How does this power supply work

[FlapJack]

Does anyone know how this power supply works. It has a 144W 24v center tapped transformer with SCR pre-regulator in to a 3 terminal regulator to make a 0-35V and 0-7.5A power supply.

I do not understand how this configuration can be a boost converter. There is no switch after the inductor.

page 13, 7.5A Variable Regulator

 

[esp1]

hi Jack,
Looking at the d/s for the transformer its rated at 144W, 6A at 24Vrms with a 115Vac input.

I think the app engineer who wrote the d/s for Page #13 circuit was 'over optimistic':???:

Doing the maths for 110Vac, thats 23Vrms * 1.414 =32.5V . minus two 'diode' drops' say 1.4V and as the d/s states 1.7V across the LT1011 , I make that 29.5Vpk.

You would not get 7.5Amps or as the transformer spec says 6Amps ay 29Vdc.

The size of the smoothing cap is 50,000uF,!
I would estimate that you could draw 6A max at ~22Vdc

Do you agree.?
E

 

[Venkadesh_M]

Here only I am seeing a +15V, -15V extra supply for a PSU.

It will not boost any voltage.

But Might be It can go upto the maximum of 35V in low load and high input voltage conditions.

 

[FlapJack]

This is the way i see it.

The SCR's can only lower the duty cycle, hence the need for a huge smoothing capacitor of 50,000 uF.

If the SCR's were replaced with Diodes then the output would be full wave rectified at 17v peak.

12 v x 1.414 = almost 17 vdc maximum voltage, and the LT1083 has to drop something just below 2 V to keep working.

 

[esp1]

hi jack,
That Triad F269u transformer appears in lots of 'ambitious' circuits in many data sheets.
This circuit claims 10Amps from a 6Amp transformer.

I suspect that they just copy the same transformer symbol across many datasheets, must be very confusing for hobbyists.
I did find a d/s for the F269u this morning, but now I cannot re-find it.!
E

 

[FlapJack]

I think the problem is i have the wrong specs on the transformer. I looked for a couple of hours and could not find the correct data sheet but i did find this info.

triad F-269U = THORDARSON 23V562

- - - Updated - - -

OK, i finally found a archive of an old triad catalog. The specs are 35vct 10A. So i would say this power supply never gave an output of 35 vdc.

Center tap of 35 vac = 17.5 vac
17.5 vac x 1.414 = 24.745 vdc - 1 diode drop = about 24vdc - about 2v drop on LT1083 = 22 vdc max output.

 

[ALERTLINKS]

Maybe it is 35V-0-35V output. It is also called 35V CT, which is actually 70V across.

 

[esp1]

I interpreted the 35Vrms CT the same as 'alert', it would have to be so to get ~30Vdc out.
but also the VA rating is 144VA, so, 144/70 = ~ 2Amps.!

If jack ever gets a triad F269U TXR perhaps he could post what he measures.:wink:

E

 

[FlapJack]

This is the closest thing to a properly specked transformer that you can buy.

$138 Ouch!
F-282U
360 VA
60 VCT at 6A

For the math to work you have to assume the 60 vac is the outside winding.
60 v x 6 A = 360 VA

For a center tap you get 1/2 the voltage and double the current.
30 v x 12 A = 360 VA

 

[FvM]

Instead of guessing about the F269U specification, it seems more promising to consider what would be a useful specification to achieve the intended 35V/10A.

It's has been already stated that the SCR preregulator can only reduce the input voltage, not boost it. The text describes the preregulator operation principle, maintaining 4 V accross the linear regulator. Considering the inductor averaging effect (I read it as 1 mH), the transformer must have at least 2x35 V, better 2x38V output. According to double wave rectifier form factor, you need about 10 Arms of each winding to provide 10 A DC, resulting in a 700 - 800 VA transformer rating.

 

[Venkadesh_M]

If you going to design this circuit, its better you have another secondary for +/- 15V.