Does a cheap DC-DC converter have significant parasitic draw?

[MJSfoto1956]

Watching every Amp, so I have this question:

• Does your typical cheap Chinese DC-DC converter have any significant parasitic draw even when nothing on the DC side is active?

I'm talking about this kind of cheap ubiquitous converter seen everywhere:

M

 

[spinningmagnets]

This is a good question, and I will also be looking for the answer. however, have you considered adding a switch to the input, so you can have the DC/DC converter off when it is not in use?

 

[billvon]

Does your typical cheap Chinese DC-DC converter have any significant parasitic draw even when nothing on the DC side is active?

It is entirely dependent on design. It can be microwatts; it can be watts.

 

[MJSfoto1956]

This is a good question, and I will also be looking for the answer. however, have you considered adding a switch to the input, so you can have the DC/DC converter off when it is not in use?

You read my mind. I'm considering a relay.
The use-case is to power extra-loud horns -- I'll need 6A for a pair of PIAA dual horns.

M

 

[MJSfoto1956]

It is entirely dependent on design. It can be microwatts; it can be watts.

That's the trouble -- with Chinese stuff there is no documentation.

M

 

[billvon]

That's the trouble -- with Chinese stuff there is no documentation.

Yep. Easiest way without documentation is to just measure it. If you can get a picture of the board and determine what controller IC they are using that will also tell you.

 

[MJSfoto1956]

If you can get a picture of the board and determine what controller IC they are using that will also tell you.

Well I sure as hell don't know what I'm looking at.
Apparently it is this Fairchild chip: http://www.mouser.com/ds/2/149/KA3842B-1010724.pdf







M

 

[billvon]

Apparently it is this Fairchild chip: http://www.mouser.com/ds/2/149/KA3842B-1010724.pdf

Which is the grandchild of the venerable Unitrode UCC1842, one of the first current mode PWM IC's.

From that data sheet, minimum current is going to be about 14ma. Driving that FET will add about another 10-20ma. So I'd guess about 30ma total no-load draw.

 

[MJSfoto1956]

From that data sheet, minimum current is going to be about 14ma. Driving that FET will add about another 10-20ma. So I'd guess about 30ma total no-load draw.

so 30mA * 72V = 2W ? (basically nothing)

M

 

[billvon]

so 30mA * 72V = 2W ? (basically nothing)

That would be a decent SWAG (subject to testing, of course!)

 

[MJSfoto1956]

That would be a decent SWAG (subject to testing, of course!)

...and what would SWAG mean in this context?

M

 

[amberwolf]

Scientific Wild Ass Guess.

The use-case is to power extra-loud horns -- I'll need 6A for a pair of PIAA dual horns.

If those are like car horns, their initial draw is a much larger spike than their noisemaking draw. It's short, but it always shutdown the various converters I tried, that would've easily supported their continuous draw. I ended up always using a battery to run the horns on CrazyBike2, and used that for the car headlight too since it was there. All the rest of the lighting ran off a converter.

So if you have trouble with them, you may have to get a much larger converter, or add a lot of big capacitors in parallel wiht the converter output, to support that initial spike. (I never tried teh capacitors with mine, as I'd rather use that amount of space for battery).