PDP-11/70 Boards

[Wayne S]

The 2 variac’s I’ve worked with were fused and if you exceeded the fuse rating it blue no matter the voltage. You want to protect the variac if you get a short circuit.


Sent from my iPhone

> On Dec 8, 2021, at 15:14, Mike Katz via cctalk <cctalk at classiccmp.org> wrote:
> 
> That would depend on the type and size of wire (solid core, stranded, strands of stranded, etc.).
> 
>> On 12/8/2021 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
>> 
>