In spec'ing a replacement transformer for the failed one in my preamp, I'm being asked for the current requirements on the secondaries. Every calculator I find online is different to the others and am a bit confused at this point. I don't want to come up to speed on all the theory right now, I just want to the right value to give the transformer manufacturer, or a calculator I can easily use to get it.
The original Trafo is 15VA with a 1amp fuse on the live input, even at 110vac.
Secondaries:
23vac > diode bridge > 10k uF cap > 15v reg (+) > 10k uF cap
_________________> 10k uF cap > 15v reg (-) > 10k uF cap
9vac > diode bridge > 4700uF cap > 5v reg (single)> 4700uF cap
The 11DQ10 diodes each show a max handling of 1.1amp,and the 15v regs are LM2940/LM2990 with slide on heat sinks. The 5v reg is a basic 7805.
It looks like the 5v rail merely handles the remote motor on the volume pot. The motor uses 150ma plus circuit. So I imagine most the current is for the 15v legs.
The original Trafo is 15VA with a 1amp fuse on the live input, even at 110vac.
Secondaries:
23vac > diode bridge > 10k uF cap > 15v reg (+) > 10k uF cap
_________________> 10k uF cap > 15v reg (-) > 10k uF cap
9vac > diode bridge > 4700uF cap > 5v reg (single)> 4700uF cap
The 11DQ10 diodes each show a max handling of 1.1amp,and the 15v regs are LM2940/LM2990 with slide on heat sinks. The 5v reg is a basic 7805.
It looks like the 5v rail merely handles the remote motor on the volume pot. The motor uses 150ma plus circuit. So I imagine most the current is for the 15v legs.
Last edited:
Sorry but your data is still incomplete/confusing.
23vac reads as: " a single 23VAC winding"
10k uF cap one/two parallel/series?
15v reg (pair)
15v reg (pair) so what do you have: single +15V, dual +15V or +/-15V?
same doubts with the 9V>5V supply.
You mention 3 regulators, in 2 types, do not specify which is which .
Besides: what kind of preamp is this?
Discrete? uses Op Amps? How many and what type? That will let us stimate current needs.
Did you say it has a motorized volume pot?
150mA rated consumption? Then supply capacity must be way above that, such motors typically consume way more when starting or stalled.
Plus 5V supply makes me think you also have some logic there, even a microprocessor, with its own power needs.
So please recheck what you have, and repost.
The 9v secondary merely handles the circuit feeding the remote motor on the volume pot. The specs on the motor say "power = 0.05w 4-6v. Motor current at end slip: 150mA at 4.5Vdc." So that would mean I would not need more than 2.5-4.5VA from transformer?
If so, then I could say 2.7VA for the 9v winding (volume remote), and 12.3VA for the 23v windings feeding the main circuit -- 535mA split between the 23v pair?
If so, then I could say 2.7VA for the 9v winding (volume remote), and 12.3VA for the 23v windings feeding the main circuit -- 535mA split between the 23v pair?
Sorry for the confusion. I corrected & clarified the first post a bit more. Perhaps that will help. But the clarification on the remote motor (just above) may be more helpful.
I've also attached the schematics for your ref.
I'm getting a nice one built by Canterbury Windings, probably better than the original, so am meaning to go with the three windings as the original had. Just identifying how much to peel off for the remote circuit.
Basic maths then, the current drawn by the motor plus the driver circuit. The only way to know for sure is to measure it, connect it to a 5V supply with an DMM in series
Last edited:
I ended up buying a cheapo trafo 4.5vx2 and wired it to the crcuit with my DMM to measure capacitance. In use, the remote volume control circuit used 180mA and peaked at 190mA sometimes when initiating a volume change. So it would seem that 200mA should be sufficient, leaving all the rest of the 15VA to the main circuits.
180mA to a remote volume control seems enormous !
Have you measured that correctly?
What voltage is that running at?
Have you measured that correctly?
What voltage is that running at?
Winding 11vac unloaded, meant to be 9vac loaded (current measured a winding). 5vdc after psu.
