I'm needing a voltage regulator or DC-DC converter or something similar. I'm not too much of an electrical engineer so I need help figuring out WHAT I need and WHERE to get it. If a part like this is unavailable, I'll need some circuit design ideas to help me make one. Here's the deal...
I'm needing to run a monster servo (Tone Seiko PS-050 if you've seen it) off a 12V battery. I know, I know, just run it off the 12volts - but I'm sticking to the manufacturer's recommendation of 8.4V max. Max current draw is expected in the 4 to 5 amps range.
So that's it. All I need is something to convert the 12V to 8.4V and handle 5 amps. Something efficient would be nice (switching?) I thought this would take about five minutes to find online but I'm having a helluva time.
On a sunny day (Tue, 21 Aug 2007 22:35:42 -0000) it happened snipped-for-privacy@aol.com wrote in :
No efficient, but something simple is 5 silicon 5A diodes in series, that will drop about 5 x 0.7 = 3.5 V, and give you 8.5 V. Less then 2 dollars. Compare to the price of a switcher, substact, calculate electricity cost for both solutions, expected lifetime, and see what is cheaper in the end. hehe
I was suggested this in another forum and it sounds like a simple and cheap solution and I do love the KISS principle. A couple followup questions...
1) Is there any heat concern? 17.5 watts max dissipation according to my math, should I use a heatsink somehow? How would I accomplish that?
2) Do I just wire the three diodes in series inbetween the power source and the component on the positive lead? A multimeter should verify that its 8.4v alright?
3) I may also do this to run a 6v radio receiver off the 12v battery. Same deal? 9 .7v diodes (of smaller current) will do the trick or some similar configuration thereof?
Many thanks! I don't use Usenet often but it's where I go when I've exhausted all my other options cuz I always know I'll get the answers I'm looking for!
You will actually get higher performance from the motor using the full 12v and limiting the current with big resistors. Although 12v isn't that much above the 8.4v it takes for 5A motor current and you won't see a huge improvment.
It's a bit tricky to heatsink axial lead diodes even with a PCB, so I'd shop around for some stud mount or TO-220 case diodes and mount them on some kind of heatsink. If you need to use a high torque stepping sequence then you'll have to double that power dissipation.
You really shouldn't care as much about the voltage (within reason) as about winding current. As long as do not excede the rated motor current you will not harm the motor. The motor is an inductive load and when the current is switched on it will not jump instantaneously to maximum. The change in current over time is given by di/dt = V/L so for a given inductance the current will ramp up faster given a higher supply voltage. With .72ohm, (12-8.4)/5, series resistance at time=0 there is no current flow so the full 12v would appear across the winding and as the current rapidly ramps up the resistor will start dropping the voltage untill the maximum current of 5A is reached where the voltage will be down to
8.4v. This results in higher torque available from the motor. Using diodes is a different matter, the motor never sees the full 12v due to junction drop in the diodes. You have to dissipate the heat either way, so I'd go for the extra performance.
Just FYI, here's a simple example of a switched mode current limited driver
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And if you're curious heres a link to an excellent stepper introduction.
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Mike
"The scientist is possessed by the sense of universal causation...His religious feeling takes the form of rapturous amazement at the harmony of natural law, which reveals the intelligence of such superiority that, compared with it, systematic thinking and acting of human beings is an utterly insignificant reflection." Albert Einstein (theoretical physicist)
There are a lot of reasonably simple switching supply solutions out there. For example, go to
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and you can enter you specifics and get a recommendation; you can then simulate the design, and for a price they'll even supply you with a circuit board with parts. Suggest LM2679 as a reasonable part to start with. Quite a bit more complicated than the KIS,S solution, but dissipates lots less power, and is a lot safer with respect to the output voltage, since it's actually regulated.
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