How to adjust dc voltage

[Fishmasterdan]

I ran into some LARGE 120vac to 48vdc power supplies while doing a search for a bench power supply.

I accuired 3 of these and they weigh about 50 pounds apiece.

I have some larger motor's I like to tinker with but getting consistant results with batterys is a pain.

So my questions is would it hurt the power supply to use a dimmer on the AC side to regulate the dc output, or do I need to regulate the dc voltage.

I would like to get a range from 2volts dc to the full 48vdc.

Not being an electroics expert I am not real sure how to regulate the dc side.

Can anyone help out here??

I would like to put something together like you see on the commerical one's. The only issue here is I think I can deliver over 100a dc current (will know forsure when I get the tech data back from the manufacture) if I connect them in parallel.

[Richard Ingram]

Are they unregulated 48 VDC supplies ? That would be a supply that has 120 VAC input through a transformer with a rectifier / capacitor on the low voltage side. This type could be controlled with a varactor and possibly to some extent with a dimmer, but it would not be regulated. The other problem would be the current that would be delivered. You could use a non regulated supply as a source for a regulator. Again if you want high current capacity you would need a substantial regulator design with multiple pass devices. I suspect that you could locate a design with a google search.

Richard

[slipstick]

Depends what sort of supply you have. From the weight they sound like classic linear regulated supplies. In which case much of the circuit in there is about making sure that the output voltage does NOT change when the input voltage changes.....so a dimmer will not do any good.

And an add-on regulator which will work between 2V and 48V at 100A is going to be interesting. If it's a standard linear design at 2V it will need to dissipate around 4.6 KW of heat (that's really a big electric heater ;)).

Steve

[vintage1]

Yup. Its one thing to regulate to 48V DC and quite another to regulate lower..in terms of heat..

I think you are on a hiding to nothing here..

[Fishmasterdan]

I will try and get some pictures and post them and see if you guys can identify what it is exactly.
These power supplies came from a junk dealer and what they were used for I have no idea.

[Fishmasterdan]

Well I cant find my darn camera.

They are older no solid state stuff inside.
They are " Regulated power supply" made by Power mate corp.

110v in and 48 v out. I can get the output voltage down (using a dimmer on the line side) pretty easy but it falls off to nothing as soon as I put some current on it.

So I what I need is something that can regulate voltage on the output side.
Anything on the market that a person could purchase to regulate DC voltage. I will deal with the current issues when I find out what kind of heat loss I am dealing with.

[Fishmasterdan]

Well I currently have a variac coming from a junked out welder and 2 from a demoed out paper mill. I will put on the dc output side and see what happens. My favoriate part of this project is everything has been free so far :).

If I have problems with the regulated part of the circuit I will remove it. The power supply has 2 30000 micro farads in there and a capacitor (I am not sure what the capacitor does it is direct connected to the transformer).

I also got (for free) a 120v to 12v ac transformer rated at 62 amps. I could easily run this up to 100 a in short bursts.

Anyone know where you can buy a large rectifier?? I dont know what I am looking at with those things.

pictures
http://i41.photobucket.com/albums/e274/Fishmasterdan/powersupply002.jpg
http://i41.photobucket.com/albums/e274/Fishmasterdan/powersupply001.jpg

[Richard Ingram]

There are some bridge rectifiers (35A, 50A) on ebay. Digikey would have what ever you need. You could also look at Jameco and Allied.

[jeffs555]

Well I currently have a variac coming from a junked out welder and 2 from a demoed out paper mill. I will put on the dc output side and see what happens.

A variac is a transformer and won't work on dc. If the variac will handle the current, you could put it on the ac input, and reduce the input voltage. If you remove the dc regulator, that will allow you to vary the dc output voltage.

A variac being a variable transformer, will allow you to control the voltage to your supply. The ac waveform will not be modified, it just reduces the voltage.

A dimmer just chops cycles out of the ac, reducing the average power, but the peak voltage remains the same, so the output voltage would be much more load dependant.

[z-matrix]

Well I currently have a variac coming from a junked out welder and 2 from a demoed out paper mill. I will put on the dc output side and see what happens. My favoriate part of this project is everything has been free so far :).

If I have problems with the regulated part of the circuit I will remove it. The power supply has 2 30000 micro farads in there and a capacitor (I am not sure what the capacitor does it is direct connected to the transformer).

I also got (for free) a 120v to 12v ac transformer rated at 62 amps. I could easily run this up to 100 a in short bursts.

