# DC DC fun



## davidmillin (Dec 14, 2013)

Hi everyone I have a 350 watt meanwell SD350D 12 cheap and cheerful dc dc converter and are currently installing it. I am using a 60watt light bulb as a precharge resistor and a 3 position switch to operate it. Position 1 off, Position 2 on via the light bulb, and Position 3 directly on. The trouble is I can only operate it by turning the voltage output adjust pot down and then turning it back up to 13.8 volts once it is running. Without the voltage output turned down the lightbulb fails to dim and input voltage measured at the terminals only gets to 61 volts (mine is a 120 volt system) and the converter cuts out for about 30 seconds when I move the switch to position 3. Is my precharge too much? any insight gratefully received.

Thanks David


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## GerhardRP (Nov 17, 2009)

davidmillin said:


> Hi everyone I have a 350 watt meanwell SD350D 12 cheap and cheerful dc dc converter and are currently installing it. I am using a 60watt light bulb as a precharge resistor and a 3 position switch to operate it. Position 1 off, Position 2 on via the light bulb, and Position 3 directly on. The trouble is I can only operate it by turning the voltage output adjust pot down and then turning it back up to 13.8 volts once it is running. Without the voltage output turned down the lightbulb fails to dim and input voltage measured at the terminals only gets to 61 volts (mine is a 120 volt system) and the converter cuts out for about 30 seconds when I move the switch to position 3. Is my precharge too much? any insight gratefully received.
> 
> Thanks David


I know nothing about these systems, but will comment anyway. Is your switch make before break on 2-3 transition?


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## davidmillin (Dec 14, 2013)

Hi Gerhard



> Is your switch make before break on 2-3 transition?


In fact the light bulb stays connected on 3 but another current path is available bypassing the bulb in the same way a precharge resistor across the main contactor stays in place once the contactor has closed.


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## GerhardRP (Nov 17, 2009)

davidmillin said:


> Hi Gerhard
> 
> 
> 
> In fact the light bulb stays connected on 3 but another current path is available bypassing the bulb in the same way a precharge resistor across the main contactor stays in place once the contactor has closed.


Again speaking from total ignorance, what is it that you are trying to protect with a precharge sequence? The traction battery has enough capability...is it just the connecting contactor? The manuals for this device make no mention of precharge.
G.


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## davidmillin (Dec 14, 2013)

> Again speaking from total ignorance, what is it that you are trying to protect with a precharge sequence? The traction battery has enough capability...is it just the connecting contactor? The manuals for this device make no mention of precharge.
> G.


The contactor and the converter itself. Maybe it is unnecessary but I thought it was a simple cheap circuit that could protect my components.also I do not have a suitable relay at this stage so by using precharge resistance I could get away with using a switch.


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## davidmillin (Dec 14, 2013)

I have decided to do away with the switch and precharge and let my secondary contactor do the job off switching the dc dc converter on and off.
The contactor will certainly stand up to the inrush current and hopefully the dc dc converter will still last well. The cutting out seems to have been resolved by removing an output diode. it is discussed in this post http://www.diyelectriccar.com/forums/showthread.php?p=390667#post390667

Cheers
David


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## dougingraham (Jul 26, 2011)

davidmillin said:


> Without the voltage output turned down the lightbulb fails to dim and input voltage measured at the terminals only gets to 61 volts (mine is a 120 volt system) and the converter cuts out for about 30 seconds when I move the switch to position 3.


If you turn down the output voltage you are lowering the load on the input side. You may have too much load on the output which is what keeps the input from comping up to full voltage. A 60 watt bulb would have a resistance of around 52 ohms when up to full temperature. It is probably a bit lower than that because it is not up to full temperature yet. Lets assume it is 52 ohms and with a 59 volt drop (120 volts minus 61 volts) that means 1.13 amps load on the input. At an assumed 90% efficiency this means that the output is pulling about 5 amps and the DC-DC itself is using about 100 ma.

