8 Dec
2021
8 Dec
'21
3:13 p.m.
Variable transformers (generic. Variac is a trademarked brand of them) are rated in amps
output, not in watts or, more accurately, volt-amps. That is because they are rated to
deliver that current at ANY voltage. The main limiting factor is flux density in the core.
The flux density is proportional to the current. So, the rated current (e.g. 2.5 amps in
the original question) can be drawn at ANY output voltage without overloading the core. At
1V out, you get 2.5 volt-amps. At 120V out you get 300 volt-amps. The current is the same,
so the IR (resistive) losses of voltage are the same.
If you draw more current than the rated current, then the core will be overloaded. It will
most likely overheat and will possibly be permanently damaged. In addition, the IR losses
will increase, causing more overheating.
On 12/08/2021 5:06 PM pbirkel at gmail.com wrote:
Seems to me that the actual coil resistance will limit the max-current at lower voltages.
20 A through wire sized for 1 A seems ... unlikely?
-----Original Message-----
From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Mike Katz via cctalk
Sent: Wednesday, December 8, 2021 5:58 PM
To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>;
wrcooke at wrcooke.net
Subject: Re: PDP-11/70 Boards
More accurately up to it's rated wattage "Power = Voltage * Current"
after all.
If you have a 100W max variac you can draw 20A @ 5V (approx) but only 1A at 100V.
On 12/8/2021 4:44 PM, Mike Katz via cctalk wrote:
> "As a general rule, a variable transformer (Variac) can provide full
> rated current at any output voltage. So a 2.5A unit can provide 2.5 A
> at 1V, 10V, 120V, etc. With a 20V output, that is 50 VA (Watts, sort
> of)."
> Up to the current rating of the variac. When you draw more current
> than the transformer can deliver then the voltage will sag.
> On 12/8/2021 4:22 PM, Will Cooke via cctalk wrote:
> >
>>> On 12/08/2021 3:58 PM Rob Jarratt via cctalk <cctalk at
classiccmp.org>
>>> wrote:
>>> So, to supply the bricks on the bench, would a variac rated at 2.5A
>>> be OK? I am not sure I know how much current the bricks will draw at
>>> 20VAC, and at what voltage the 2.5A rating is given. Otherwise,
>>> would this do the trick?
>>> https://cpc.farnell.com/block/steu250-48/transformer-250va-230-400v-
>>> 2-x/dp/TF01418?st=24v%20transformer
>>>
>>> Thanks
>>>
>>> Rob
>>>
>>>
>>>>> JRJ
>> As a general rule, a variable transformer (Variac) can provide full
>> rated current at any output voltage. So a 2.5A unit can provide 2.5
>> A at 1V, 10V, 120V, etc. With a 20V output, that is 50 VA (Watts,
>> sort of).
>>
>> Will
>>
>> "I was born not knowing and have had only a little time to change
>> that here and there."
>> Richard Feynman
>
"I was born not knowing and have had only a little time to change that here and
there."
Richard Feynman