# Need circuit design help - DC-Dc converter / voltage divider / scaler



## jtgreeson (Aug 14, 2009)

I would like to build a DC-DC converter/voltage divider/scaler circuit that will take the voltage from my traction pack and accurately divide it by 12 with a variable load. The output should vary proportional to 1/12 to input voltage (output around 14 volts with an input of about 160 volts) and the output load current will vary between 30 and 60 microamps. (.03-.06 amps) Is there an IC or circuit that will do this? My experience with electronics is not zero but limited. I've done some reading and research but have not found the solution. A "resistive divider" circuit would be unusable due to the variable load. I plan to us this to power my EV Display that uses the input to estimate total pack voltage. Thanks for any help!


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## samborambo (Aug 27, 2008)

Strange request. What for?


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## Inbred_Weasel (Aug 6, 2010)

You could use your simple voltage divider, and hook the output up to a voltage follower made out of an op-amp. That way the op-amp will draw almost no current from the voltage divider because of its high input impedance, yet the op-amp can also deliver variable current to your sensor. 

Check out http://en.wikipedia.org/wiki/Buffer_amplifier for some more ideas on how to buffer the output of your voltage divider.


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## jtgreeson (Aug 14, 2009)

Thanks Mr. Weasel, great idea! I was not familiar with unity gain buffer, voltage follower or op amps. After a brief research I now think a properly designed voltage divider made from two resistors fed into an op-amp (maybe a LM641?) would allow me to Power my EV Display which has a varying power requirements.

To reply to Mr. SamboRambo: I've intalled Dimitri's EV Display ( http://minibms.mybigcommerce.com/products/EV-Display.html ) into my conversion. It draws 30 to 60 microamps. It is designed to be powered from 4 of the pack cells (about 14 volts) and multiplies that voltage by the user supplied number to calculate and display the total pack voltage. I know, a little screwy but Dimitri had good reason to do it. If I can design a "Scaler" that reduces the pack voltage by 12 (I have 48 cells) I can get accurate pack voltage display and keep my pack from discharging unequally due to the constant small draw from this EV Display.


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## zwmaster (Nov 23, 2009)

Hello.

You could use some current source and measurement resistor instead of standard voltage divider. Voltage follower is still needed like described before (not in some cases where ADC input impedance is very high or measuring resistor is very low value).
In any case USE stable resistors for voltage divider (TK50 (50ppm/degC) or less).


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## Tesseract (Sep 27, 2008)

jtgreeson said:


> ...
> To reply to Mr. SamboRambo: I've intalled Dimitri's EV Display ( http://minibms.mybigcommerce.com/products/EV-Display.html ) into my conversion. It draws 30 to 60 microamps. It is designed to be powered from 4 of the pack cells (about 14 volts) and multiplies that voltage by the user supplied number to calculate and display the total pack voltage....


It actually draws 30-60mA (that's milliamps, not microamps). Is there a reason why you aren't going to power the display from the bottom-most (closest to the negative terminal) four LFP cells (perhaps because you won't be top-balancing the pack?)?

The reason I ask is because while the resistor divider feeding a buffer is a perfectly reasonable solution at 30-60uA, it is not so attractive at 30-60mA. If you need to drop ~160V down to ~12V and supply up to 60mA that works out to (160-12)*0.06 = 8.88W of power. Much of the dissipation in the op-amp can be avoided by supplying it with a voltage only slightly higher than the maximum it is expected to see on it's input - that would best be accomplished with, eg., a 12V to 15V dc-dc converter powered by the vehicle's 12V battery. You still need to supply the operating current for the display through the op-amp buffer, though, and requiring 60mA of output eliminates about 90% of the parts choices. The LT1010 (Linear Technology) or BUF634 (TI) come immediately to my mind.

I know this is a design decision Dimitri struggled with when developing the product. I personally wish he would have used an isolated dc-dc converter module inside the display to derive power from the 12V system and then directly read the pack voltage through a plain old resistor divider; he chose to skip the dc-dc converter (IIRC, he was running out of space) and go with this method instead. Don't want to put too many words into his mouth, especially since these were off-the-cuff discussions between us from several months ago.


