[time-nuts] HP5370 power supply measurements

Poul-Henning Kamp phk at phk.freebsd.dk
Mon Jul 11 14:46:23 EDT 2016

Inspired by the recent thread about power trouble, I spent an hour
to actually measure why the HP5370 gets so warm as it does.

I'm running on 230VAC here, with the 230/240VAC power setting.

With A6 and A7 removed and turned off: PAC= 4.2 W

That is the loss in the transformer and rectifiers, since the "power"
switch on the HP5370 really isn't that.

Turning it on, PAC= 19.9W

So the fan uses 15.7W.  (it's also noisy, I should find a replacement)

I also measured the four unregulated DC voltages:

	-20V -> 28.9V
	-10V -> 13.4V
	+10V -> 13.4V
	+20V -> 28.9V

Inserting A6 does not change the OFF state PAC measurably.

Turning on however:  PAC = 140 W


	-20V -> 27.5V
	-10V -> 10.9V
	+10V -> 11.9V
	+20V -> 27.1V
	+15V -> 14.99V
	-15V -> 15.04V
	+5V  -> 5.05V
	-5.2V-> 5.4V
	+10V -> 10.0009V
	-20V -> 123mV ripple
	-10V -> 2.02V ripple
	+10V -> 960mV ripple
	+20V -> 312mV ripple

The unregulated busses are still waay over what the schematics
would have you belive (10/20V).  The ripple on the -10V seems
pretty high, but the four electrolytic capacitors seem healthy

Next I tried measuring the current on the four regulated voltages,
which all have short-circuit measuring resistors:

	A6 pin  5- 6 188mV, 0.07R 3% -> 2.68A   +5V
	A6 pin  7- 8 498mV, 0.07R 3% -> 7.11A   -5.2V
	A6 pin  9-10 109mV, 0.4R  1% ->  .2725A +15V
	A6 pin 11-12  55mV, 0.4R  1% ->  .1375A -15V

Since I measured this on the bottom of the motherboard, the contact
and PCB trace resistance to/on A6 is also included so these currents
are high estimates.

Measuring with a clamp milliamp-meter instead I find:

	+5V   -> 2.2A
	-5.2V -> 5.5A

Doing the math:

		 Brutto    Netto 
	Trafo:    4.2 W    4.2 W
	Fan:     15.7 W   15.7 W
	OCXO:    14.0 W   14.0 W   Netto  Brutto
	-20:      3.8 W    2.1 W   2.1 W   3.8 W
	-10:     60.0 W   29.0 W  29.0 W  60.0 W
	+10:     31.9 W   13.4 W  13.4 W  31.9 W
	+20:      7.4 W    4.1 W   4.1 W   7.4 W
		137.5 W   82.5 W  48.6 W 103.1 W

Modern efficient DC/DC converters should therefore be able to
reduce the PAC by about 50W or one third.

I tried load-testing the unregulated busses, and it looks like all
four of them can be loaded up to about 5A individually before the
voltage falls out of spec (10/20 V)

So I've been playing around with KICAD and come up with a design
with three Traco TEN40WIR DC/DC converters (2x2411 + 2423) running
off the +/- 10V rails, and I'm seriously tempted to try order a
PCB just to try it out...

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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