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Basic Electronics » Voltage Divider, Do the resistors heat up?

February 11, 2010
by treymd
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When you regulate voltage with a regulator, the regulator heats up. I was wondering if the same happens with resistors in a voltage divider circuit.

Resistors limit current, would that not, in and of itself cause some heat to be generated?

February 11, 2010
by mongo
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Depending on the wattage that you need to handle, the resistors can certainly heat up.

Voltage regulators do a better job at it and usually have means to attach a heat sink where the resistors have some difficulty. Unless they are power resistors, which do have heat sink ability.

February 11, 2010
by treymd
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The white ceramic resistors that have a power rating stamped on them eg: 5W0-3ohmK is that a power resistor? I'm trying to figure out how the treadmill manufacturers get away with transformerless power supplies. I've read that the best(safest) way to step down power is an isolation transformer followed by the rectifying bridge then filter it through capacitors, and finally a regulator. But they seem to skip the transformer all together and feed household voltage into the rectifier and then filter it through caps, but I'm not seeing a regulator on this board I pulled out of the unit....

Note: I'm not about to build my own power supply folks, just wondering how it works.

February 11, 2010
by mongo
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The DC motors use the power in a different manner than what might be thought.

When the power comes in and rectified, it creates about 145 VDC at the capacitors. Many treadmills run on 24VDC and there are some that use 90VDC as well. The speed control actually sends a series of pulses and as they average out with the feedback voltage from the motor, the speed can be controlled.

It's a little more complex than that in reality but it's a simple idea.

February 11, 2010
by treymd
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I assumed that some sort of pulsing needed to be at play here, since well, you don't get variable speed with constant voltage, and it is probably easier to pulse than it is to actually vary the voltage. I'm wondering if maybe what I was seeing was basically a set of relays that pulsed DC charging up 2 560uF caps and the caps kinda smoothed out the power.

Or perhaps the caps were just there to smooth out the rectified wave and the very large mosfet was in charge of pulsing the motor and the caps were a filter and nothing more.

I pulled this http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=IRFP250N-ND

off the control board, it was on the ground side of the motors power circuit.. hey! that's probably EXACTLY what that did, it pulsed the voltage to the motor! And the caps were supplied with constant rather than pulsed power so they must have been filtering the rectified wave. Thanks!

I guess I need to stare at the board some more.

February 12, 2010
by mongo
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One way to pulse the motor is to actually trigger a pair of SCR's in the rectifier bridge at different points in the phase of the incoming power. If you trigger at the top of the wave, you get a higher voltage than if you trigger on the downward slope of the wave. Kinda like a light dimmer would operate.

February 13, 2010
by BobaMosfet
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treymd,

You need to study RMS Power to understand how voltage changes. Voltage from your outlet is much higher than you think, and (atleast in most of the USA) is about 11VAC RMS at the outlet. You don't see the peaks, you only see about 77% of the peak. This varies depending on a number of factors, but it's a place for you to start.

A transformer's sole purpose is two-fold: adjust the ratio of voltage to current one way or the other, and to provide a non-physical coupling (current field) for isolation.

Rectified power is not smooth. It usually has to go into a capacitor to help keep the sinusoidal drops (upstream of the bridge-rectifier) from affecting the rectified amplitudes coming out (downstream side of the rectifier).

Something else to bear in mind-- if a treadmill uses a HEAVY motor, that means it will require a lot of current. In which case, it may be designed to tap 117V and all the associated amperage (within the limits of how the entire treadmill is supposed to work).

And yes, resistors can get hot-- they dissipate current by generating heat. In order to keep a resistor from burning up, it must be rated to withstand the expected current it will be resisting-- you can use Watt's law (PIE) to determine the size of resistor necessary, as they are rated (from 1/8th Watt and up).

BM

February 16, 2010
by treymd
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OK Boba, I'll make my next task to read RMS more closely, I've been seeing it pop up quite a bit, so I should.

I have read a little bit on rectification, so the part about rectified power not being smooth makes sense to me. Basically the bottom phase of the SIN wave is flipped up and the caps sort of smooth the wave out a little (supposedly good enough for motors) but beyond that the power should then be further filtered with a regulator if being used with more sensitive components.

this page is similar to what I read but I cannot seem to find the great reference I originally studied, sadly....

http://www.electronics-tutorials.ws/diode/diode_6.html

I know the motor I eventually want to drive is rated at 70v 3A. I am reading up on triacs right now hoping that perhaps that is the method suited for pulsing AC, I'm just unsure about how to step down the voltage to 60ish without spending a ton of money on a transformer. It's worth noting that the control board originally in the unit did not transform the power before it was fed to the motors.

How would you do such a thing?

AC > rectifier > Cap Filter > voltage divider???

then pulse the power before with a triac or after with a larger mosfet? So many things I need to be more sure of before I actually attempt this.

February 16, 2010
by treymd
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I meant to say that the the original controller did not use a transformer to step down the voltage, but another means entirely. The AC was transformed in the sense that it was reduced and filtered, but a transformer was not used in the process. Only a rectifier, 2 large capacitors, and what appears to me to be pulsed power using a large mosfet.

The voltage reduction is what is eluding me right now. I will continue to stare at the board and try to reconstruct the schematic until I see how they did it.

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