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Substituting a 50uH inductor for a 100uH inductor ?

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picbits

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I'm about to build a batch of power supplies for internal workshop use (current/voltage regulated for a bunch of stepper motors).

I've got a couple of tubes of LT1074 and a big box of brand new 100uH inductors (8 amp).

The datasheet shows that a 50uH should be used for the LT1074 and 100uH for the LT1076 but as I have the 100uH in abundance it seems a shame not to use them.

Any ideas ? I've got a fantastic datasheet/writeup from Linear Technology which basically says:

Choosing an inductor .....

Weigh it
Look at it
Measure the resistance
Toss a coin
Use it if all above within parameters :D

I'm not worried if I lose a couple of % efficiency or don't use the 100uH to its full capacity but it will sure beat trying to drop a 30V DC supply down to 2.5 volts at 4 amps in a linear fashion.
 
I'm about to build a batch of power supplies for internal workshop use (current/voltage regulated for a bunch of stepper motors).
Why bother?

If you have chopper based stepper drives it does the down conversion for you and will keep your torque up at high step rates to boot. You just set them for your desired drive current and go.

Dan
 
Indeed - parts of their "more interesting" datasheet show it with a 150uH inductor.

I'll screen grab some of their datasheets in a minute and post them up - they are written how datasheets were meant to be written. ;)
 
Why bother?

If you have chopper based stepper drives it does the down conversion for you and will keep your torque up at high step rates to boot. You just set them for your desired drive current and go.

Dan

Building my own drivers but would like current limiting on the PSU side as well just in case. The motors I'm using are rather expensive to replace if anything with the controlling side went a bit wrong.

The actual PSU is capable of 10A @ 30V and I have 10 of them.

Its all being done on a bit of a budget and I want to have a play with these LT1074s. I also want to see if I can interface them to a PIC to set the current/voltage output digitally so this is a good opportunity to try them out.
 
hi,
Providing the two inductors are not mutually coupled you can connect two 100uH in parallel to give 50uH.
Keep the phasing the same.
 
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Here are some cracking extracts from the LT datasheet
 

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hi,
Providing the two inductors are not mutually coupled you can connect two 100uH in parallel to give 50uH.
Keep the phasing the same.

I've had a read through their "less official" datasheet and it shows the LT1074 being used with a variety of inductors from 25uH to 150uH so I think a 100uH shouldn't trouble it too much.
 
I've had a read through their "less official" datasheet and it shows the LT1074 being used with a variety of inductors from 25uH to 150uH so I think a 100uH shouldn't trouble it too much.

Hi,
I have found that the range of the inductor from one 'designe'r to another does vary, without much impact on the circuit.
 
Building my own drivers but would like current limiting on the PSU side as well just in case. The motors I'm using are rather expensive to replace if anything with the controlling side went a bit wrong.

The actual PSU is capable of 10A @ 30V and I have 10 of them.

Its all being done on a bit of a budget and I want to have a play with these LT1074s. I also want to see if I can interface them to a PIC to set the current/voltage output digitally so this is a good opportunity to try them out.
How much current does the stepper motor take?
 
Each motor takes 2.8 amps at 2.5v per winding so 5.6amps total per motor (Bipolar steppers).

I've got some interesting ideas on the chopper circuit side of things for the actual motor controller but want a decent current regulation on the PSU side in case of firmware failure.
 
Each motor takes 2.8 amps at 2.5v per winding so 5.6amps total per motor (Bipolar steppers).

I've got some interesting ideas on the chopper circuit side of things for the actual motor controller but want a decent current regulation on the PSU side in case of firmware failure.
I was actually thinking hardware, but that much current is marginal from my normal preferred drive chip:

https://www.electro-tech-online.com/custompdfs/2008/12/3977.pdf

This one, on the other hand, drives external FETs, and with 4 $1 duals will easily drive 3A into 2.5V from 30V.

https://www.electro-tech-online.com/custompdfs/2008/12/3986.pdf

https://www.electro-tech-online.com/custompdfs/2008/12/FDS8949.pdf

Dan
 
Indeed - I've considered the dedicated stepper ICs and have had a play with some high side mosfet driver ICs (bootstrapping etc).

I'm more looking at making the SMPS for being able to reduce the voltage from the crude 30v 10A supplies (unregulated) to lower voltages for some of the ideas I've got for adapting the Linistepper idea - i.e. to provide linear current regulation for the steppers rather than using chopper circuits but still have the facility to use choppers within the same driver.

Bit complicated but I like the robustness of power transistors in linear mode in H Bridges so will have a play with the 100uH inductors and see if I can get some kind of hybrid bipolar driver knocked up.
 
chopper circuits use the motor's inductance as the switcher element. Linear current sources are going to be lossy and unreliable. The 30V unregulated, on the other hand, requires at least 60V on the power switches, maybe more.

Dan
 
Bottom line. Try it, see how it works. Hook up an oscilloscope if ya got one. I work with power supplies and thats what we do *most* of the time. You can't beat the old "experimentation" method.
 
Hi there,


Going from 50uH to 100uH is actually better for a number of
reasons and only bad really for one reason.

Assuming a continuous circuit design...

For one, you get one half the ripple current which usually means
one half the ripple voltage which is usually desirable. This also
makes it easier for the cap to work because the change in
current is less, which means the ripple current rating for the cap
can be lower which means the cap runs cooler.
The ripple current is easy to picture:
For a 50uH think about a ramp from say 6 amps to 10 amps
for some design, but for a 100uH inductor the ramp would go
from 7 amps to 9 amps (one half the ripple current). This
is usually a good idea and even necessary in some designs.

So with all this good news what could possibly be wrong?
The bad news (which isnt that bad really) is that the efficiency
for a given inductor design is not as good for a value 2x times
as large, because it takes 1.4x more wire and a heavier gauge
or a larger core size which also makes it bigger physically.

So in view of all that and being that you probably dont need
super high efficiency, you have nothing at all to worry about
and everything to GAIN.

There is a catch, but this probably doesnt apply to your
circuit, and that is that if the design is for a discontinuous
circuit then you have to be more careful about the value
of the inductance or else you might accidentally force the
circuit to work in a continuous mode in which case the
circuit theory is different and so the circuit probably wont
work the way you would expect it to.

As i said though, your circuit probably works in a continuous
mode so i wouldnt worry. If you check the output voltage
and it seems to regulate with a reasonable change of load
and maybe input voltage then it's probably a continuous
mode design. Most DC - DC converters are with somewhat
large dc current flowing through the inductor all the time.
You can double check the data sheet most likely and find
out that way too.

If you happen to have a circuit that depends on inductor
timing to set the operating frequency however then the
frequency will end up being lower, which still probably
wont hurt as long as the output cap is correctly sized.
 
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Hi again,


You're welcome :)
 
Linear has an excellent simulation program. Model your circuit and take a quick look at the difference between the inductors. What you see will confirm what you have read here.
 
Linear has an excellent simulation program. Model your circuit and take a quick look at the difference between the inductors. What you see will confirm what you have read here.

Funny you should post that - I was having a play with LTSpice last night and having a mess about with various values.

Turns out the 100uH seems to work fine as does a MUR420 instead of the MBR745 although the output has a few more spikes (around 0.1V Peak to Peak)
 
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