245V on 220V apparatus - is the voltage drop resistor value I computed correct ?

[kagliostro]

I want to tame a bit the line voltage (around 245V) to supply a 220V Record Player

The consumption at max power and with the plate rotating is around 0.020mA

I computed 25v / 0.020mA = 1250R @ 0.5W

Will be a 1200R @ 2W adequate for the use ?

Thanks

Franco

[sluckey]

The consumption at max power and with the plate rotating is around 0.020mA

That seems to be an unrealistic amount of current. Even if you really mean .02 amps, still seems way to low for an ac motor capable of turning a record platter.

[kagliostro]

Ciao Steve

My fault, it is 0.02A not mA (20mA is what I read)

I'm not practice on Record Players

20mA x 220V = 4.4W

I'll try to measure with one other tester

Thanks

Franco

[sluckey]

Your calculations seem logical to me. I'd use 1200Ω as a starting point and be prepared to adjust up/down as needed.

I'm pretty good with these kind of calculations for dc circuits but ac calculations don't always work out for me. 

[kagliostro]

I measured newly the current and the correct value is 170mA with motor ON and 150mA with motor OFF

When I measured previously the current I did a very stupid thing, I set the tester for DC current instead of AC current 

So

25v / 0.17mA = 157.059ohm @ 4.25W

and the resistor I think to use is

150R @ 10W

What do you think about ?

more

Is 4.25W in heat too much to be inside a wood record player case ?

Thanks

Franco

[sluckey]

I think 170mA is totally believable. I wouldn't worry about the resistor heat. Just give it plenty of air space.

[PRR]

Or a 230V:24V >5VA transformer wired auto-transformer.

Is 245VAC the actual measured wall-voltage there? I was unsure if "240V" lands were trending away from the 240V-250V we get here, or not.

My city house, where lines are short, has 122V each side so 244V total. This house in the woods where lines are long has 250V at the street which works out less after 500 feet of wire, so 240V-245V typical.

[jjasilli]

Ditto to a bucking tranny.  A resistor may send over voltage to the amp when the motor is not running.

[kagliostro]

Thanks to ALL

I can put the resistor in an adequate place, but the case has no vent holes, only few thin cracks/loopholes

A buck transformer ... something like this ?



or this ?



5VA .... is it enough ?

At the moment the official line voltage for domestic use here is 230V, 400V for industrial use, before we had 220V and before 125V & 220V, for industrial use the old line was 380V

At my house that is in a town (Treviso) the line is around 224V, at the house of my friend, that is in a village not very far (Quinto di Treviso), the line is around 245V


Actually a 150R resistor (if set for the whole consumption of 170mA) may send an over voltage to the circuit when the motor is OFF but I computed anyway a voltage drop of 18V when the consumption is "only" 120mA (when the motor is OFF) this give 227V ( 245V - 18V) that isn't very far from what is desired

Franco

[jjasilli]

Sluckey has a page somewhere showing how to wire a bucking transformer; or google it to make sure.  A resistor is simpler if it works.  OTOH, a bucking tranny could be put in a standalone box with a SW to select different secondary taps.  Then it could be used to compensate for your varying supply voltages > to a better output voltage for various devices.


It also complies with your personal, innate attraction to transformers! 

[kagliostro]

Ciao Jjasilli

In this situation an external buking transformer will be not appreciated, may be an internal one if small

I'm sure the two schematics I posted can do the job, only I don't know which will be the better solution between those

More, I'm not sure I correctly understand PRR about the correct VA rate of the required Buck transformer

he wrote 230V:24V >5VA and is that > that confuses me ( )

Today I'll experiment a bit with resistors (I've a large collection of big resistor)

Franco

[92Volts]

The second schematic is a "bucking transformer". The secondary directly "subtracts from" line voltage, and we can easily find it supplies 170ma at 24v. That's 4VA, so 5VA or larger should work (that's what was meant by >5VA).

The first schematic is a true "autotransformer". I'm less familiar with these but it should work too. Note the "primary" sees the reduced voltage in this configuration while the primary saw full voltage in the bucking configuration. Therefore voltage across the secondary will be lower and voltage reduction will be lower. You could choose one or the other to fine-tune the resulting voltage, or to use a 240v-24v or 220v-24v transformer.

[jjasilli]

he [prr] wrote 230V:24V >5VA and is that > that confuses me (  )

The bucking (or an auto) tranny needs to handle current into a 5W load.  Your math says you need 4.25W.  Looks like PRR rounded up. W = V x A.  But that's for DC.  We have AC.  The AC formula is more complex, but the DC formula works well enough for general use.  However, AC current handling is more correctly expressed in VA, rather than W.

[kagliostro]

Thanks 92Volts & Jjasilli

My doubt was about the fact that > is used to indicate "more than"

I know there is difference between VA and W, but I'm only able to have simple computation about W

--

Today I tried with a 150R resistor and the drop in voltage (the line here is around 224V and I measured the voltage out of the 160V winding of the autotransformer, the schematic of the circuit is here http://el34world.com/Forum/index.php?topic=22686.msg242350#msg242350) with no resistor on the 160V tap there are 164.5V (motor ON - max consumption)

with 150R resistor 154.2V (10.3V drop)

with 200R resistor 150.7V (13.8V drop)

with 330R resistor 141V (23.5V drop)

the 150R and 200R resistor were 17W resistors and the 330R resistor was a 9W resistor, this 9W resistor became hot in a few time

math I used is totally unsafe, it gives odd results (I suppose it is due to the fact here we talk about AC, not DC)

I'm considering to give a look at my recovered junk box for a small transformer ............

Franco

[PRR]

The bucking transformer only has to handle the difference, not the whole load.

Because you need about 10% drop to a 50 Watt load, a 5Watt/VA transformer should be OK for home use. If you are running a 24 hour disco, use 10VA.

[kagliostro]

Many Thanks PRR

This morning I was looking for an usable transformer on my recovered junk box but till now nothing, however I've other box to examine for this search

--

I did also other investigations using resistors, two 150R 17W in series seems to be usable,  they warm but the temperature is affordable

--

I hope to find an usable transformer

Thanks again

Franco

[kagliostro]

I've find a 230V / 18V 11VA PCB transformer

Will this configuration work correctly or will result in problems when the voltage change switch is set for 220V instead of 240V ???





EDIT:

The friend that suggested me the previous solution had also this idea



This seems better to me, which is your opinion about ?

Thanks

Franco

[kagliostro]

I installed the transformer following this schematic and all is fine

the drop of voltage is OK and the transformer is cold also after some miutes of work




Many thanks to All


Franco

[jjasilli]

  (I knew you'd have a suitable tranny on hand)

[kagliostro]

Thyis is the schematic of the whole thing



Franco