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All, I am new to Circuit labs and have been finding it great. I have a DC-DC converter from TRACO which take a 12 volt DC supply and then outputs a max of 300 volts DC at a max of 10ma. funny thing is when I connect a 12 volt dc power supply it seems to short circuit the supply and pull the input voltage down to 5 volts (this is what my multimeter reads). However , when I connect a 12 volt battery everything works great. For the life of me I can't get my head around this. Does any one have any ideas of what might be happening? And any suggestions on what I might try.

Thanks, Andy M

Update, In talking to the TRACO group it appears this issue may be due to inrush current.

Are you trying to drive a load at the same time the converter is starting up?

300V * 10mA = 3W

Assume the converter is about 75% efficient (unlikely to be better than 80% with such a high step up ratio) then the steady state power taken from the 12V supply will be about 4W, i.e. the current drain will be 333mA.

If the converter has a poorly designed inrush current/soft start control then the input current drain as the 12V is applied could be much greater than this.

How much current can your bench PSU supply before it either goes into current limit or just runs out of steam?

Excellent, thanks for the reply. This is exactly it. I turned my current up on my bench PSU to 60 mA and no more problems.

Now that I have the circuit working with the Battery and the bench PSU, I need to see if I can do something to get the standard 12v plug in (the type that would be used to charge a phone, or power a clock radio) power supply to work.

The label says it's specs are; input voltage of 120V @ 0.4A with and output of 12V @ 1A.

Any suggestions on what I can use to reduce the inrush. It looks to me like the DC-DC converter must be pulling above 1A inrush, which my bench PSU is able to handle for the breif time it is there, however the plug in power supply can not. I've been reading about thermisters any thoughts.

Put a large capacitor across the 12V supply rail just before the TRACO PSU inputs.

Then wire a switch between the +12V across the cap and the +12V i/p of the TRACO PSU.

Turn on the 12V supply 1st then turn on the switch to connect the TRACO.

The inrush current will then not drop the 12V supply so much because it's coming from the cap not the PSU.

You could measure the inrush current by connecting a small R in series with the input supply ground and then looking across it with a scope to measure the voltage drop across the resistor. This will then show you how big (I=V/R) and how long (t) the inrush current is.

From that you can scale the capacitor from CV=It where V is the voltage drop you can allow across the cap due to the charge (It) being drawn out of it.

You can model this in CL.

Be very wary of using thermistors etc., to "throttle" the inrush current. If you limit it the TRACO may spend too long in some inefficient startup state, unable to get to a more efficient state before it overheats and damages itself.

Discuss inrush limiting with TRACO before trying anything like that.

Or buy a PSU with a proper, fully specified, controlled startup.

If you're using one of these:

http://www.tracopower.com/fileadmin/medien/dokumente/pdf/datasheets/thv.pdf

or

http://www.tracopower.com/fileadmin/medien/dokumente/pdf/datasheets/phv.pdf

then their datasheets are rubbish. There's no mention of input currents or power at all.

Utterly useless.

Great thanks for the feedback, I'll have to spend some time working on this. For reference the unit I am using is a THV 12-300P. Yeah no mention of input currents or power. That said when the thing works it works well, I guess they just need to improve the documentation and ideally there inrush/soft start circuit.

I have the TRACO group looking into this as well so we will see what they comes back with.

Update: I finally received the inrush current that TRACO measured. The current spikes to a max of 10.5 A max in then drops to -3A lasting 50 micro-sec. With smaller spike of what looks like 1.5 A. all over a 200 micro-sec.

At any rate I managed to find a 12 volt 4A computer type power supply which work great. Thanks for your help with this signality.

My pleasure

:)