Anyone know where you can buy a large rectifier?? I dont know what I am looking at with those things.

pictures
http://i41.photobucket.com/albums/e274/Fishmasterdan/powersupply002.jpg
http://i41.photobucket.com/albums/e274/Fishmasterdan/powersupply001.jpg

buy some very thick wire for your design,
those on your puffer capacitors will melt at 50 amp continuous

[stall warning]

If that cap is on a separate winding on the transformer, it might be
something similiar to the Sola constant voltage transformers. The cap
may be saturating the transformer as a form of voltage control. Maybe
as a method from preventing power company spikes from getting thru.

It could also be modifying the waveform to something that looks
like a square wave. Cable companies use a modified square wave as
their switching powersupplies are more efficient.

[Fishmasterdan]

Hmm..

All I am really after hear is something that can test out a few larger motors and run my several battery chargers at once.

The capacitor has 552v ac going to it??

If I am seeing this correctly I can modify this power supply by:
1: Use a Variac on the input side to regulate the voltage via the transformer.
2: I need to get rid of the circuit board (I am assuming that is doing something to the circuit telling it to stay at a certain voltage).

For a power supply all I really need is a transformer (to change voltage)
A full bridge recifier (to change from ac to dc)
and a capacitor (to filter out any voltage spikes)

Is that correct?

From that general power supply you can add other things to get more feature you want. ???

Z-matrix;
Thanks for the tip.

Stall;
If it was a square wave would it have to be on the dc side?? Doesnt the rectifier chop off half the sign wave? It has over 500 volts ac on that thing and it comes off a seperate set of tap all by themselves on the transformer.

Dan

[z-matrix]

Hmm..

All I am really after hear is something that can test out a few larger motors and run my several battery chargers at once.

The capacitor has 552v ac going to it??

If I am seeing this correctly I can modify this power supply by:
1: Use a Variac on the input side to regulate the voltage via the transformer.
2: I need to get rid of the circuit board (I am assuming that is doing something to the circuit telling it to stay at a certain voltage).

For a power supply all I really need is a transformer (to change voltage)
A full bridge recifier (to change from ac to dc)
and a capacitor (to filter out any voltage spikes)

Is that correct?

From that general power supply you can add other things to get more feature you want. ???

Z-matrix;
Thanks for the tip.

Stall;
If it was a square wave would it have to be on the dc side?? Doesnt the rectifier chop off half the sign wave? It has over 500 volts ac on that thing and it comes off a seperate set of tap all by themselves on the transformer.

Dan

hmm, 48V telecom power supplies?
you probably will need a graetz rectifier on a heat sink, or synchronous rectifier for that current - 60A and large capacitors capable of the output voltage plus a little more.
you might easily build a buck regulator for regulating output voltage.
linear regulation is a no-go.
for poor regulation, you can use a lamp/motor dimmer at the primary side.

[Fishmasterdan]

Z-matrix;

Graetz rectifier ??
http://www.answers.com/topic/diode-bridge
I believe there is a center tapped on it there now.
Linear regulation
http://en.wikipedia.org/wiki/Linear_regulator
I dont think I am trying to do linear regulation. I would like to use a Variac. But I dont think I understand Linear regulation all the way, (in the smiplest form) it is just changes voltage via electronics like transistors.

Buck regulator
I need to do alot of reasearch on that one.

[jeffs555]

Sounds like that is a constant voltage transformer like StallWarning suggested earlier. It would have a high voltage non-polarized capacitor across some extra windings. In that case, you wouldn't want to connect a variac to the primary input of the transformer. You could connect the variac between the secondary and the rectifier, but it would have to handle the full supply current.

[z-matrix]

Z-matrix;

Graetz rectifier ??
http://www.answers.com/topic/diode-bridge
I believe there is a center tapped on it there now.
Linear regulation
http://en.wikipedia.org/wiki/Linear_regulator
I dont think I am trying to do linear regulation. I would like to use a Variac. But I dont think I understand Linear regulation all the way, (in the smiplest form) it is just changes voltage via electronics like transistors.

Buck regulator
I need to do alot of reasearch on that one.

yes, well if it has 2 times the output winding with a center tap that is a special case, you may only need 2 diodes each positive connected at one output for example, negatives are connected to the positive of the puffer capacitor, and center tap to the negative, this can be done given the center tap is really center, not many volts difference.

You need switched mode regulation, or you will have very low efficiency.

[flatfour]

Your supplies have ferroresonant transformers. They are not suitable for varying the input voltage to lower the output.

http://www.generaltransformer.com/ferro.htm

http://www.allaboutcircuits.com/vol_2/chpt_9/6.html

[Fishmasterdan]

You guys are great.

Looks like a Variac on the output side of the transformer is the way to go. Dont have to worry much about burning up the transformer now :).

I was suppose to get 3 of these decives.
Turn's out one of them is broken and went to the recycle yard and the other is a 10a max out put.

I