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DC/DC converter

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2PAC Mafia

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Hi,

I had a problem on a yacht, the remote control system failed so I disconnected a station which I thought could be the cause of the problem. After that the problem disappeared so I removed the control lever to repair it.

Inside I found a DC/DC from RECOM R05A05:

https://www.alldatasheet.es/datasheet-pdf/pdf/35378/RECOM/R05A05.html

As the problem on board was only happening sometimes I decided to replace this component and test, my experience showed me some of this intermittent failures comes from DC/DC.

I choosed this one, Murata NMH0505DC:

https://docs-europe.electrocomponents.com/webdocs/0b73/0900766b80b73e57.pdf

So my doubt comes when I was testing the component, without load any of them gives 5V, one gives 6,4V and the other 8V. I connected 100 ohm resistor as a load and they give 5,3V and 5,45V. Are they manufactured for a specific load to have at its output the exact +5V and -5V?

The original one is for +100mA and-100mA output and the one I bought is for +200mA and -200mA output so I´m not sure if I should choose the same +100mA and -100mA.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
The Murata that you loinked to, seems to suggest that you need the unit loaded 10% Max at all times, but it does not explicitly say that. It does say to meet specs, it has to loaded 10% which is essentially the same thing.

RECCOM kinda does too, but you have to look at the graphs.
 

Pommie

Well-Known Member
Most Helpful Member
if you add a 5.1V zener in series with a 10ohm resistor to the output then it will load it 10mA at 5.2V and should keep it within an acceptable voltage range.

Mike.
 

2PAC Mafia

Member
OK, but I usually tested these kind of DC/DC without any load and they kept the voltage output properly.

The Murata that you loinked to, seems to suggest that you need the unit loaded 10% Max at all times, but it does not explicitly say that. It does say to meet specs, it has to loaded 10% which is essentially the same thing.

RECCOM kinda does too, but you have to look at the graphs.

In this case the one I bought is higher wattage so may be I should buy same wattage, 1W. The Murata one is 2W so to keep the same relationship I should but the Murata 1W.

if you add a 5.1V zener in series with a 10ohm resistor to the output then it will load it 10mA at 5.2V and should keep it within an acceptable voltage range.

Mike.

Should I put resistor and zener 1 W type? Or just buy the same DC/DC wattage which is calculated for this load?
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
[quote-th OP]So my doubt comes when I was testing the component, without load any of them gives 5V, one gives 6,4V and the other 8V. I connected 100 ohm resistor as a load and they give 5,3V and 5,45V. Are they manufactured for a specific load to have at its output the exact +5V and -5V?[/quote]

This kinda refutes what you just said?

I agree with Pommie. You can use 10 (you found 8) as the worst possible case: P>= (10-5)^2/10 ohms for the resistor.
 

Pommie

Well-Known Member
Most Helpful Member
The idea with the resistor and zener is to draw 10mA whenever the voltage starts to rise. Once the voltage gets to 5.2V then 0.1V will be across the 10 ohm resistor and 10mA will flow thus keeping the circuit loaded. The power rating therefore needs to be 5.1 * 0.010 = 0.05W for the zener and 0.1 * 0.010 = 0.001W for the resistor. Use something like 1/4W should be more than adequate.

Mike.
 
Last edited:

dr pepper

Well-Known Member
Most Helpful Member
Some are yes, I've a few old dc dc blocks which are unregulated, therefore the voltage goes up and down.
If its regulated the o/p voltage should be fixed, allthough some smps's do expect a min load, you'd have to check the datasheet, or stick a resistor or lamp on the o/p that gives a bit of a load, beware a lamp has a high inrush current, which sometimes is usefull.
 

Flyback

Well-Known Member
Are they manufactured for a specific load to have at its output the exact +5V and -5V?
some smps, on no load, give well above their nominal loaded voltage output...this is because they switch continuously, and due to something called "leading edge blanking" in the pwm controller, the power fet (switch) stays on for a short period each switching cycle even if their is no load, and as a result, you get the rising output, because some energy gets transferred to the output every switching cycle even if their I no load to use the energy..so the output cap gets charged up...but this doesn't happen with all of them.
 
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