Current consumption? - UK Vintage Radio Repair and Restoration Discussion Forum

Geoff 555 · 15th Feb 2010, 12:09 pm

Hello I have a chassis that uses the following valves UCH21,UBF80,UAF42,UL41.
This is a AC/DC set and I was trying to find out how many watts it would take to power it ,now according to my meter it takes 0.1 amps. Surely this can not be right, the reason I want to know is that it is 220 V max, and the mains voltage here can be 251V which it really does not like at all (only run for a couple of minutes.) I need to buy a transformer and did not want to go to more expense than necessary being as I have to make a cabinet and things to house it in.
I know it's not worth it but just hate throwing things out.
Thanks all.
Cheers.
Geoff.

AlanBeckett · 15th Feb 2010, 12:13 pm

Geoff,
That's more than likely an AC/DC set with a resistive dropper for the HT and heaters. Some added resistance should fix it.
What's in there at the moment?
Alan

Boom · 15th Feb 2010, 12:20 pm

More than likely a European AC/DC set. I'd add 60mA for the HT to your figures Geoff.

Ideally a 187 Ohm 5 Watt dropper section would bring it up to the UKs 250VAC.

Dave

Geoff 555 · 15th Feb 2010, 2:20 pm

Hello thank you all, I did try a resistance and it was getting very hot and being a coward and thinking of setting fire to something though I would ask first.
I have found a auto transformer 210V primary to 240V secondary is there any reason not to use it back to front as one might say?
Cheers.
Geoff.

paulsherwin · 15th Feb 2010, 2:42 pm

Power resistors do run hot when loaded - it's how they dump the energy they're absorbing. If the resistor is adequately rated, you just need to provide ventilation and not touch it. 5W is a bit optimistic though, and 10W would offer more headroom. Some resistors can be bolted to a chassis so that it can act as a heatsink.

You could use an autotransformer as you suggest, but it's a bit of a sledgehammer to crack a nut.

Paul

Sideband · 15th Feb 2010, 4:55 pm

Quote:

Originally Posted by Geoff 555

I have found a auto transformer 210V primary to 240V secondary

Puzzled by this

. Auto-transformers only have one winding so you'd have one winding with different taps...no secondary winding. If it is an auto-transformer, there is no reason why you cannot feed 240v across the whole winding and take 210v from the tap. It will act as a step-up or step-down depending on how you use it.

Rich.

Geoff 555 · 15th Feb 2010, 9:52 pm

Hi about the auto transformer I was puzzled as well but it has a plate on it which states that it is an auto and has a primary and secondary, it will take a few days to unearth it but that is how I remember it.
I will use a resistor at the moment just to get it running.Thank you.
Cheers.
Geoff.

Boom · 15th Feb 2010, 10:03 pm

I have a toroidal isolation transformer that is clearly marked 240 in/out but connected one way around it gives 220 out for 240 in and the other way it gives 260 out for 240 in.

Lovely unit but I'm very wary of using it on vintage equipment. Being toroidal I can't see how the makers got the windings wrong.

Dave

Dave

jimmc101 · 15th Feb 2010, 10:31 pm

A small 2 x 15V mains transformer with the secondaries connected in series opposition to the primary will give 220V from a 250V supply.
Since only the secondaries will carry the current to the set, even a small 6VA transformer will handle 0.2A.

Quote:

Originally Posted by David Tilley

I have a toroidal isolation transformer that is clearly marked 240 in/out but connected one way around it gives 220 out for 240 in and the other way it gives 260 out for 240 in.

Dave

Were these voltages measured off load?
Remember that the secondary voltage will drop as the transformer is loaded, at full rated load the secondary voltage may drop from 260 to 240.
(8% regulation, a bit high but feasible especially if it's a small one).

Jim

Herald1360 · 16th Feb 2010, 12:26 am

Quote:

Originally Posted by jimmc101

A small 2 x 15V mains transformer with the secondaries connected in series opposition to the primary will give 220V from a 250V supply.
Since only the secondaries will carry the current to the set, even a small 6VA transformer will handle 0.2A.

Ah ha! An elegant engineering solution. Cheap and no significant waste of power either.

jimmc101 · 16th Feb 2010, 8:26 pm

I've been thinking a little more about this...
The mains current of an AC/DC set will have a DC component caused by the HT rectifier.
(Say 100mA AC for the heaters plus 50mA DC for the HT.)
This might cause core saturation problems in a small transformer so a more generous rating may be advisable especially with a toroidal transformer.

Does anyone know how close to flux density limits modern transformers run?

Jim

Geoff 555 · 17th Feb 2010, 8:55 pm

Just to say a big thank you to every one it's been sorted by a small 20 VA 12-0 , 12-0 transformer, brings it nicely down to 217V with no problems at all.
The auto transformer mentioned before does have primary and secondary marked on the plate, the actual tappings are N, 240, 230, 220, 210. may be they just used up a spare plate?
Cheers.
Geoff.

kalee20 · 18th Feb 2010, 2:16 pm

Jimmc101 is right to be cautious, this effect can arise and would result in the little transformer running somewhat hotter than you'd expect. But in practice, I'd be confident using a transformer of double the rating.

