Power supply Questions.

[jack0987]

Hi:

I need a dc power supply for some stepper motors.

I have a bunch of transformers with a 110V primary and a 18V secondary @ 4 amps..

Could I connect them together to give me say 24V to 36V @ 8 amps?
I think that will be enough for stepper motors that are not too big.
What would the connection look like?

I also have a beefy, high quality microwave that I was going to toss. The transformer is still good. It is the heat element that went. Would that transformer be a possiblity?

Please bear with me. One other small question. I have a wall plug power supply that the label says puts out 5VDC. When I checked it with a volt meter, I get about 8VDC. Why is that and can I use it for TTL components safely?

 

[Hero999]

Not quite.

If you connect two 18V 4A secondaries in series you'll get 36V@4A out, if you connect them in parallel you'll get 18V@8A out. You can't get 24V unless you have two 12V transformers.

 

[jack0987]

Not quite.

If you connect two 18V 4A secondaries in series you'll get 36V@4A out, if you connect them in parallel you'll get 18V@8A out. You can't get 24V unless you have two 12V transformers.

I have about 8 of these 18V transformers. How about series-parallel?

 

[Hero999]

You can do that but the total current capacity for both windings in series is always equal to the one with the lowest current rating. For example if you connected two 18V 4A transformer in parallel, then connected another one in series, the total maximum current rating for both the coils would onle be 4A. This is because the current for all circuit elements connected in series is equal so all the current flows through the single transformer winding which is only rated at 4A. For 8A @ 36V, you need four transformers connected in series parallel. The only reason why one might connect three transformers in series parallel is if they needed a higher voltage at a lower current rating.

 

[jack0987]

For 8A @ 36V, you need four transformers connected in series parallel.

Thanks. My pockets not being very deep, I will go with four T's in series parallel for now provided the controller boards and motors will accept it. I am trying to figure that out now. Some have used two old PC power supplies in series to get theirs working at 24V. If I understand correctly, 36V is better. Still learning this and thinking of using these Driver Boards.



Do you think the T in my old micro might do? It's good size, I remember that much.

 

[BrownOut]

The microwave transformer won't work. Forget about it. Try connecting the secondaries of the 18V transformers as show below, and measure the VOUT. I'm thinking it might be 26-28V, hopefully, then you can use the two 18V transformers to get your 24VDC.

 

[Hero999]

Oh yes, I didn't think of that, are you aware that the filtered DC output will be near 24VDC?

I'd probably recommend full wave rectification over biphase though.

**broken link removed**

 

[MrAl]

Hi there,

The microwave transformer has a 120vac primary, so if you can get the secondary off you can add your own turns to make whatever voltage you want to make, but it wont be easy. I actually have two of these around i saved from old microwaves, but havent gotten around to using either one of them yet

 

[BrownOut]

Oh yes, I didn't think of that, are you aware that the filtered DC output will be near 24VDC?

I'd probably recommend full wave rectification over biphase though.

The reason was to save a diode drop. It will still be full-wave. I was also thinking that if the transformers are 18V @ 4 amps, they might be slightly more if lightly loaded, maybe 19-20V. If so, then he might have enough headroom for a regulator. Of course, he won't get the 8 amps he was looking for, probably the origianl 4 amps is more reasonable.

 

[jack0987]

I do intend on adding a hefty bridge rectifier and a large cap.

As an afterthought, the microwave transformer may not be a good choice because it most likely is a step up T.

I'll hunt about a bit for parts. (Got kicked out of the dump scrapping for computer parts.) I have an old scanner I was thinking of getting a stepper motor out of for experimenting but it is a four wire motor (bipolar) and I want unipolar (5 to 8 wires). The rails and sleve bearing would be helpful if I have at least three of the same scanners for enough parts. Anyone have an old Microtek scanmaker 4 to get rid of.

 

[BrownOut]

I doubt small steppers will require much current. BTW, it's real easy to make a simple stepper controller, and you can actually just use old, low density CMOS logic, and your computer's parallel port as a controller. Just a suggestion. Don't give up on the bipolar steppers, they only require a simple H-bridge to drive them. You might be able to scavage some power transistors to make the bridge. I also have an old scanner I wanted to ravage for parts. Just haven't gotten around to it.

 

[Ubergeek63]

Thanks. My pockets not being very deep, I will go with four T's in series parallel for now provided the controller boards and motors will accept it. I am trying to figure that out now. Some have used two old PC power supplies in series to get theirs working at 24V. If I understand correctly, 36V is better. Still learning this and thinking of using these Driver Boards.



Do you think the T in my old micro might do? It's good size, I remember that much.

you should NOT do that directly. mismatches between the transformers will cause problems. think in terms of an extreme case - a 1:1 in parallel with a 1:1.1.

on the other hand if you use separate rectifiers for each you will be able to run even if it does not share current equally at low loads.