The precharge resistance is too high. Switch to a higher wattage light bulb or disconnect the DC-DC output during the precharge.

Are you certain you need to precharge this device? Do you experience an alarming spark when you connect it to the traction battery without the light bulb in the line?


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## EVfun (Mar 14, 2010)

... or another idea if you have already added the output relay to the DC to DC converter in place of a diode. 

The problem is that the DC to DC is presenting a load to the traction pack as soon as it's powered up by the secondary contactor closing. When you turn DC>DC voltage down the 12 volt battery takes all the load because the target output voltage of the DC>DC ends up less than your 12v battery. If you parallel the coil of the relay that connects the DC>DC to the 12v battery with the coil of the main contactor then the DC>DC won't have a load on it until the main contactor closes. Your existing precharge system will precharge both the Curtis 1221 and your DC>DC.


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## davidmillin (Dec 14, 2013)

> ... or another idea if you have already added the output relay to the DC to DC converter in place of a diode.
> 
> The problem is that the DC to DC is presenting a load to the traction pack as soon as it's powered up by the secondary contactor closing. When you turn DC>DC voltage down the 12 volt battery takes all the load because the target output voltage of the DC>DC ends up less than your 12v battery. If you parallel the coil of the relay that connects the DC>DC to the 12v battery with the coil of the main contactor then the DC>DC won't have a load on it until the main contactor closes. Your existing precharge system will precharge both the Curtis 1221 and your DC>DC.


Good idea it saves the dc dc starting under load. My main contactor is being switched on and off by the potbox microswitch though so I will connect it to the main contactor coil input just prior to the potbox as shown below.

Thanks
David


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## davidmillin (Dec 14, 2013)

> Are you certain you need to precharge this device? Do you experience an alarming spark when you connect it to the traction battery without the light bulb in the line?


Probably not. The inrush current is listed as 45A dropping down to 6A. While the spark is slightly alarming when I touch the wire connected to my pack to the load I think an EV200 contactor won't have too much trouble. It seems that most people don't bother with precharge for their converters. Please let me know if you think precharge is necessary at the above mentioned figures.

Thanks
David


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## dougingraham (Jul 26, 2011)

I don't know if a 45 A inrush will cause enough damage to the contacts in an EV200 to be an issue. I would expect eventual pitting of the contact faces leading to an increase in resistance. At some point the contacts could weld together preventing them from opening.

But you could pretty easily move the DC-DC feed to the controller side of the main contactor and let the controller precharge do the work.


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## davidmillin (Dec 14, 2013)

> I don't know if a 45 A inrush will cause enough damage to the contacts in an EV200 to be an issue. I would expect eventual pitting of the contact faces leading to an increase in resistance. At some point the contacts could weld together preventing them from opening.
> 
> But you could pretty easily move the DC-DC feed to the controller side of the main contactor and let the controller precharge do the work.


I thought his was something to be avoided so the dc dc wasn't robbing precharge from the controller. I have come across other threads where people recommend installing the feed to the dc dc on the battery side of the main contactor.


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## dougingraham (Jul 26, 2011)

davidmillin said:


> I thought his was something to be avoided so the dc dc wasn't robbing precharge from the controller. I have come across other threads where people recommend installing the feed to the dc dc on the battery side of the main contactor.


There is no perfect answer because the device you are using as a DC-DC was not designed for this role. There are downsides no matter what you choose to do.

I think I would try to use the controller precharge. It won't hurt the controller, the worst that would happen is it will fault because the voltage never comes high enough for it to decide to close the main contactor.


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## EVfun (Mar 14, 2010)

dougingraham said:


> There is no perfect answer because the device you are using as a DC-DC was not designed for this role. There are downsides no matter what you choose to do.
> 
> I think I would try to use the controller precharge. It won't hurt the controller, the worst that would happen is it will fault because the voltage never comes high enough for it to decide to close the main contactor.


