Multi Tap Transformer wiring Question

[EmmKay]

I was hoping that I might be able to find my answer in the forums anywhere before posting but I couldn't hence my question here..

I have a multi tap transformer. Primary is 220V secondry is 0,12,60,90,110 V
The transformer is rated for 20AMPS max which is more that enough for my needs.

I need 12Vdc and 60Vdc from the transformer. Any suggestions as to how I would go about doing that!.

I used one full bridge rectifier for 0-60 to give me one dc conversion and another full bridge for 0-12 to give me the other dc conversion. As I wanted a common ground for both outputs.. I connected the grounds together. Here things went in a different direction. the 0-60V reading was good.. but the 0-12V reading went up to 76V

I used 3300uf 80V caps for filtering on both connections.

It could very well be that I am going about it in completely the wrong way.. any help and guidance is appreciated

cheers

emmkay

 

[Boncuk]

I don't know what you did wrong wiring the transformer.

It should work that way (see attachment).

Please make sure I1 + I2 don't exceed the maximum total current of the transformer.

Boncuk

 

[EmmKay]

@boncuk Thank you for your input.. This is exactly how I wired the transformer.. R1 Load is 317T regulator giving 6 Volts dc to a 74HC08.. while R2 is a DC motor rated at 60V 11 amps. The total consumption is well below the transformer rating..

The problem is that I am getting 76V reading for 12V supply with no load.. and on load it settles on 32V..???????

For testing I have used 5K 5W resistors as dummy loads on the PSU..

one thing that is not in your schematic is filter Caps .. 3300uf 80V for both supplies..

The 60V supply is good .. no issues there..

Cheers

 

[ericgibbs]

I don't know what you did wrong wiring the transformer.

It should work that way (see attachment).

Please make sure I1 + I2 don't exceed the maximum total current of the transformer.

Boncuk

Good Morning Hans,
Are you sure about your circuit drawing, I think there is an error.

EDIT:
Consider the potential at the -neg of bridge B2 its at ~-60V and this potential is being superimposed on the -neg end of bridge B1.

 

[EmmKay]

following are the two options that I have tried...

both do not give the desired result.

Help!

 

[ericgibbs]

following are the two options that I have tried...

both do not give the desired result.

Help!

hi,
You could do it this way, note the 60Vdc is half wave, should be OK depending upon what you are using it for.??

 

[EmmKay]

Cheers Eric,
Will try it out in the next few mins. and let you know if it did the job or not!.

The dc motor is driven by an H Bridge configuration. so I guess Half wave shouldn't cause too much of a problem!

 

[EmmKay]

@eric..
I justy tried the method you suggested.. no joy I am afraid.. now I get 17VDC no load correctly as per your schematic for 12 V but now the 60V has dropped to just around 40 no load..
instead of 3000u Caps I am using 3300 uf 80V..

 

[ericgibbs]

@eric..
I justy tried the method you suggested.. no joy I am afraid.. now I get 17VDC no load correctly as per your schematic for 12 V but now the 60V has dropped to just around 40 no load..
instead of 3000u Caps I am using 3300 uf 80V..

hi,
Can you recheck your wiring.?

An LTspice simulation shows that the circuit should work.

You say the transformer is rated at 20Amps,, but what is the total VA of the transformer... is it 10A at 110v !!!!

 

[EmmKay]

Well first thing first.. no idea about the actual VA.. the way we buy /get transformers here are by specifying the final voltage and amps.. i.e. the max voltage and amps the transformer will be sujbected to. in this case it was 110V and 20Amps before rectification.

Some thing really interesting happend btw..
I left the PSU on wired to the configuration that you suggested. there was a dummy load on the 12V section and nothing on the 60V section.. low and behold there was a bang... the cap on the 60V section blew up!..

a 3300uF 80V cap makes a hell of a noise .. actually very similar to a 9mm going off (get to hear them going off every night!)!.

so this is where I stand..... I have checked and rechecked the wiring.. I even have 200V 470uF caps sitting on the table to avoid the bang! so don't know what more to say..! I am a mech engineer with electronics as a necessary evil in my bag of skills.. and first time dealing with this type of an issue i.e. PSU's..

 

[ericgibbs]

hi,
The fact that you show only ~ 40V and the 80V cap blew, suggests that your half wave rectifier is short circuit.

