# PDP-11/70 Boards

pbirkel at gmail.com pbirkel at gmail.com
Wed Dec 8 17:06:31 CST 2021

```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
>

```