I think to use the controller precharge he would have to connect the DC>DC positive traction pack input after the main contactor, then not cycle the contactor with the release of the throttle or cycle the DC>DC output relay with the main contactor. Perhaps instead he could run an inrush limiter on the positive DC>DC input.

I never cycled the main contactor when I used a Curtis 1221, but that is recommended and I did have a runaway event.


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## davidmillin (Dec 14, 2013)

Thanks Doug and EV fun for continuing to come up with solutions. I am intrigued by inrush current limiters but don't know a lot about them. As I understand it an inrush limiter is a thermistor that is a resistor whose resistance changes as current flows through it and it heats up and placing one of these say a 9amp 10 ohm one in my DC DC input line will limit inrush current to about 12amps for a 120volt supply. I can buy one for $2.13. Sounds like a perfect solution. What are the drawbacks?


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## EVfun (Mar 14, 2010)

The peak input current draw of the DC>DC must be less than the rating of the inrush limiter. Inrush limiters run hot -- you should keep the leads as long as practical and keep them away from other parts. They have some resistance even when hot so they will cause a small voltage drop on the DC>DC input. I just pulled one number up at Mouser that looks, at first glance, like it may work. Ametherm SL22 30005 has a 5 amp rating, a cold resistance of 30 ohms, a hot resistance as low as 0.4 ohms, and costs less than $2 each. 

I have used these to soft-start headlights so my small DC>DC didn't complain. I used a relay to short them out after their resistance drops to a low value. That is easy to do, you put one end of the relay coil between the inrush limiter and the load and the other end of the relay coil to ground. So long as most of the voltage drop is across the inrush limiter the relay won't turn on. When most of the voltage drop is across the intended load the relay sees 12 volts and pulls in, which causes the relay contacts to short out the inrush limiter (thanks to Lee Hart for that idea.) I used either the Amphenol CL-21 or CL-30. I may have to resort to that method again.


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## davidmillin (Dec 14, 2013)

> The peak input current draw of the DC>DC must be less than the rating of the inrush limiter. Inrush limiters run hot -- you should keep the leads as long as practical and keep them away from other parts. They have some resistance even when hot so they will cause a small voltage drop on the DC>DC input. I just pulled one number up at Mouser that looks, at first glance, like it may work. Ametherm SL22 30005 has a 5 amp rating, a cold resistance of 30 ohms, a hot resistance as low as 0.4 ohms, and costs less than $2 each.
> 
> I have used these to soft-start headlights so my small DC>DC didn't complain. I used a relay to short them out after their resistance drops to a low value. That is easy to do, you put one end of the relay coil between the inrush limiter and the load and the other end of the relay coil to ground. So long as most of the voltage drop is across the inrush limiter the relay won't turn on. When most of the voltage drop is across the intended load the relay sees 12 volts and pulls in, which causes the relay contacts to short out the inrush limiter (thanks to Lee Hart for that idea.) I used either the Amphenol CL-21 or CL-30. I may have to resort to that method again.


I really like this idea am I correct in thinking that if I use a relay that switches after the voltage drop is gone to short across the limiter that heat and the requirement for long leads becomes less of an issue as the limiter is only active for a few seconds?

Thanks
David


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## dougingraham (Jul 26, 2011)

davidmillin said:


> I really like this idea am I correct in thinking that if I use a relay that switches after the voltage drop is gone to short across the limiter that heat and the requirement for long leads becomes less of an issue as the limiter is only active for a few seconds?


This is called a precharge circuit. And if you are including a relay to short across "the limiter" you probably don't need to use an expensive device that changes resistance with temperature, you can just use a resistor of the appropriate value.

The negative temperature coefficient devices like you are talking about here will limit the inrush current but in normal operation they get hot. One problem with them is that it takes a while for them to cool off and they need to be cool to perform their limiting function. So if you turn on the device the limiter will get hot and if you turn it off and then back on before it can cool it doesn't do its job the second time.

