30-07-2015, 10:20 PM
That's quite a range of series resistance - 1.7 to 264 ohms. It would be interesting to plot these on a graph - there would be quite a big gap, with nearly all of them clustered at one end.
Personally, if using a pot like this, I wouldn't wire it as a rheostat; I'd use it as a potential divider. This will be better behaved, but it will dissipate a fair amount of power when it's just doing nothing.
With some very basic calculations (too late in the day for anything else!), you could probably get by with a 10 ohm potentiometer. Higher values would work but again you might find that the scale is cramped at one end. Actually, I might be tempted to add a fixed resistance between the lower (anti-clockwise) end and ground, so that the minimum is just below 1.4V rather then 0V, so at least you're using more of the track.
Anyway, on the assumption we have 6.3V AC going in from the transformer - and we'll keep it as AC for simplicity - then we are wasting 4 watts of heat in the pot alone. Perhaps a bit less if a resistor is added. Perhaps a bit more when a valve is being powered. Probably worse if you converted to DC first, depending on how much smoothing you include.
At least here the circuit is presenting a reasonably low source impedance to the heater, so compared to the rheostat connect, the voltage will hopefully vary rather less during warm-up. Hopefully.
Ultimately, I keep returning to the variac. With that, you have the magic of transformer action, meaning losses are minimised. With a resistance, good old I-squared-R is impossible to avoid.
This is the sort of thing: http://www.ebay.co.uk/itm/Regavolt-variable-Transformer-Type-301-0-5-Amps-/252033736171?pt=LH_DefaultDomain_3&hash=item3aae619deb - it would be on the primary side of a 240->6.3V transformer. Shame it's a bit pricey, but no-doubt one would turn up at the right price eventually. The current rating of 0.5A is generous for this application, as the total load is only 6.3VA (plus a bit of safety margin). On the primary side, that's mere milliamps.
I suppose there is another alternative. You could use a light dimmer ahead of a mains transformer. It's crude, and the waveform won't be pretty, but that doesn't actually matter (perhaps the meters might be unhappy, depending on what sort you choose?). Such things are available commercially: http://www.elenco.com/product/productdet...upply=NTc2 - I've attached the schematic so you can see how it works.
Personally, if using a pot like this, I wouldn't wire it as a rheostat; I'd use it as a potential divider. This will be better behaved, but it will dissipate a fair amount of power when it's just doing nothing.
With some very basic calculations (too late in the day for anything else!), you could probably get by with a 10 ohm potentiometer. Higher values would work but again you might find that the scale is cramped at one end. Actually, I might be tempted to add a fixed resistance between the lower (anti-clockwise) end and ground, so that the minimum is just below 1.4V rather then 0V, so at least you're using more of the track.
Anyway, on the assumption we have 6.3V AC going in from the transformer - and we'll keep it as AC for simplicity - then we are wasting 4 watts of heat in the pot alone. Perhaps a bit less if a resistor is added. Perhaps a bit more when a valve is being powered. Probably worse if you converted to DC first, depending on how much smoothing you include.
At least here the circuit is presenting a reasonably low source impedance to the heater, so compared to the rheostat connect, the voltage will hopefully vary rather less during warm-up. Hopefully.
Ultimately, I keep returning to the variac. With that, you have the magic of transformer action, meaning losses are minimised. With a resistance, good old I-squared-R is impossible to avoid.
This is the sort of thing: http://www.ebay.co.uk/itm/Regavolt-variable-Transformer-Type-301-0-5-Amps-/252033736171?pt=LH_DefaultDomain_3&hash=item3aae619deb - it would be on the primary side of a 240->6.3V transformer. Shame it's a bit pricey, but no-doubt one would turn up at the right price eventually. The current rating of 0.5A is generous for this application, as the total load is only 6.3VA (plus a bit of safety margin). On the primary side, that's mere milliamps.
I suppose there is another alternative. You could use a light dimmer ahead of a mains transformer. It's crude, and the waveform won't be pretty, but that doesn't actually matter (perhaps the meters might be unhappy, depending on what sort you choose?). Such things are available commercially: http://www.elenco.com/product/productdet...upply=NTc2 - I've attached the schematic so you can see how it works.