It is a motorised remote, also has the LED and mute switch in the circuit. The circuit current at idle, motor not actuated, is 140mA. DMM set to mA and inline with one of the winding leads.
It is a motorised remote, also has the LED and mute switch in the circuit. The circuit current at idle, motor not actuated, is 140mA. DMM set to mA and inline with one of the winding leads.
inline with one of the winding leads means you are trying to measure the pulsing AC current before the rectifier.
I suspect your DMM cannot read that correctly and if it did it would be measuring either an average or an rms equivalent. You need to measure the DC current after the smoothing capacitor and don't use the ammeter function. Use a low value resistor 1r0, or 5r0 and measure the Vdrop to find the average DC current through the reistsor.
If you set your DMM to mVac you may be able to get some idea of the AC voltage range on that DC value.
I suspect your DMM cannot read that correctly and if it did it would be measuring either an average or an rms equivalent. You need to measure the DC current after the smoothing capacitor and don't use the ammeter function. Use a low value resistor 1r0, or 5r0 and measure the Vdrop to find the average DC current through the reistsor.
If you set your DMM to mVac you may be able to get some idea of the AC voltage range on that DC value.
Hm. I don't have low value any resistors lying around. Why would I not use the ammeter function even on DC?
it's not accurate.
Read your instructions and specifiactions the ammeter function is commonly ten or more times less accurate than the DC voltmeter function.
Read your instructions and specifiactions the ammeter function is commonly ten or more times less accurate than the DC voltmeter function.
I'm not trying to be difficult, but to understand. The related specs on my DMM follow:
ACV, 4-40V range, 10mV res, +- 0.8%+6d acc
DCV, 4-40V range, 10mV res, +- 0.5%+4d acc
ACA, 40-400mA range,100uA res, +- 1.5% +5 acc
DCA, 40-400mA range,100uA res, +- 1.0% +5 acc
So if the accuracy of AC current is still 1.5% with a similar level of digit uncertainty to voltage, is that really going to make a meaningful difference in spec'ing the current needs from the transformer winding when the ACA reading is 180mA? 1.5% would be +-3mA, right?
ACV, 4-40V range, 10mV res, +- 0.8%+6d acc
DCV, 4-40V range, 10mV res, +- 0.5%+4d acc
ACA, 40-400mA range,100uA res, +- 1.5% +5 acc
DCA, 40-400mA range,100uA res, +- 1.0% +5 acc
So if the accuracy of AC current is still 1.5% with a similar level of digit uncertainty to voltage, is that really going to make a meaningful difference in spec'ing the current needs from the transformer winding when the ACA reading is 180mA? 1.5% would be +-3mA, right?
the DC range on a current of 180mAdc will be ~+-2.3%
The AC range on a current of 180mAac will be ~+-2.8%
Your ammeter is a lot better than 10times worse than the voltmeter tolerances.
But I still recommend you measure Vdrop across a series measuring resistor.
The AC range on a current of 180mAac will be ~+-2.8%
Your ammeter is a lot better than 10times worse than the voltmeter tolerances.
But I still recommend you measure Vdrop across a series measuring resistor.
I've got something wrong here. I put a 1ohm resistor in series between the positive of the second PSU cap and the PCB (at C8 on attached schematic). Measuring 5VDC through the circuit still. Good.
Then I measure AC voltage drop across the resistor and get only 12mV, which seems a bit too small, but equally concerning is that the voltage drop does not get any larger when actuating the remote volume control motor. And when I remove the resistor and replace it with the ammeter, I get 0mA.
Then I measure AC voltage drop across the resistor and get only 12mV, which seems a bit too small, but equally concerning is that the voltage drop does not get any larger when actuating the remote volume control motor. And when I remove the resistor and replace it with the ammeter, I get 0mA.
Sorry that I neglected to mention that. The DC measured at 0mV. I just measured again with the same result. The circuit works, with LED, motor moving the potentiometer. DCV of wire to ground is always 5.3VDC. But across the 1R resistor, 0mVDC, even when actuating the vol ctrl motor.
- Status
- Not open for further replies.