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## dimitri (May 16, 2008)

Jim,

I put quite an effort to resolve this issue, yet keep the product simple and affordable, but I could not find an acceptable solution. Every approach I tried had significant drawbacks. I am not alone, most comparable products on the market have the same issue, their voltage scaling is either very limited or non-existent, some offer multiple models, each for specific voltage range, but I can't accept this approach, its not cost effective for me. 

Also, have you even considered the benefits? I ran my own EV Display in my EV powered from 4 cells for couple of months, then I switched it to 12V aux and still have it this way. I see absolutely no benefit of monitoring actual voltage of LFP pack, its the least useful bit of data in EV with LFP pack ( assuming there is BMS or some other protection mechanism ensuring the pack is within operating voltage window, such as controller's LVC cutoff, etc ).

I do not recommend to power display from first 4 cells in the pack. Long term imbalance is significant enough to cause a concern. Its possible to setup top balancing BMS to correct this imbalance, but not all chargers can be fine tuned to make it work properly long term. Plus it would waste energy for nothing.

Consider how small the voltage swing in your pack as you drive. There is virtually no benefit in precision tracking of this voltage swing. If you power display from 12V aux and setup cell count to show correct nominal pack voltage, your kWh and kW reading will be within 7% error margin. Do you really need more accurate reading than this?

If you find a good solution, please share.


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## EVfun (Mar 14, 2010)

30 milliamps is 5 amp hours per week. If the car is not driven and charged every day it will require cell regulators that can bypass quite a bit of power. The TS lithium cells don't have enough voltage rise to fire off any regs until they are very close to fully charged. 

My balance time is just the charger constant voltage time setting for the most part. In my case that is set for 1 hour after 144 volts (3.6 vpc) so I can only balance about 1/2 amp hour per charge.


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## jtgreeson (Aug 14, 2009)

Tesseract: You are so correct, my typo - it is 30 milliamps. Thanks for the other suggestions.

Dimitri: Thanks for your comments. I still agree with you that the pack voltage display has limited use while driving. My motivation to have the EV Display powered by the whole pack was:

1. Have an easy way to monitor the total pack voltage during charging. (not while driving)

2. Prevent the EV Display from discharging my small (18Ah) 12volt battery.

EVfun: That 5 Ah is exactly what I am concerned about. The constant drain on either the 4 pack cells or the 12v aux. is an issue.

I think Dimitri's advice is what I will go with initially and and power the EV Display from my 12v bat. It will be a little inconvenient to make sure to charge the 12v once a week or so. My Zilla controller pulls 70 milliamps at rest and I had to rewire so it wouldn't be "on" all the time. It ran my 12v dead in three days. Not an option with the EV Display since it has to be on all the time.

Dimitri: Since the "total pack voltage" display is not that useful, would it be OK to just power the display from the total pack using a DC-DC converter? Would that mess up any of the EV Display's functions other that total pack voltage? That solution would at least solve my problem #2 and eliminate having to remember to charge the 12v if the car sits for a while.

I've been unable to spend time on the conversion for a month do to traveling but needed to figure this out first anyway. I'm getting back on it in two weeks.

Thanks for everyone's help!


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## dimitri (May 16, 2008)

jtgreeson said:


> 2. Prevent the EV Display from discharging my small (18Ah) 12volt battery.
> 
> EVfun: That 5 Ah is exactly what I am concerned about. The constant drain on either the 4 pack cells or the 12v aux. is an issue.
> 
> ...


I have even smaller 10AH aux battery and I had exact same issues with it even before I made EV Display. I even started a poll thread to see how many people hardwire DC-DC to be always on, and found that many do and it works well. So, for past 3 months or so, I had my DC-DC always on, keeping aux battery always full, and it works great.


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## EVfun (Mar 14, 2010)

I'm going to second Dimitri here. I have always wired my EVs so the DC to DC is always on. I generally use a 16 t0 20 amp hour battery, the same common little size AGM that was used in the Zappy Scooters.


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## jtgreeson (Aug 14, 2009)

Any negatives to having the DC-DC on all the time?


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## JRP3 (Mar 7, 2008)

Potentially you could drain your main pack if you left the vehicle uncharged for a very long time. Other than that I don't see any problems, I leave mine on all the time.


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