Another option exists and some people have added them to their DC-DC converters to prevent the converter from blowing up. This solution is to place an inductor in the input line to the DC-DC. The problem is that when the motor controller turns on the batteries sag and when the motor controller turns off the batteries bounce back up. This typically happens at 8 khz to perhaps as high as 20khz rate. When the DC-DC is connected to the battery the input filter caps will see an inrush when the controller turns off the current to the motor and then when the motor controller turns on the current to the motor the DC-DC input caps will heroically try to keep the battery voltage from sagging by having an equivalent outrush of current. AC power supplies don't have the outrush current because the bridge rectifiers that convert the AC to DC block the outflow current. But the caps discharge some because the converter is taking the energy out of them during the battery sag. An appropriately sized inductor will limit the inrush current and decrease the sag the input filter caps see. The down side is a large increase in output current from the converter will deplete the charge on the input filter caps and starve the converters output. Likewise a sudden decrease in current demand from the converter will see a spike in voltage across the input caps. So you don't want too large of an inductor and too small will do nothing. I think Evnetics has a recommendation for an inductor in their FAQ or maybe it is in the Soliton manual. I would go with their recommendation as all the different factors have most likely been considered.

None of these suggestions are more than bandaids because the DC-DC device itself is flawed for use in this application. It should handle its own inrush current limiting at power up.


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## hbthink (Dec 21, 2010)

I have used both Meanwells and TDKs as DC/DCs for my vehicles. I always used a diode on the output to prevent backfeeding the dc from aux battery. I also had limiting toroid on input from main pack battery to reduce the ripple. In both cased the dc/dc did not last very long and stopped working after 1 year. I think my brake vacuum pump did them both in. I now use a TDK unit from the Ford Transit its water cooled and sealed against the elements. I first noticed that it sags way less than both the other units. I also have a disable pin which when pulled to gnd disables its output. I use this to turn the unit off/on using a relay which is switched via my Soliton which gets its input from the ignition switch. This has proved to be very reliable solution and with the cooling system runs way cooler and with more authority than either of the standard dc/dc units I used before. Also its designed for car use and is sealed against the elements. Both the other units were quite weathered from the water and air that enters my engine compartment. Air cooling also did not work effectively.

Steve


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## EVfun (Mar 14, 2010)

davidmillin said:


> I really like this idea am I correct in thinking that if I use a relay that switches after the voltage drop is gone to short across the limiter that heat and the requirement for long leads becomes less of an issue as the limiter is only active for a few seconds?


Heat is a much smaller issue, but you need a relay that pulls in near your pack voltage. A 120 volt AC relay generally pulls in well below 120 volts DC. Since your converter presents a capacitive load any early pull in on the relay would cause an inrush and stress the relay contacts. 

With my headlights a 12 volt DC relay was common and the load is resistive. My problem is that headlights look like about 0.2 ohms cold but 1.4 ohms hot. The 60 amp inrush sometimes trips my Zilla controller off (it catches the brief sag on the 12 volt system.)


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## davidmillin (Dec 14, 2013)

This is what I think I will do as shown below. when the key is on accessory position the secondary contactor engages and the DC DC runs with the inrush limiter and no output.Then on key on position the limiter is shorted across and the output switched on. This means I can just use a 12 volt coil relay to short the limiter and don't need to deal with working out what the right relay and resistor I need to activate the relay from voltage drop. Thanks for all the information and suggestions. I am thinking I may have been better to buy a more expensive DC DC converter as by the the time I add these circuits and an enclosure to protect it the price begins to exceed that of a more purpose built one.

David


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## EVfun (Mar 14, 2010)

I don't see any problem with that setup. You will want to verify that the DC>DC is up to pack voltage before your precharge timer closes the input and output relays. You could pick a fixed resistor in place of the inrush limiter.


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