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Need help with diode bridge circuit

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jboulasii

New Member
What I need to do is have a diode bridge circuit that will rectify 110 VAC to ~100VDC. They part im struggling with is that after 5 seconds or so I need to have to voltage dropped to 55VDC. I think I can do this by bypassing one of the diodes in the bridge circuit and going to half wave but i'm not sure how to do that. I need the package to be small and compact. The current draw of the circuit is about 10A. Any ideas.
 

Hero999

Banned
Just rectifying 110VAC will give 155VDC after the filter capacitor.

You need a switched mode power supply to convert the 155VDC to 55VDC. It also needs to be isolated for safety reasons.

Why do you want to do this? What's it for?
 

jboulasii

New Member
I have a sytem voltage of 110VAC but there is a device in the circuit that required 100VDC for inital operation but after 5 seconds I want to drop the voltage in half to reduce the heat load.
 

Hero999

Banned
What device would that be?

Please post a schematic?

It sounds like you're doing something the hard way.
 

jboulasii

New Member
the device is a brake. There is already a unit that does the same thing im trying to accomplish. The unit is a molded unit about 2 inches square and about an inch high. The problem with the current unit is that the time dealy is not long enough. It was designed long before I got there and all the old timers have retired, the company does not have a schematic.
 

crutschow

Well-Known Member
Most Helpful Member
Can you trace the circuit? It doesn't sound like it would be too complicated
 

jboulasii

New Member
I wish i could. All I have is a black box (molded plastic) with 4 terminals on it. When I apply 110VAC to the input terminals i get 100VDC at the output terminals. After .5 seconds the output drops to 55VDC.
 

marcbarker

New Member
Maybe it's a bridge rectifier, with no reservoir capacitor. After a 0.5 s time period a relay NC contact goes click and one of the diodes is disconnected?

If it's potted, you can use methylene chloride (paint stripper?) to remove it.
 
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solis365

New Member
If it's potted, you can use methylene chloride (paint stripper?) to remove it.

Thats some nasty stuff...


can you measure outputs with a scope to see whether its filtered and well-regulated DC, or just rectified DC?

measure open circuited and under load. if you know what typical load its designed for you could try various loads around that point to see what happens (perhaps the DC will start to ripple under higher load...
 

jboulasii

New Member
My first thought was the same as yours, its a bridge with some type of NC. Do you have any ideas on what type of timing relay it might be? Like i mentioned its all im a small package about 2" square and 1" thick. I need to change the specification and make it a little more robust.
 

jboulasii

New Member
I put a scope on the output yesterday. When I apply 115VDC to the input terminals I get 105VDC out the signal has some ripple to it so i'm assuming they have a CAP on the output. After about 1/2 second the voltage drops to about 55 VDC again with some ripple. Also it takes about 1/2 second for the voltage to change from 105 to 55. Does anyone have any ideas on how to accopmlish this. I was thinking a bridge rectifier with a Cap to smooth the output but i'm unclear on how to drop the voltage. Could I use a Thyristor as one of the diodes in the bridge and use that to drop my voltage down.

Another idea would be a zener to limit the voltage but I'm not sure how to do the switching.
 

WME_bill

New Member
Seems very curious. Small box, yet smoothing mains at 10amps normally needs a large capacitor, much bigger than your box.
What is the load? Is it resistive, or a coil, which could be "self smoothing".
Is the transformer centre tapped, for disconnecting half of a bridge rectifier setup doesn't by my reckoning halve the voltage. Sounds like a couple of thyristors, with a capacitor which charges up after a few seconds and then cuts to half-wave, using a transformer with centre tap 55-0-55. I am most intrigued. You may have to cut or dissolve away the potting on the box. Methylene Chloride = Nitromors Original is not nice, but ok if you use gloves and have some ventilation. I agree with Nigel Goodwin, more details please, as he is obviously thinking of a change from full wave to half wave rectification, (or even switched mode). Bill
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
I agree with Nigel Goodwin, more details please, as he is obviously thinking of a change from full wave to half wave rectification, (or even switched mode). Bill

With no information on what's happening we can only guess, we need full and accurate information to try and reverse engineer it.
 
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