DIY PSU & Regulator

[CageyH]

Hi Guys,

I recently purchased a modded second hand SL 1210 which came with a Time Step PSU, under the hood it has one of these;

Attachment 12024

(http://uk.rs-online.com/web/p/products/2238500/).

I'm guessing combining the 2 25V secondary's results in a 50V output? If this is correct would the output of 50V would be too high for the rectifier board and if so would something like this (http://uk.rs-online.com/web/p/products/2238550/) with 2 18V secondary's be suitable?

I have attempted to take a reading from the transformer (with / without load) but cannot get a stable voltage .

I only ask as I have eventually acquired a Aluminium housing for the PS and fitting this tansformer would be; i, easier, ii, provide better shielding(?), iii, allow me to fit the stock transformer in the spare SL 1210 for re-sale. Is this worthwhile perusing or with my limited knowledge should I just leave things as they are (if it ain't broke...)?

Darren,

If you are not confident with playing with electrickery, leave it be!
It's not worth the risk, unless you have someone close by who knows what they are doing.

What exactly are you trying to do?

[Qwin]

Darren - Bear in mind electronics is not my field but..........

Do not use the combined secondaries of 50v it is too high a voltage.

Using the formula that Neal has given to show the increase in voltage that the rectifier process produces you will end up with 70.7v less some losses. This is too high for the LM317 which while having no real limit on input voltage does have a limit on the differential between input and output, which is an absolute maximum of 40v difference according to the spec.

We are outputting 20.6v so anywhere near 70v input has to high a differential (approx 50v).

I think what Neal is saying is that just one of the 25v secondaries is enough to do the job as it will produce a rectified 35.35v less some losses along the way. This will be ok for the regulator to further reduce to 20.6v as the differential is now down to approx 14v.

The stock Techie Tranny does produce more than required as was said further back. The result of using a smaller input voltage at the regulator will be less heat produced by the reduction process.

You could certainly try using a single 25v secondary without harming anything and see what the regulator produces before connecting its outputs to the board.

I have simplified things and not taken load etc into consideration but I think it should work.

But do not connect the secondaries and use 50v.

If someone more knowledgeable sees a flaw in what I am suggesting please jump in.

[DarrenHW]

Thanks for the input guys and thank you Ken for putting this in a language I can understand! With this in mind is there are advantage in using one of the toroidal secondary's at 25V? Is ~32V from the stock tranny better than ~25V from the toroidal? Does higher voltage provide more overhead / redundancy or will the lower voltage allow the rectifier and regulator to run more efficiently?

I will hook 1 x 25V secondary to the PCB (disconnected from the motor board), take some readings and report back.

Thanks everyone.

[6L6]

Ken, you are spot on in your caluulations.

The LM317 has much less 'dropout' -- the voltage lost to the regulator itself, the minimum input voltage required to get a certain output, than the stock discrete regulator. Usually the LM317 needs just a couple volts (2-3) more on the input than the output you ask of it. SO, as we only need 20.5-21.0V on the output, if the input is as low as 24V it will be just fine. Some similar regulators have only 1V of dropout. Cool.

Darren, if your unregulated PSU voltage is in the 24-35V range, you will be fine.

Now, as to the transformer, don't hook up only one secondary, but parallel them. Red+Yellow together, that's your AC1, then Black+Orange, that will be AC2

Does higher voltage provide more overhead / redundancy or will the lower voltage allow the rectifier and regulator to run more efficiently?

Higher voltage is less efficient, as the regulator needs to burn the excess voltage into heat. (That's it's job) So more is not better in this case.

But do not connect the secondaries and use 50v.

If you connect them in series, I.E., Red AC1, Black + Yellow, Orange AC2. That would give 50V

[The Grand Wazoo]

The AoS warning about working on the mains is wisely posted at the beginning of this thread. However, I think it is worth posting a reminder here.....I promise you that even the very best upgrade will be a total waste of time if you have to listen to it from the inside of a coffin at the bottom of a 6 foot deep hole in the local cemetery.


STOP!
Before you put yourself or others in danger, think very carefully about whether you are qualified to carry out any of this work.

Art of Sound Electrical Safety Advice

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[NRG]

You'll get about 32v out of the 25v secondary with full wave rec and smoothing cap. So will drop about 11v across the reg. I think power draw is what 120mA for the 1210? Call it 150mA @ 11v or 2w as near as...heat sink it well.

[DarrenHW]

Safety warning received and understood, I take full responsibility for my actions and all reasonable precautions to avoid a Darwin nomination.

[DarrenHW]

Hi Jim, Thanks for the response.

Now, as to the transformer, don't hook up only one secondary, but parallel them. Red+Yellow together, that's your AC1, then Black+Orange, that will be AC2

This is how the tranny was originally hooked up but I couldn't get a stable reading on the multimeter, any idea why? I could correctly read all voltages from the stock tranny.

[DarrenHW]

Mystery of fluctuating voltage solved! A case of user error with the multimeter, I had it set to dc not ac

With the multimeter set to ac the tranny reads a constant 31.5V in parallel. Thanks for the input and sorry to have wasted everyone's time.

Now just got to fit it into the new enclosure, boy do I feel like a cock

[Stratmangler]

Mystery of fluctuating voltage solved! A case of user error with the multimeter, I had it set to dc not ac

With the multimeter set to ac the tranny reads a constant 31.5V in parallel. Thanks for the input and sorry to have wasted everyone's time.

Now just got to fit it into the new enclosure, boy do I feel like a cock

Did the same thing with my DMM yesterday - talk about having a senior moment
At least you found the cause of the problem.