Parallel Volt Regs And Ballast Resistors

[Michael]

Hi,

I'd like to put some volt regs in parallel and the datasheet advises a
0.015R resistor in the output. Is this a requirement or can I get away
without it? If I need it, is there a 'trick of the trade' to getting the
right value as the datasheet suggests 2ft of 18ga wire which I'd rather not
use due to space constraints.

Cheers,

Michael

 

[Chuck]

Hi,

I'd like to put some volt regs in parallel and the datasheet advises a
0.015R resistor in the output. Is this a requirement or can I get away
without it? If I need it, is there a 'trick of the trade' to getting the
right value as the datasheet suggests 2ft of 18ga wire which I'd rather not
use due to space constraints.

Hello Michael,

Can't comment on the importance of that
resistor, but fractional ohm resistors
like that are available commercially if
you want to avoid the space of a
home-made resistor.

Do a Google search on "current sensing
resistor .015 ohm" and also try Ebay.
I've seen them there from time to time
fairly cheap. Obviously, not all current
sensing resistors are fractional ohm values.

Good luck.

Chuck

 

[Michael]

Hello Michael,

Can't comment on the importance of that resistor, but fractional ohm
resistors like that are available commercially if you want to avoid the
space of a home-made resistor.

Do a Google search on "current sensing resistor .015 ohm" and also try
Ebay. I've seen them there from time to time fairly cheap. Obviously, not
all current sensing resistors are fractional ohm values.

Good luck.

Chuck

Thanks Chuck, I looked at sites like Newark and they didn't turn up anything
so figured they didn't exist.

Cheers,

Michael

 

[kell]

Thanks Chuck, I looked at sites like Newark and they didn't turn up anything
so figured they didn't exist.

Cheers,

Michael- Hide quoted text -

- Show quoted text -

Go to www.digikey.com click site map, click part search, scroll down
to resistors and click current sense resistors. Check out the Ohmite
brand resistors for the best prices.

But you might not need to spend any money at all if you just want some
ballast resistance. Try using paper clips. The cheap steel in paper
clips has much higher resistance than copper does, so you can use
short pieces. Dab the end of the paper clip in some flux to make it
easier to solder (electronics flux, not plumber's flux).

About the only difference between cheapo steel versus current sense
resistors is that the steel has a big temperature coefficient, but
that's actually a good thing for current-limiting.

Another thing you could try is get a hold of some nichrome wire,
perhaps by cannabalizing a power resistor, but nichrome doesn't solder
well.

 

[Chuck]

But you might not need to spend any money at all if you just want some
ballast resistance. Try using paper clips. The cheap steel in paper
clips has much higher resistance than copper does, so you can use
short pieces. Dab the end of the paper clip in some flux to make it
easier to solder (electronics flux, not plumber's flux).

Great idea!

Michael, if your ohmmeter doesn't let
you measure down to 15 milliohms, you
can pass a current of, say 1 amp,
through a piece of clip and using a DMM,
measure the voltage drop. Most DMMs
ought to read 15 millivolts OK. This is
a four-wire resistance measurement FWIW.

Chuck

 

[Phil Allison]

"Michael"

I'd like to put some volt regs in parallel and the datasheet advises a
0.015R resistor in the output.

** Really ?

Post the link.




........ Phil

 

[Jonathan Kirwan]

I'd like to put some volt regs in parallel and the datasheet advises a
0.015R resistor in the output. Is this a requirement or can I get away
without it?

If you are going to try putting regulators in parallel, the resistors
are generally necessary. Each regulator will be trying to control its
output and doesn't expect other regulators to be doing the same thing
to the same output, where some conflict can arise. The better value
of the resistors will depend on the load and the regulators
themselves, but the resistors provide a little bit of "working room"
for each of the regulators to have some measure of control over their
own outputs, as they were designed to have, while allowing their
aggregate current handling to be summed into the load. Just wiring
their outputs together isn't wise.

What kind of current are you talking about, load-wise? What's the
regulator? (And are they just trying to sell you regulators, when a
bypass transistor around it might do a similar job for you?)

Jon

 

[Chuck]

** Really ?

Post the link.




....... Phil

I don't know what the OP was using, but
here's one link to a datasheet that
recommends the use of 0.015 ohm ballast
resistors:

javascriptpenreq('http://www.ortodoxism.ro/datasheets2/0/00hl8utol4qogo8hlqilf35r0opy.pdf')
LT1083-FIXED datasheet

Bottom of page 9.

Chuck

 

[Jasen]

But you might not need to spend any money at all if you just want some
ballast resistance. Try using paper clips. The cheap steel in paper
clips has much higher resistance than copper does, so you can use
short pieces. Dab the end of the paper clip in some flux to make it
easier to solder (electronics flux, not plumber's flux).

I've never had a problem soldering zinc plated paperclips using only
electronics solder. I use pieces of them them between D connectors to
make custom adaptors

About the only difference between cheapo steel versus current sense
resistors is that the steel has a big temperature coefficient, but
that's actually a good thing for current-limiting.

Another thing you could try is get a hold of some nichrome wire,
perhaps by cannabalizing a power resistor,

or a toaster etc...

but nichrome doesn't solder well.

also 0.015 ohms would be a very short piece.