[Heathkit] IP-28 Issues

[Bill Cromwell]

On 01/22/2014 09:17 AM, Gary H. Harmon, Jr. wrote:
> A engineering friend of mine locally has been working hard to resolve issues
> with an IP-28 power supply.  His findings follow:
>
>   
>
> "I have come to the conclusion that the circuit design on the IP-28 has some
> serious issues with the power rating of the current limit pot.  All power
> must flow through this series device and power dissipation of the pot would
> be around 30W at full voltage.  The ten turn pot is only rated at 2W.  I
> honestly do not know what the Heath engineers were thinking when they
> designed this one!  I would suspect that other components in the load
> current path are also under rated; I know my unit took out three transistors
> in addition to the pot and another series resistor."
>
>   
>
> Any thoughts on this problem?
>
>
Hi Gary,

I do not have one of those supplies and I certainly don't have access to 
a manual or schematic. I *HAVE* built several power supplies with 
electronic current limiting including supplies with fixed current to the 
limit of the power transformers capability to furnish the required voltage.

That sense resistor does indeed have most of the output current flowing 
in it. The voltage drop across that resistor, however *NEVER* exceeds 
0.7 volts. And there is part of a transistor circuit across it. Unless 
there is a total, catastrophic failure elsewhere in the power supply. 
And that, sir, is what fuses are for. To prevent burning down your building.

So if that is a 12 volt (or 13.8 volt) supply you can't do the math 
using 12 volts and the output current. It's 0.7 volts times the output 
current. For very high current power supplies that can require a 
wirewound power resistor. So for 300 watts dissipated in this resistor 
the output current would exceed 40 amps. Is this a 40 amp supply? A 2 
watt resistor would be maxed out in that position at somewhat under 3 
Amps - worst case.