# Is a diode necessary on DC DC output



## davidmillin (Dec 14, 2013)

Hi Everyone

A fellow EV builder told me that it was necessary to install a diode on the positive output of my DC DC converter and I am wondering whether this is the case I haven't seen it mentioned on any threads in these forums and I am wondering whether it is the source of my DC DC cutting out. I observe a 0.7V forward voltage drop between each side of the diode once I have the converter running and I am wondering whether I should remove it so the DC DC doesn't need to work as hard.

Thanks in advance
David


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## z_power (Dec 17, 2011)

I used automotive relay rated at 70A, the reason is to prevent battery drain when dcdc is off (some models can sink a few mA from battery when not working).


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## davidmillin (Dec 14, 2013)

Thanks for that. what about a fuse? do I need a fuse on the output?


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## Caps18 (Jun 8, 2008)

What I have learned is in my "entire wiring diagram" thread. I'm not sure what happens without it.


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## z_power (Dec 17, 2011)

davidmillin said:


> Thanks for that. what about a fuse? do I need a fuse on the output?


Fuse is always a good idea; I managed to connect my dcdc output in reverse and without protection there would be some smoke left during first start. Thankfully 80A "mega" fuse did its job


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## EVfun (Mar 14, 2010)

You can check and see if you need a diode on the output of your DC>DC pretty easily. You should have a fuse on the output of your DC to DC and if it is external that can be an easy place to do for this test. What you want to do is disconnect the traction pack input from the DC>DC and insert a low value resistor in the output of the DC>DC before the vehicle's 12 volt battery. If the DC>DC draws power from the 12 volt battery when disconnected from the traction pack you will read a voltage across the resistor. If you read 0.1 volts across a 10 ohm resistor that would be 10 milliamps of current (voltage equals amps time ohms.)


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## davidmillin (Dec 14, 2013)

Hi EV Fun



> You can check and see if you need a diode on the output of your DC>DC pretty easily. You should have a fuse on the output of your DC to DC and if it is external that can be an easy place to do for this test. What you want to do is disconnect the traction pack input from the DC>DC and insert a low value resistor in the output of the DC>DC before the vehicle's 12 volt battery. If the DC>DC draws power from the 12 volt battery when disconnected from the traction pack you will read a voltage across the resistor. If you read 0.1 volts across a 10 ohm resistor that would be 10 milliamps of current (voltage equals amps time ohms.)


I hooked up the DC DC converter as suggested using a flashlight bulb 4W about 8 ohm as in the circuit below as i didn't have a resistor and the bulb came on brightly does this mean I definitely need a diode or does it suggest a fault with the converter?


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## EVfun (Mar 14, 2010)

Provided the input power to the DC>DC was disconnected...

You certainly need the diode (or relay) if you ever plan to have the traction pack disconnected from the DC>DC converter without disconnecting the 12 volt battery. That back flow of power suggests that the DC>DC will pull at least 300 milliamps from the battery if it isn't powered. It could suggest a problem with the converter, but it could be normal too. 

The relay trick could work for you to eliminate the diode drop. You would tie the coil to the ignition circuit (pretend it is the ignition coil) so the contacts would only connect the output of the DC to DC to the rest of your 12 volt system when the car is on. I would be inclined to use the relay because if you choose a diode it would be rated for more than the maximum current output of your DC>DC and that diode will need a heat sink.

Here is how I could choose between the diode and the relay. What voltage does the DC>DC put out when it isn't connected to anything? If that voltage is <13.8 volts the DC>DC will not be able to charge the 12 volt battery fully with a diode drop in the way. If the DC>DC puts out more than 14.8 volts you will need the relay because you need to disconnect the charger when the battery isn't in use to prevent overcharging. If you choose to float the 12 volt battery (leave it charging all the time) the charge voltage needs to be between 13.2 and 13.8 volts.


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## davidmillin (Dec 14, 2013)

> Provided the input power to the DC>DC was disconnected...
> 
> You certainly need the diode (or relay) if you ever plan to have the traction pack disconnected from the DC>DC converter without disconnecting the 12 volt battery. That back flow of power suggests that the DC>DC will pull at least 300 milliamps from the battery if it isn't powered. It could suggest a problem with the converter, but it could be normal too.
> 
> ...


 Yeah the input was disconnected when I tested. I really appreciate everyone's input on this. After some more testing I have removed the diode and will install a relay on the output. It seems to be working perfectly now with the diode removed. The DC DC converter has variable output (from about 10V to 17V) so in theory either a diode or relay should work but as the converter was playing up with a diode and it sounds like it needs heatsinking I will go for a relay. Also I had intended to have a dashboard switch to operate the converter but I have removed the extra complexity of another relay rated for pack voltage and precharge for the converter as I can just use my secondary contactor switched by key accessory position to do that job. Key accessory position will also simultaneously switch the output relay on. Thanks for everyone's help.

David


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## dougingraham (Jul 26, 2011)

The only reason to place a diode in the output of the DC-DC converter is to prevent leakage back into the converter when it is turned off. You need to verify that there is leakage before you think about the addition of a diode. Make sure your buffer battery is fully charged and that all vehicle systems are turned off. Place an ammeter in the line where you would insert the diode. Read the current. Divide the buffer battery capacity by the measured current and decide if you can live with that many hours.

A diode of sufficient capability could still have enough leakage to be unacceptable. And as pointed out it will probably need a heat sink of some kind. It also lowers the output voltage between 0.7 and 1.4 volts depending on load. I am using a Vicor as my DC-DC and it has some leakage but I can tolerate it because I am not using a lead acid battery as my buffer. I plan to be one of the first to buy Tesseract's DC-DC when he gets it done. Leakage is just one reason why I would replace it.


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## charlespe (Jul 19, 2014)

davidmillin said:


> Thanks for that. what about a fuse? do I need a fuse on the output?


Using the fuse is always providing you the benefit , otherwise you could fry it


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