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This topic comprises 2 pages: 1 2
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Author
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Topic: Strong 2020 turret R10 resistor q.
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Rick Long
Jedi Master Film Handler
Posts: 759
From: Toronto, Ontario, Canada
Registered: Nov 1999
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posted 05-22-2001 12:10 AM
Gotta question perhaps some of you electronic whizzes out there can answer for me.Reference: Manuals Section;
Simplex 35 Model PR1060 with TU2020/2030 turret parts (Oct.93) Manual page 35 (turret controller schematic) I have been having some luck working on repairing these things lately. While searching the schematic, however, I have to wonder about the purpose of resistor R10. Basically, it is placed at the output of the bridge D6 and (to my way of thinking) does nothing but dissipate heat. Further it is only on the output feeding the 5-volt regulator, there is not one on the 12-vot side. While testing this circuit, on the repair bench, I can see no reason for its existence, other than burning my thumb when I turn the board over to press the buttons on the board . My son feels that it may be necessary to discharge the capacitors prior to the regulator, after powering down. Not sure if I agree. Any ideas?
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John Walsh
Film God
Posts: 2490
From: Connecticut, USA, Earth, Milky Way
Registered: Oct 1999
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posted 05-22-2001 12:17 PM
I think it is to load down the 1000uF cap, which reduces the DC ripple into the voltage regulator, thus improving it. I understand that adding a resistor there would usually make the ripple worse, but since it is on the input, the ripple is still above the 5v. The 5v is pulling more amperage, and I'm guessing that after the circuirt was done, they found the ripple too high, and this was a quick fix. There isn't a 100 ohm resistor on the other (12v) supply, because it requires less amperage, and was smooth enough.Or, maybe they found that there was a race-condition "glich" when powering off, (the turret motor would start to spin just a little, or the solenoid would unlatch, for example) and wanted to kill the 5v quickly. What I'd like to know is what the code is in that programmable part, so I can figure the turret operation out.
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 05-24-2001 05:12 PM
Ok....here is Steve's guess...R10 is, for sure, a bleed off resistor...it isn't to help with charging the capacitor (it would be in series then) nor can it help with ripple. I think the answer lies in pin 1 of IC 1...it is labled "Reset". R10 probably forms the "reset" on power off...without R10...there is VERY little stuff to bleed off the 1000uF capacitor (the regulator, oscillator and IC1)...the size of R10 was probably choosen such that it will achieve an active low on a typical power cycle. Steve ------------------
"Old projectionists never die, they just changeover!"
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 05-26-2001 03:41 PM
A couple of flaws in that reasoning....First, if one wanted to "load the input" then the resistor should be on the output of the trasformer, not the diodes....why would you load your diodes (and make them heat up more) without reason. Furthermore, diodes are can fail from spikes as much as anything...often there will be a supressor capacitor across the input of the rectifier to clamp said spike (typically low capacitance but high voltage). The only logical reason for R10 is bleed off the main filter cap at a fast enough rate. Since I don't see it in the parts listing (ie what is it's wattage) I suspect that it was added after "field testing". Do you know what the output of the transformer is? The 7805 with proper heatsinking can take a lot. It also doesn't seem to be loaded very heavy. Steve ------------------
"Old projectionists never die, they just changeover!"
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John Walsh
Film God
Posts: 2490
From: Connecticut, USA, Earth, Milky Way
Registered: Oct 1999
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posted 05-27-2001 10:06 AM
This is a spec sheet for that voltage regulator (I think.) There is no actual listing for a "VR7805" although it seems like a standard part. CLICK HERE TO DOWNLOAD
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 05-28-2001 12:36 PM
From an engineering standpoint, the concept of putting a loading resistor in circuit is just plain silly (unless it is required for stabilization...some IC amplifiers are designed for low impedance outputs and must be loaded in order to not oscillate themselves to death).If the goal was to drop the voltage the other safter solutions are many...including series resistance (since the load is known and the regulator gives wide range)...a series zener diode can also drop the voltage well and again current load on the output isn't high so it needn't be huge (heck Strong's igniter circuits work using said zeners to drop the open circuit voltages). I'm sticking with simple bleed off. Steve ------------------
"Old projectionists never die, they just changeover!"
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