Hi, I purchased a used 540 VA 18-0-18 V transformer and I would like to test whether it has operating problems under load. Not having armored resistors, can I create a load with a boiler resistor with the following characteristics? R 27.5 ohm, 1950 W, 230V. Rectifying 36 V I would have around 48V which with the 27.5 ohm resistor should make around 1.75A. It's definitely not a heavy load for this transformer, do you think it's enough to understand if it works well? Thank you
I tested with a load of 1.75A and there is a slight hum that can be heard even at a distance of about one meter and the same hum can also be heard without load. I would like to use it for a class A amp, do you think it is possible to reduce the hum in some way or is it better to return it to the seller? The price is only €50, but if you tell me it's unusable I'll return it. Thank you
If it were a transformer you already had or recycled or bought for peanuts then it would be worth trying to improve it, you risk little $
But for 50 Euro I would expect it to work flawlessly, and if not send it back.
The standard cure for a humming/buzzing transformer is re-soaking it in transformer varnish and worst case household/wood varnish, BUT:
* as Mooly mentioned, it may work or not, 50/50 % , at home there is no guarantee varnish will fully penetrate to the deepest windings or laminations.
* IF it does not work, seller will not accept back a transformer dripping sticky smelly varnish.
So best is to send it back as-is.
But for 50 Euro I would expect it to work flawlessly, and if not send it back.
The standard cure for a humming/buzzing transformer is re-soaking it in transformer varnish and worst case household/wood varnish, BUT:
* as Mooly mentioned, it may work or not, 50/50 % , at home there is no guarantee varnish will fully penetrate to the deepest windings or laminations.
* IF it does not work, seller will not accept back a transformer dripping sticky smelly varnish.
So best is to send it back as-is.
Put it on a dim bulb with NO load. A good toroid has enough primary inductance that it won’t buzz or draw significant current. If you see any glow at all it draws too much magnetization current and isn’t going to be suitable for audio (at least not what you’re used to).
The “fix” is to add primary turns, and put out a little less voltage.
The “fix” is to add primary turns, and put out a little less voltage.
If you were to try soaking the transformer in varnish, pulling a vacuum on the container would improve penetration. Putting a system together is obviously not the answer to improving a $50 xfmr but you may know someone who uses one for composites/degassing epoxy, a wood turner who uses epoxy or acrylic to stabilze wood or someone who extracts thc/essential oils with solvents.
Provided the total voltage across the two cap is below around 1 volt then its as good as it gets. The diodes only protect the caps from excess voltage.
Remember that mains purity (how close to a perfect sine with no distortion) plays a big part in transformer noise so be sure to test at different times of day.
Remember that mains purity (how close to a perfect sine with no distortion) plays a big part in transformer noise so be sure to test at different times of day.
I found many discussions online, in some cases they talk about problems (fire) if a diode fails, what can be done to eliminate this problem? I found other schemes that are similar, could you explain to me in the attached scheme the usefulness of R1 and C3? I understand that more diodes raise the DC voltage withstand value, right? Thanks
If a diode fails short nothing bad will happen. If they fail open then the caps could see a massive voltage spike at switch on and could fail by going 'pop'. As with any mains circuit, it should be appropriately fused and located in the equipment such that it could not cause damage if the worst happened.
Worst case if you connect a 2200/6.3 or 10 volt cap (which is a suitable voltage rating here) across the mains it will explode instantly. If it located correctly and safely in the equipment it will do no further damage.
FWIW, large power diodes... I'm going to say I've never had a large mains rectifier fail open.
R1 and C3. I can't see they serve any purpose whatsoever. Normally we bypass electrolytics to help preserve low impedance at high frequency due to the caps self inductance. My first thought here is that doing that is a negative thing because if there were any high frequency rubbish on the mains its surely better to keep it out.
6 diode version... measure your voltage across the caps first. If you really need a 6 diode version then I think you have bigger problems than buzzing toroid
and you need the cause of the mains asymmetry (that cause this misleadingly named 'DC' shift) diagnosing and fixing.
Those are my quick thoughts on it
Worst case if you connect a 2200/6.3 or 10 volt cap (which is a suitable voltage rating here) across the mains it will explode instantly. If it located correctly and safely in the equipment it will do no further damage.
FWIW, large power diodes... I'm going to say I've never had a large mains rectifier fail open.
R1 and C3. I can't see they serve any purpose whatsoever. Normally we bypass electrolytics to help preserve low impedance at high frequency due to the caps self inductance. My first thought here is that doing that is a negative thing because if there were any high frequency rubbish on the mains its surely better to keep it out.
6 diode version... measure your voltage across the caps first. If you really need a 6 diode version then I think you have bigger problems than buzzing toroid
and you need the cause of the mains asymmetry (that cause this misleadingly named 'DC' shift) diagnosing and fixing.Those are my quick thoughts on it
Based on the information provided, it seems like you're on the right track to test your transformer. Using a 27.5 ohm resistor with a voltage of around 48V should indeed draw approximately 1.75A, which might not be a heavy load for a 540 VA transformer, but it should be sufficient to test whether the transformer operates properly under load.
However, there are a few considerations to keep in mind:
However, there are a few considerations to keep in mind:
- Safety: Make sure to exercise caution when working with high voltage and current. Ensure proper insulation and take necessary precautions to avoid electric shock or fire hazards.
- Monitoring: While testing, it's essential to monitor the transformer for any signs of overheating, unusual noise, or voltage fluctuations that could indicate potential problems.
- Gradual Increase: If possible, start with a lower load and gradually increase it while monitoring the transformer's performance. This approach can help identify any issues more effectively.