# DC-DC wandering voltage... howto stabilize?



## dtbaker (Jan 5, 2008)

I have an ACME DC-DC.... 
http://evolveelectrics.com/dc-dc-converters/636w-ac/dc-dc-converter/

'always on', doing its job fairly well. battery pack is 156v nominal in the Miata. Lately, the output voltage has been wandering; requiring occasional tweaks of the pot to get it back 12.0-13.0 where it should be to make the Zilla and 12v car stuff happy. Seems like if it goes over 13.5 or under 11.0, Zilla is not happy.

the way I can see the 12v system voltage is a little meter I keep plugged into the cigarette lighter.

The problem seems to have gotten worse this winter, when its cold, and several times at night has been an issue when I've turned on headlights and began driving.... I'd get about ten feet and the DCDC would turn itself OFF! weirdly, if I reset and start driving with headlights off, and turn them on after 10 seconds of driving or so, then it was ok. I've only had the dcdc click off when I had headlights on, or not allowed enough time after key-on before driving (to pre-charge zilla I think?)

Even with everything on, I should be well below the 600watt output limit of the dcdc.

so..... what are some good ways to stabilize the voltage output?
shall I put a little 12v battery in parallel?
could this be some kind of grounding issue?


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## azdeltawye (Dec 30, 2008)

dtbaker said:


> ...
> shall I put a little 12v battery in parallel?...


This is always a good idea in my opinion. 
You don't want a single point failure to cause you to stall in busy traffic on on the railroad tracks...


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## Tesseract (Sep 27, 2008)

dtbaker said:


> I have an ACME DC-DC....
> http://evolveelectrics.com/dc-dc-converters/636w-ac/dc-dc-converter/


That product was designed for use in office/home equipment. It has no business being used in industrial or automotive applications. Primary reasons: enclosure with multiple openings (not dust/moisture proof), trimpot for voltage adjustment, probably cheap paper-phenolic PCB that will absorb moisture over time and delaminate, components not secured to withstand high shock/vibration.




dtbaker said:


> so..... what are some good ways to stabilize the voltage output?
> shall I put a little 12v battery in parallel?
> could this be some kind of grounding issue?


There's no point putting a 12V battery in parallel with the converter if the voltage drops below the minimum float voltage (and note that you can't really recharge a 12V battery with a float charger - that's only meant to stop self-discharge). So at a minimum I would replace the pot with two resistors of similar total value and equivalent ratio to deliver 14.0V (if the Zilla really can't handle more than 13.5V - which doesn't seem likely - then put a diode or two in series with its 12V supply connections to drop the voltage by 0.6V or 1.2V). I would also consider spraying the PC board with clear acrylic paint to provide at least a minimum level of protection against humidity/condensation. Finally, yes, wire a 12V battery in parallel with the converter, but only if you set the voltage to at least 13.8V, and preferably 14.0 to 14.2V (you don't want to go too much above the float voltage if using a constant voltage supply vs. a "proper" 12V battery charger).


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## dtbaker (Jan 5, 2008)

Tesseract said:


> That product was designed for use in office/home equipment. It has no business being used in industrial or automotive applications. Primary reasons: enclosure with multiple openings (not dust/moisture proof), trimpot for voltage adjustment, probably cheap paper-phenolic PCB that will absorb moisture over time and delaminate, components not secured to withstand high shock/vibration.
> 
> 
> 
> ...



yeah.... I am beginning to think that the DC-DC is showing signs of falling apart internally, and it may be best to spend the extra dollars to get a more dependable unit. After many more cases where the DCDC just clicks off.... it doesn't seem to be the controller affecting it at all, just the DCDC turning itself off; usually just after I've keyed-on and started driving. I don't see how that puts a load on the 12v system, but thats when it usually clicks off, and usually when the radio is on (which is not a very big load either). This ACME DCDC is supposed to be able to output close to 600 watts, which should be more than enough.

Begs the question of course.... which ones ARE suitable and solid?!

300-400 watts are probably enough for the average small car without electric power steering. I had a Curtis 300 watt unit in my Geo when it was 96v, but swapped it out when I went to 120v. The Miata is 156v nominal, so I probably need the DC-DC to handle up to 170vDC input


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## Sunking (Aug 10, 2009)

Tesseract said:


> There's no point putting a 12V battery in parallel with the converter if the voltage drops below the minimum float voltage (and note that you can't really recharge a 12V battery with a float charger - that's only meant to stop self-discharge).


