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Any SMPS from 115/230 Vac to 6.3Vdc?

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Z

zintolo

Member
Joined 2012
#1
Hi,

I've seen many DC/DC converters to get 6.3Vdc for heaters from 6.3Vac, but why have an e tra winding on the power transformer if it can be directly done through SMPS from the main supply? Less weight, less money, less heat.

But... I haven't found any available on the market.
Does someone know if there's someone producing them? Or if a 5V SMPS can be modified in the voltage divider to get the results?

Due to the initial overcurrent due to low temperature of the heaters, I would consider a safety factor of 4 for the needed current, in order to be on the safe side.

Thank you in advance for your answers.

Kind Regards
Roberto
 
#6
audiowize said:
Meanwell makes a bunch of 7.5V SMPS that can be turned down to provide lower voltage. All you need is an appropriate resistor on the output of one of those and you are in business.
Click to expand...
A resistor and a proper cap might be the right combination to eliminate
any disturbances from the SMPS.
The resistor will also make the coldstart easier as the SMPS will not
experience "short" during start but will "see" the resistor. By time the filamants
will warm up and voltage/current stabilize.
 
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#7
Find a 5 volt model with a trimmer pot. It probably has the trimmer pot in series with a resistor already so you will have to change that resistor to get into a higher range. Watch out for cheap supplies with low voltage filter caps and remember to keep track of the wattage now that you increased the voltage.
 
#9
audiowize said:
Meanwell makes a bunch of 7.5V SMPS that can be turned down to provide lower voltage. All you need is an appropriate resistor on the output of one of those and you are in business.
Click to expand...

Thanks audiowize, they have a local distributor as well: Distribuzione componenti elettronici - RAFI srl
The 7.5V SMPS with a resistor is a good idea also for the issue I've read everywhere on the net: heaters' inrush current is not well tolerated by the SMPS, but an high wattage low value resistor could help.

I my case I would need to drop 1.2 V with a current of... four EL84 and two 12AX7 would be... 4x 760 mA + 2x 600 mA = 4.24 A so 0.283 Ohm (two 0.56 Ohm in parallel) of 5 W each (they will dissipate 5 W totally), like this one: 603-SQP500JB-0R56.

I have to say that I would still prefer the adjustable SMPS, with higher current capability, without wasting power and increasing chassis temperature, but I will check this solution as well.
 
#10
astouffer said:
Find a 5 volt model with a trimmer pot. It probably has the trimmer pot in series with a resistor already so you will have to change that resistor to get into a higher range.
Click to expand...
Thanks astouffer, I've emailed to an ebay seller how much his SMPS can be adjusted, but I don't expect that stock will vary more than 10%. Change the reference to the chip seems a good solution.

astouffer said:
Watch out for cheap supplies with low voltage filter caps and remember to keep track of the wattage now that you increased the voltage.
Click to expand...
I've planned to have a safety factor of 4 on the needed current (so buy a 5 V 20 A SMPS) just to be on the safe side.

I've seen (and probably I already have one somewhere at home) those DC-DC converters, that can be supplied by 6.3 Vac, but I would prefer to avoid heaters winding in the power transformer.
 
#11
petertub said:
A resistor and a proper cap might be the right combination to eliminate any disturbances from the SMPS.
Click to expand...
Thanks petertub, do you think it will inject alot of noise in the heaters and the audio circuit? Only low quality ones or I have to expect it from all? It will be installed inside the chassis.

petertub said:
The resistor will also make the coldstart easier as the SMPS will not experience "short" during start but will "see" the resistor. By time the filamants will warm up and voltage/current stabilize.
Click to expand...
The resistor will be quite low in value (0.28 Ohm), will it have any of the two effects as well?
 
#13
baudouin0 said:
Some SMPS can produce audio switching noise if only lightly loaded as they go discontinuous. But I think you would be OK at 25%. Most SMPS will restart if they hit current limit so you may be fine with 2-2.5x.
Click to expand...

Thanks baudouin0, so it could be enough 10A if the SMPS was already foreseen for 6.3V, but being most probably designed for 5V, 20A is better.

In the meanwhile I've asked to the ebay seller that has been linked before if he has a complete SMPS to supply BH-EL84 for heaters, B+ and powerdrive. That could simplify even more the power supply.
 
#14
astouffer said:
Find a 5 volt model with a trimmer pot. It probably has the trimmer pot in series with a resistor already so you will have to change that resistor to get into a higher range. Watch out for cheap supplies with low voltage filter caps and remember to keep track of the wattage now that you increased the voltage.
Click to expand...

The 5V supplies I have tried won't even get up to 6V, most of them top out around 5.8V or so.
 
#15
zintolo said:
Thanks astouffer, I've emailed to an ebay seller how much his SMPS can be adjusted, but I don't expect that stock will vary more than 10%. Change the reference to the chip seems a good solution.

I've planned to have a safety factor of 4 on the needed current (so buy a 5 V 20 A SMPS) just to be on the safe side.

I've seen (and probably I already have one somewhere at home) those DC-DC converters, that can be supplied by 6.3 Vac, but I would prefer to avoid heaters winding in the power transformer.
Click to expand...

Do also be aware that many of these supplies expect the negative output terminal to be earthed, or at least close to it. Often you will see in the datasheets that there's an internal capacitor between the case of the SMPS and one of the output terminals. If you need to bias up your heaters to avoid heater cathode insulation problems, a SMPS starts to not look like such a great idea.
 
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#20
Im using an XP 48V to 9V 1A dc/dc converter to power the heaters of 2 x ECC88's for a phono stage. Im dropping the 9V to 6.3V first via a pi filter then a simple active gyrator as a second filter. Will dissipate a bit of heat though.

I would have liked 7.5V to reduce the heat but I couldnt find anything. Anyone know of anything suitable ? I need 48V because of the 48V to 230V HT boost converter.

Sorry if its slightly off topic.
 
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