Need Component Supplier

lardom

· Is Mouser one of the simpler companies to order components from? I haven't figured out how to order what I need from Digi-Key.
· To keep a ULN2803 from sizzling I used a Radio Shack 33 ohm 1/2 watt resistor in series with a 12V lead acid battery. The resistor got hot in a hurry.·The guy at Radio Shack recommended a cooling fan....'No fan.'· I want to keep things as simple as possible. I an willing to remove the resistor from the small enclosure I used·and solder it in-line to the battery positive to safely dissipate heat. I talked to a guy from a local TV repair shop and he immediately said 6 watts and gave me a 47ohm 3 watt ceramic resistor he had laying around in his shop. He doesn't order them anymore.
1)·Is a ceramic resistor the way to go?
2) Is soldering a resistor to battery power wire a good idea?
3) What is a good company to order from?
······················································ ·...The best ideas come from this forum...

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔

HollyMinkowski

When I was in the US I used DigiKey up until
they started charging state tax :-(

Then I switched to mouser, it was indeed simple
ordering. I like that they don't charge your card
for any parts you backorder..until they ship.
And they pay the first 10.00 or so of the shipping
when they do send out the backorder parts!

For lots of parts you should order from Tayda Electronics
in Thailand. They ship fast and register the shipments.
They accept paypal and cards. They have amazing
prices!!! i.e. board mounted pots for .10, resistors
for .01, xtals for .23, 1N4007 diodes .01, LM317T .21
etc etc.

www.taydaelectronics.com/servlet/StoreFront

mctrivia

Digikey is great once you get use to it. If you can order enough for free shipping and get something back ordered. you can get free shipping on every item you buy from then until they ship the back ordered part [noparse]:)[/noparse]

bad idea to place a hot resister near a battery. If you need a lower voltage or current consider a switching power supply to drop the voltage or limit the current. this will increase your battery life since you will not be wasting power to heat.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Lots of propeller based products in stock at affordable prices.

mctrivia

probably about the simplest switching option.




▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Lots of propeller based products in stock at affordable prices.

zoopydogsit

Hi Larry,

Hmmm the ULN2803 should be able to handle 50V, so the voltage from your 12V battery should not be an issue. Though it looks like it can only drive 500mA. So I'm wondering what is frying it. How have you placed the back transient (back EMF) diodes? If they are back to front then they'll suck as much current they can handle (1N4001 would suck 1A, so more than the ULN2803 could handle), so it may be a possibility. Alternatively what are you driving? it may be the motor is sucking as much as the battery can give it (a lot).

Could you please post a circuit diagram of what you are doing, just the driver & power side is what we need to see. ie. Battery, driver & motors. Also please measure the resistence of the motors and any useful specs on them. This will help to understand the problem and give you better recommendations.

Dave

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
It's all a function of time.

lardom

@Holly, thanks. I needed that information.

@mctrivia and Zoopydogsit, I attached pictures. I say I like to keep things simple but it's better to make it reliable. What do you suggest?

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔

mctrivia

for myself I used this preassembled driver board it is pretty inexpensive at $35 already assembled but my stepers have only 4 wires and yours have 5 so may not work.

Here is there schematic(you really only need the bottom right if doing yourself and you have a 5V source)


the main part on this board is the A3982


So that is the most reliable method, and will give you more speed then the chip you are planning.

For cheaper and simpler using the high wattage resister will be fine. You do need to make sure the resister does not exceed 180 C during standard operation or it will melt an ABS box. This is not likely.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Lots of propeller based products in stock at affordable prices.

zoopydogsit

Hey Larry,

Thinking about it, it's probably your motor is wanting to draw too much current and the ULN2803 can only handle 500ma.

You should·check your motors specs to see it's operational voltage. If it's a 3V motor and your dropping 12V
across it then it'll·suck lots of current and fry your ULN2803.

Alternatively limit the current to it. To do this you need the following equations.

R=V/I··Resistence = Voltage divided by·current

P=IV··· Power = current multiplied by voltage.

Where V = 12V battery, I is 500ma

So the resistor you would need in series with the ULN2803 power pin would be 24 ohm 6Watt resistor
you could make this by placing a 6 x 1 watt 144ohm (or greater resistence value) resistors in parallal.

Or some form of series shunt current limiter. I liked McTrivia's switcher, very nice ;-)

Or Google give a whole bunch of examples. (I just googled under Images "current limit circuit", I love google!).

http://www.google.com.au/images?hl=en&source=imghp&q=current+limit+circuit&gbv=2&aq=f&aqi=&aql=&oq=&gs_rfai=

Or put resistors in series with the motor windings.

Or try a less meaty motor (higher resistance windings) if you don't need a motor with as much torque.

Again, it depends on the problem and what you need to do to correct it.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
It's all a function of time.

mctrivia

to be fair I did not draw either schematic. The first comes from the chips data sheet as the recommended configuration. I did though spend a fair bit of time looking for the simplest chip that needed the least number of extra parts.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Lots of propeller based products in stock at affordable prices.

zoopydogsit

A great suggestion. One that I wouldn't have considered, switches have come a long way since I last looked at them.

Larry, Another possible solution could be to do it in software (I love the Propeller!), use PWM to vary the on time :-)
It would be a bit of a franken-stepper solution, as you are driving an inductive load it may reduce the need for
some of the extra devices, you may want a cap in their somewhere to clean it up a bit. It would be worth the play if you
have the time and an Oscilloscope. Best testing it with microstepping and a current meter in series (just a multi-meter
in current mode). Then tune PWM·duty cycle·and·phase to get the result on one winding working at the desired current
before doing the others. Sounds like fun :-)

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
It's all a function of time.

lardom

·@mctrivia, I have a Sparkfun·'EasyDriver'·that uses an A3967 as the main chip. I was still experimenting
with bipolars at the time. I could not get the L293 to work so I gave up. I discovered it was a lot easier
to control a unipolar. I see where the A3982 could handle the current but both chips look like they are
designed for bipolars. I have more knowledge now so I will give it another try. My first choice was a DC motor
with a gearhead and a quadrature encoder which I set aside because the stepper was simpler. I will study
what you have posted.·I looked at your schematics and thought "Gosh, there is still an awful lot I have to learn".
You have sent me back to the books.
·@Zoopydogsit, please tell me more about pwm. I used pwm for my first machine which used a modified servo
and a 556 timer chip. (Forrest Mims was my hero.) Are you saying I could·reduce the current by controlling the
duty cycle? I'd like to know what that looks like.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