I'm building a small test power supply for playing with op-amps, and had a small question...
I'm using a (2x)15v transformer (measured output of 15.2~15.6v).
After passing through the bridge rectifiers and the initial 680mf capacitors (pre-regulator), I measure a (+-) 20v; no load.
Would I be dumb (please just don't quote me as dumb...^o^
to use 7815/7915 regulators... Or would I be better off using 7812/7912...? (I've been reading the 78xx series need at least 3v over the rating to work properly)
Also, would 12v vs 15v be in any way noticeable when building something like a headphone amplifier...?
Thanks,
bluesmoke
I'm using a (2x)15v transformer (measured output of 15.2~15.6v).
After passing through the bridge rectifiers and the initial 680mf capacitors (pre-regulator), I measure a (+-) 20v; no load.
Would I be dumb (please just don't quote me as dumb...^o^
to use 7815/7915 regulators... Or would I be better off using 7812/7912...? (I've been reading the 78xx series need at least 3v over the rating to work properly)Also, would 12v vs 15v be in any way noticeable when building something like a headphone amplifier...?
Thanks,
bluesmoke
Hi,
a 15Vac transformer is ideal for a 15Vdc regulated supply.
I am surprised that you have only 20Vdc after rectifying and smoothing on no load. I would expect 22Vdc to 24Vdc when on no load. Vdc=[ Vac * sqrt(2) * (1+regulation) ] - 1.4
How did you find out you had 15Vac? Did you buy it that way or measure it into no load? If measured it may be that the transformer is actually a 12Vac unit.
You are right about leaving an adequate overhead for the "dropout" from the regulator.
The spare voltage should allow for the lower voltage from the transformer when under load and also for the ripple coming from the smoothing. The DMM will measure the average level coming out of the smoothing caps. When on load the ripple will be plus and minus half the ripple voltage either side of the measured voltage. You can see this on an oscilloscope. When on no load the ripple is almost zero (<=10mVpp)
a 15Vac transformer is ideal for a 15Vdc regulated supply.
I am surprised that you have only 20Vdc after rectifying and smoothing on no load. I would expect 22Vdc to 24Vdc when on no load. Vdc=[ Vac * sqrt(2) * (1+regulation) ] - 1.4
How did you find out you had 15Vac? Did you buy it that way or measure it into no load? If measured it may be that the transformer is actually a 12Vac unit.
You are right about leaving an adequate overhead for the "dropout" from the regulator.
The spare voltage should allow for the lower voltage from the transformer when under load and also for the ripple coming from the smoothing. The DMM will measure the average level coming out of the smoothing caps. When on load the ripple will be plus and minus half the ripple voltage either side of the measured voltage. You can see this on an oscilloscope. When on no load the ripple is almost zero (<=10mVpp)
Thanks for the reply.
I had bought the transformer thinking it was 15v AC (as the sticker on the transformer detailed the leads and values). Tested it no load as outputting 15.5~v... (came from a creepy downtown electronics store that I always hate going to... will stop)
Next time will go for an 18v just to be safe.
Not sure if it makes any difference, but I used some heavy-duty bridges just because I tend to over-spec. Not sure if that would lessen the pre-regulated voltage any...
Would love to have a scope for my projects... a luxury for this lifetime...
Cheers,
bluesmoke
I had bought the transformer thinking it was 15v AC (as the sticker on the transformer detailed the leads and values). Tested it no load as outputting 15.5~v... (came from a creepy downtown electronics store that I always hate going to... will stop)
Next time will go for an 18v just to be safe.
Not sure if it makes any difference, but I used some heavy-duty bridges just because I tend to over-spec. Not sure if that would lessen the pre-regulated voltage any...
Would love to have a scope for my projects... a luxury for this lifetime...
Cheers,
bluesmoke
Hi,
+-12Vdc should be enough to run a headphone amp. It depends very much on the design of the amp.
Potentially, you can get 7 or 8Vac out of that and that is enough for the headphones to blow your brains out.
Try wiring it all up and remeasure the pre-reg voltage when signal is present.
Since phones are very sensitive, you require a VERY quiet output and this usually requires a VERY quiet supply.
+-12Vdc should be enough to run a headphone amp. It depends very much on the design of the amp.
Potentially, you can get 7 or 8Vac out of that and that is enough for the headphones to blow your brains out.
Try wiring it all up and remeasure the pre-reg voltage when signal is present.
Since phones are very sensitive, you require a VERY quiet output and this usually requires a VERY quiet supply.
Paul is correct, but still I nearly never use any fixed voltage regs anymore... got a busload here somewhere in a box, its just so much more flexible being able to adjust voltages. Sometimes of course this is nto needed, like say you are building a pic programmer...
And of coarse there is much better out there than the 317 and 337, but considering how cheap they are and that you just need two 4k7 trimpots for the majority of small voltages, and 120 to 220R biasing resistors... above what you allready have.
I'd love to see thse papers if you have a link 'drew.
And of coarse there is much better out there than the 317 and 337, but considering how cheap they are and that you just need two 4k7 trimpots for the majority of small voltages, and 120 to 220R biasing resistors... above what you allready have.
I'd love to see thse papers if you have a link 'drew.
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