Luckily, Geoff has been able to do this, and it's working well!

15th Feb 2010, 12:09 pm	#1
Geoff 555 Heptode Join Date: Jan 2008 Location: Bristol, UK. Posts: 981	Current consumption? Hello I have a chassis that uses the following valves UCH21,UBF80,UAF42,UL41. This is a AC/DC set and I was trying to find out how many watts it would take to power it ,now according to my meter it takes 0.1 amps. Surely this can not be right, the reason I want to know is that it is 220 V max, and the mains voltage here can be 251V which it really does not like at all (only run for a couple of minutes.) I need to buy a transformer and did not want to go to more expense than necessary being as I have to make a cabinet and things to house it in. I know it's not worth it but just hate throwing things out. Thanks all. Cheers. Geoff.

15th Feb 2010, 12:13 pm	#2
AlanBeckett Rest in Peace Join Date: Dec 2008 Location: Burton upon Trent, East Staffordshire, UK. Posts: 1,686	Re: Current consumption? Geoff, That's more than likely an AC/DC set with a resistive dropper for the HT and heaters. Some added resistance should fix it. What's in there at the moment? Alan

15th Feb 2010, 12:20 pm	#3
Boom Retired Dormant Member Join Date: Jan 2007 Location: Westbury, Wiltshire, UK. Posts: 2,451	Re: Current consumption? More than likely a European AC/DC set. I'd add 60mA for the HT to your figures Geoff. Ideally a 187 Ohm 5 Watt dropper section would bring it up to the UKs 250VAC. Dave

15th Feb 2010, 2:20 pm	#4
Geoff 555 Heptode Join Date: Jan 2008 Location: Bristol, UK. Posts: 981	Re: Current consumption? Hello thank you all, I did try a resistance and it was getting very hot and being a coward and thinking of setting fire to something though I would ask first. I have found a auto transformer 210V primary to 240V secondary is there any reason not to use it back to front as one might say? Cheers. Geoff.

15th Feb 2010, 2:42 pm	#5
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,966	Re: Current consumption? Power resistors do run hot when loaded - it's how they dump the energy they're absorbing. If the resistor is adequately rated, you just need to provide ventilation and not touch it. 5W is a bit optimistic though, and 10W would offer more headroom. Some resistors can be bolted to a chassis so that it can act as a heatsink. You could use an autotransformer as you suggest, but it's a bit of a sledgehammer to crack a nut. Paul

15th Feb 2010, 9:52 pm	#7
Geoff 555 Heptode Join Date: Jan 2008 Location: Bristol, UK. Posts: 981	Re: Current consumption? Hi about the auto transformer I was puzzled as well but it has a plate on it which states that it is an auto and has a primary and secondary, it will take a few days to unearth it but that is how I remember it. I will use a resistor at the moment just to get it running.Thank you. Cheers. Geoff.

15th Feb 2010, 10:03 pm	#8
Boom Retired Dormant Member Join Date: Jan 2007 Location: Westbury, Wiltshire, UK. Posts: 2,451	Re: Current consumption? I have a toroidal isolation transformer that is clearly marked 240 in/out but connected one way around it gives 220 out for 240 in and the other way it gives 260 out for 240 in. Lovely unit but I'm very wary of using it on vintage equipment. Being toroidal I can't see how the makers got the windings wrong. Dave Dave

16th Feb 2010, 8:26 pm	#11
jimmc101 Heptode Join Date: Mar 2009 Location: Portsmouth, Hampshire, UK. Posts: 674	Re: Current consumption? I've been thinking a little more about this... The mains current of an AC/DC set will have a DC component caused by the HT rectifier. (Say 100mA AC for the heaters plus 50mA DC for the HT.) This might cause core saturation problems in a small transformer so a more generous rating may be advisable especially with a toroidal transformer. Does anyone know how close to flux density limits modern transformers run? Jim

17th Feb 2010, 8:55 pm	#12
Geoff 555 Heptode Join Date: Jan 2008 Location: Bristol, UK. Posts: 981	Re: Current consumption? Just to say a big thank you to every one it's been sorted by a small 20 VA 12-0 , 12-0 transformer, brings it nicely down to 217V with no problems at all. The auto transformer mentioned before does have primary and secondary marked on the plate, the actual tappings are N, 240, 230, 220, 210. may be they just used up a spare plate? Cheers. Geoff.

18th Feb 2010, 2:16 pm	#13
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,087	Re: Current consumption? Jimmc101 is right to be cautious, this effect can arise and would result in the little transformer running somewhat hotter than you'd expect. But in practice, I'd be confident using a transformer of double the rating. Luckily, Geoff has been able to do this, and it's working well!