That is not accurate. For Pb batteries there is no better charger algorithm than Float chargers. Not as fast as a 3-stage algorithm of same current capacity, but much kinder. gentler, with less gassing, and corrosion of the plates.

Every telephone switching center, cell site, data center, and all other communication systems uses Float algorithm chargers. You might be thinking of a Trickle Charger which is a constant voltage algorithm but limited to C/20 or less charge current.

Anyway to the OP this sounds more like an equipment problem in which only replacement will fix. However before doing that I have one suspicion worth looking into. You mentioned with everything turned on you draw 600 watts. At 12 volts that is one heck of a large load drawing some 50 amps. Try this. Measure the voltage when there is no load at say your headlights. Say 13.2 volts. Then measure again with everything on. If you see the voltage drop significantly would indicated 1 of a few things. Undersized wire or poor connections. If you see low voltages at the load devices, check directly on the output of the converter, If it still reads 13 volts, but load devices drop say more than 1 volt which IMO is too much, you have either undersized wiring, poor connections and/or both. In Low voltage system a good design goal is no more than 2% voltage drop at full load. On 12 volts that is only .25 volts max.


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## Tesseract (Sep 27, 2008)

Sunking said:


> That is not accurate. For Pb batteries there is no better charger algorithm than Float chargers...


Okay, yes, I guess I meant trickle charger, which is basically an extremely current-limited float charger. Otherwise my post stands.


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## Sunking (Aug 10, 2009)

Tesseract said:


> Okay, yes, I guess I meant trickle charger, which is basically an extremely current-limited float charger. Otherwise my post stands.


 No problem that is why I mentioned Trickle Charger. I have no problem wit the rest of your reply.


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## dtbaker (Jan 5, 2008)

Sunking said:


> Anyway to the OP this sounds more like an equipment problem in which only replacement will fix. However before doing that I have one suspicion worth looking into. You mentioned with everything turned on you draw 600 watts.


LESS than 600 watt max rating of the dc-dc. probably closer to 300 with high beams, radio, brake, and vacuum pump all on. There can be a spike when I flip on headlights, or the vacuum pump comes on. I'd guess the average 12v load is no more than 150watts at night.



Sunking said:


> In Low voltage system a good design goal is no more than 2% voltage drop at full load. On 12 volts that is only .25 volts max.



I don't see too much voltage drop when I flip on loads, but I do see wide swings in voltage from one time I drive to the next. even with NO load it might be 11.7 one day, and 12.8 the next, and 13.1, and back to 12.2.... with unchanging load.


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## JoeG (Jul 18, 2010)

I have been using a Meanwell HLG power supply for 2 years now to charge my 12V aux battery. They will run off of AC or DC, are PFC, are current and voltage regulated, and sealed and potted, for outdoor use. It sounds like the 15V unit would be what you need, it adjusts from 12.7-15.8V
Here is a link to the product PDF
http://www.meanwell.com/webapp/product/search.aspx?prod=HLG-600H
Check online here for prices
http://www.onlinecomponents.com/keywordsearch.aspx?text=Hlg-600h-15
Joe


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## dtbaker (Jan 5, 2008)

JoeG said:


> I have been using a Meanwell HLG power supply for 2 years now to charge my 12V aux battery. They will run off of AC or DC, are PFC, are current and voltage regulated, and sealed and potted, for outdoor use. It sounds like the 15V unit would be what you need, it adjusts from 12.7-15.8V
> Here is a link to the product PDF
> http://www.meanwell.com/webapp/product/search.aspx?prod=HLG-600H
> Check online here for prices
> ...



this looks like a nice unit! The 15v model looks perfect, and at a good price! .... settable to 13.5v with a little battery in parallel to keep onboard 12v lights, clock, and radio up when car is off.... all components should be happy.

I am contemplating changing my design slightly such that the dc-dc is on with key-on, and the battery would keep the 12v alive when key-off. This should reduce hours of use for the dc-dc and make it last longer.

nice find!


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