpg
 

blocco a spirale

Joined Jun 18, 2008
1,535
It's perfectly normal to see the voltage of an unregulated power-supply fall when a load is connected, that's why voltage regulators were invented; to provide voltages that don't drop under load.
 

Thread Starter

minkey01

Joined Jul 23, 2014
185
also. 1 more question. do you always have to have a load on the full wave bridge rectifier? can a switch come off of it before the load therefore when the switch is off there will be no load?

thanks.
 

blocco a spirale

Joined Jun 18, 2008
1,535
It doesn't need a load to be connected all the time, it's common to have a switch between this type of supply and the load.
 

crutschow

Joined Mar 14, 2008
23,349
Any type of rectifier-filter's output voltage will change with load.
This is due to the resistance of the transformer and the diode's forward-drop dynamic resistance.
 

Thread Starter

minkey01

Joined Jul 23, 2014
185
another thought. if my load is never going to change, should I just add some resistor in series with my current load in order to get the 30V down to 27V?
 

blocco a spirale

Joined Jun 18, 2008
1,535
Why do you need 27V?

You can't really regulate your 30V down to 27V because, as you know, under load the unregulated voltage drops to 27V which would leave nothing for the regulator (even an LDO) to work with.

If 30V is too high, you could add 4 silicon rectifier diodes in series to lose a few volts but the output voltage will still be load dependent.

Perhaps you could add a switch-mode psu to provide the regulated 27VDC you think you need.
 

Thread Starter

minkey01

Joined Jul 23, 2014
185
Why do you need 27V?

You can't really regulate your 30V down to 27V because, as you know, under load the unregulated voltage drops to 27V which would leave nothing for the regulator (even an LDO) to work with.

If 30V is too high, you could add 4 silicon rectifier diodes in series to lose a few volts but the output voltage will still be load dependent.

Perhaps you could add a switch-mode psu to provide the regulated 27VDC you think you need.
Can't change the power supply. I have to use that transformer and rectifier setup.

If you look at the original schematic, my new circuit is similar to theirs. I'm using that power and a bunch of switches, buttons, and relays. My circuit is just a little smaller. What I don't understand is how did they setup their total schematic using unregulated power? Wouldn't their volts be changing every time they throw a switch? I measured the old machine and somehow their volts aren't changing at the switches, but my new circuits are.

That is why I wanted to take the 30V DC (that is straight from the rectifier with no load) and regulate it down to 27V and then hook everything up. This way the 27V will not change depending on what my switches and buttons are doing. Correct?

Thanks!
 

Papabravo

Joined Feb 24, 2006
12,405
Without seeing the original schematic, it is hard to tell from your description whether you believe in elves or factories.
 

Thread Starter

minkey01

Joined Jul 23, 2014
185
let me get my full circuit together. i will start a new thread with a more clear description and question.

i'll be back. :)
 

Dodgydave

Joined Jun 22, 2012
8,389
Minkey, what are you trying to achieve, do you need a fixed voltage of 24 to27v regulated, or lower, what do you need it for?
 

Papabravo

Joined Feb 24, 2006
12,405
it's in the original post

but never mind, i'll figure something out.
No it isn't. You believe there is something that you did not understand or could not explain that should prevent the voltage from dropping when buttons were pushed.
 
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