What type of rectifier are you using.??

EDIT:
This is, both halfwave rectification.

 

[EmmKay]

hi Eric,

I used 1n4007's as we use them often and had a hand full in stock! should I be using something else..????

 

[Damo666]

1n4007s are underated for your application.

 

[RCinFLA]

You cannot use two bridge rectifiers for two separate supplies on an auto-transformer. You would have to have two separated windings.

The A.C. inputs on a bridge rectifier is alternately clamped to ground on each half cycle of the input waveform. The '60v label' will clamp to ground pushing the "12 v label' to 48v and the '0 v label' to 60v.

First get rid of the "O volt" label on the transformer. (unless you ground the transformer at this point, then you can put a half wave single diode at each of the taps).

 

[EmmKay]

@ericgibbs... any suggestions on what type of rectifier I should try..? the final load on the 60V rail would be 11Amps peak! and if both rails are sharing the common I would use similar rectifiers for both rails..


@RCinFLA .. ok the first part I kind off understood as to why it won't work.. can you elaborate on your suggestion of a solution for with an example of how to get rid of '0 volts'....???

 

[Boncuk]

Hi Emmkay,

you could try using 0 and 60V tap to gain the supply voltage for the motor and the 90 and 110V tap to gain 20V (reduced by the LM317 anyway), both voltages gained using bridge rectifiers.

That way none of the voltages shares a transformer tap.

Use separate ground connections for the beginning and connect to common ground when voltage outputs fit your needs.

Then check for any changes.

Boncuk

To Eric

Hi Eric,

I've downloade LTSpice, but I can't use it. I can't find a library to load components. BTW, ISIS gets into trouble simulating transformers.

Regards

Hans

 

[ericgibbs]

@ericgibbs... any suggestions on what type of rectifier I should try..? the final load on the 60V rail would be 11Amps peak! and if both rails are sharing the common I would use similar rectifiers for both rails..

hi,
What are the overall physical dimensions of the transformer winding and core.?

Also an approx gauge of the secondary winding wire.?

The rating of the transformer as 10A dosnt tell us anything.

 

[EmmKay]

Hi Boncuk.. I had a chance to try what you suggested. sure enough I got a stead 80V no load for the 60V supply and 26V no load between 90-110 for the 20V supply. All well and good. dummy loads applied individually.. all well adn good. reduced the 20V to 6V approx using two LM317T so that there isn't a major voltage drop across one 317 to avoid the over heating.. all well and good..

combined the common for the 6V and 60V .. 60 V remains the same 6V went up to 46.. both 317 blown..

caps are the same for both supplies for filtering 80V 3300uF..

Hi Eric..
Sorry I haven't had a chance to measure the exact dimensions of the Xformer. the over all size is 6in x 4in x 6in height.

winding size is 4.5in x 6.5 x 4.5 from outside and the winding wire diameter is 1.5 mm

Hope it helps..

 

[mneary]

RCinFLA is correct. With a tapped winding that you have, all of the other suggested configurations are wrong. You have confirmed this with explosions and smoke. [edit] My apologies to ericgibbs: Your half-wave solution is the same as I suggest - don't know how I missed it on the first reading. You are correct. [/edit]

You have two choices:

1) Use two totally independent windings. If you use the transformers that you've described, this means you need two of them. With two independent windings, you can use two independent bridges each with their own capacitor and hook them up as you like.

2) Continue to call your 0V terminal "0V". Develop each of the two necessary voltages with a half wave rectifier (single diode) and capacitor. This isn't a good idea at the current levels involved.

In either case, you should use properly rated diodes. 1N4007s are only good for about 1A. You should consider 25A bridges for 10A; if your co-workers or local merchant don't know some, come back and we'll try to help.

 

[EmmKay]

@mneary.. thank you for your kind input. Having two transformers is out of the question for this particular scenario.

Now its more of a matter of learning than anything else.

I do understand the nature and need of another rectifier instead of 1n4007 due to the loads involved.. but for testing I think these would do if the dummy loads are around 1K 0.5W resistors..

for thr final result I do have 35A bridges ready at hand for use. but until I get to that stage....!

I just have to learn now as to why.. when the common is joined after the rectfication and filtering the voltages change.. the higher one remains as it is and the lower one increases by X2 if not more......

Cheers all.