The Phonoclone and VSPS PCB Help Desk

[rjm]

The rating allows for the higher transient conditions during startup. Its the steady state conditions that are more important anyway, since the issue is thermal...

(calculate the heat dissipation as a function of input output voltage differential and you'll see what I mean)

Anyway, the optimal operating conditions for all these regs is to have the input voltage about 4-8 V higher than the output. No reason to exceed this, and no reason to worry about the max rating.

[AndrewT]

Hi RJM,
I am not criticising your choice of components nor your understanding of the problem.

I am warning against the improper selection of a higher voltage secondary that could damage the regulator/s.

[rjm]

I hadn't read any it as criticism. Your answer was perfectly correct, but I was concerned when I attempted to extrapolate back to what prompted TBs original question.

Andrew said he will use 12V windings, so he'll end up with 17 V rails, which will do just nicely for the 12V LT1085/1033 regs he plans to use... 25V rated caps will also be sufficient. So as far as I can tell theres no reason to delve into the depths of the datasheets...

[superhkm]

How about 15v secondaries Richard ? on paper it shouldn't be to much , or ?

[AndrewT]

Hi,
for the sake of an example, let's do some numbers.
In the UK the tolerance on mains supply is +6% -10% around 240Vac i.e. 216Vac minimum to 254Vac maximum.
For small VA transformers the regulation can be in the range 15% to 30%.
For a single bridge rectifier on a dual polarity supply the Vdrop into a near opencircuit (smoothing cap leakage only) can be about 0.5V and when outputting significant current approaching 0.8V.

Using those assumptions we have the following for a 230Vac:15Vac +15Vac transformer.
at 230Vac output =15Vac.
at 254Vac output =16.56Vac.
at 216Vac output = 14.09Vac.
worst case peak voltage on the smoothing caps is 16.56*1.3*1.414-0.5=30Vdc. (goodbye to long life on 25Vdc smoothing caps).
With a light load the worst case maximum PSU voltage will drop about 1.2V from peak to about 28.8Vdc. Dissipation in the regulator passing 100mA is about (28.8-12) *0.1=1.7W (it needs a heatsink).

Now let's look at minimum voltages.
at 216Vac and a heavy load (say 200mA). the minimum PSU voltage will be 14.09*1.414-(0.8+1.2+1V)=16.9Vdc.
From this we need to subtract the ripple voltage on the snoothing caps. let's say this is 1Vpp.
we have 16.9V-1V=15.9Vdc at the bottom of the worst case low voltage on the input to the regulator. Vdrop for the negative regulator is specified as 3V. Subtract this from the lowest input voltage and you have 12.9V. This is >=the required output voltage. There it is OK.

BUT,
15Vac risks damaging both the smoothing caps and the regulator if no heatsink is fitted.

The above is roughly what would result from a 6VA to 12VA 15Vac transformer.

If you use the recommended 12Vac transformer you must ensure the lowest voltage, at the bottom of the ripple exceeds the (output voltage +Vdrop). This is ensured by using a higher VA transformer and more smoothing capacitance but do not use too much to be safe. Excess smoothing brings other avoidable problems.

[rjm]

How about 15v secondaries Richard ? on paper it shouldn't be to much , or ?

Like Andrew said, or in short: Yes this will be ok, but you are starting to push it if the regulation on the transformer is poor. I'd use 35V capacictors just to be on the safe side. You could also consider 15V regulators.

[TB]

Hello RJM and AndrewT, I did not want to move the focus of this thread from the main subject. Very often datasheets are the main source of information and confusion for less skilled DIYers. I have asked that question in good faith. And your responses are great. I am 50 years old music fan who tries to catch up with old hobby.
Thanks

[superhkm]

Originally posted by rjm


Like Andrew said, or in short: Yes this will be ok, but you are starting to push it if the regulation on the transformer is poor. I'd use 35V capacictors just to be on the safe side. You could also consider 15V regulators.

I'm sorry Richard, I left this thread a long time ago, did a search now and realized that you already answered this question.

Now I realize that you claim the sound betters with lower voltages, and you may be rigth, but it takes me back to that other question you were asked once:

Have you ever opened up a Phonocube ?

[AndrewT]

Originally posted by TB
Hello RJM and AndrewT, I did not want to move the focus of this thread from the main subject. Very often datasheets are the main source of information and confusion for less skilled DIYers. I have asked that question in good faith. And your responses are great. I am 50 years old music fan who tries to catch up with old hobby.
Thanks

Hi Tb,
you are from a generation that were taught to do arithmetic, before the age of calculators and computer games.
You should find most of the arithmetic fairly easy, but in electronics calculation/modelling is essential to UNDERSTANDING why decisions have to be made.

[rjm]

Two items today:

1. I have updated the VSPS page. I was concerned about the availability of the RIAA resistors in the 732k and 105k values, so I had a look at what the response would lool like with the next closest standard values, 750k and 110k. The answer is: pretty good. Unless you are going to pay as much attention to the caps (ie measure and match to 1% of the list value) there is really no reason not to use the standard carbon film values.

2. I put together my RC Kit today, and uploaded some photos of the job to my flickr account. Have a look if you like. Everything went smoothly, and it took less than an hour.

/R