Hi Roy,
As odd as it might sound, the 2215 is very unlike the 2215A (in many ways) and what is known about adding a simple cooling fan does not apply to the 2213 and 2215 as well as many of the other small 22xx scopes that were made around the world.
Even common scopes like the 2213A do not have this undocumented fan circuitry on the mainboard.
The common 22xx scopes this fan modification applies to are the 2215A, 2235 and the Military version of the 2235, the AN/USM-488 and probably the 2235A as well.
The component numbers are printed on the mainboard of the 2215A and the 2235, but the markings are gone on the military version even though the unused fan circuitry is there.
Higher models such as 2236, 2230, 2232, 2221 come with a fan and a fan circuit that is similar. Some are half wave like the 2236, others are full wave like the 2232.
Of the ones I have seen, they all use pin 20 on the main transformer, and also use pin 22 if it is a full wave supply on that particular model.
Below is a document from 2008 that explains the fan circuit and components I used, and includes some of the good input I got from other Tekscope members back then.
(Unfortunately the photo album capability on this group will not allow posting this related document with the photos because it is not an 'image' file)
tom jobe...
PS I have installed the fan circuit on quite a few 22xx scopes in the last 10 years, and the uF value of the capacitor does not seem to matter much, nor does going to a full wave supply make much difference in the end result. Adding another diode over to pin 22 of the transformer, on the bottom side of the mainboard is all you need to get a full wave supply if you want to try it, and a photo of that will also be added to Leo's photo album after I edit the file names to help explain what is in each photo. I see someone else has posted some photos on this same subject to Leo's album but with no hint of what you are seeing in the file names.
Components I have used include:
CR965 = ultra fast diode, one amp
R965 = 3 ohm resistor, 1/4 Watt
C965 = 100uF to 1000uF capacitor
Recently the question was asked "was there an optional fan for the AN/USM-488 oscilloscope (the military version of the 2235)? and some off line discussion led to some interesting findings that a few of you might like to hear about.
There was no optional fan as far as anyone seems to know, but there are some unused traces and solder pads on the mainboards of the 2215A, 2235, and USM-488 (and other similar 22XX's), that appear to have been made for a power source to run such a fan.
On the 2215A and 2235 I was looking at, the component numbers were silk screened on the mainboard in these empty locations, and these component numbers are not in the electrical parts listings or shown on the schematics I looked at. The USM-488 does not have the markings on the mainboard, but all of the same traces and solder pads are there.
Looking at the front of the oscilloscope, these component locations are near the right edge of the mainboard, just in front of the aluminum plate that is the heat sink for the two inverter transistors and the one MOSFET of the inverter.
It is a half wave supply coming from pin 20 (the 8.6 volt winding) of the main transformer to the cathode of diode CR965. The anode of CR965 connects to the resistor R965 which becomes the minus pin of the 2 pins called P9965 (which is where the fan plugs in). The positive pin of P9965 is connected to ground and a capacitor C965 goes between the plus and minus pins of P9965 to smooth the half wave power. (the fan's positive supply is ground, and the fan's negative supply comes from this negative supply circuitry)
I did not know what to do for component values so I studied the full wave supply for the 2232 fan and my electronic text books for awhile, and came up with these values. CR965 is a one amp diode, R965 is a 3 Ohm resistor, and C965 is a 1000 uF capacitor.
The resistor limits the current to protect the diode at start up, and the capacitor was probably intended to be fairly large as the capacitor mounting holes are on 0.3" centers (7.6 mm). The capacitor for the 2232 full wave fan supply is a 4700 uF, so 1000 uF seemed like a reasonable place to start, and it physically fit in there nicely.
The fan is a 60 mm / 12 volt computer style fan, and it fastens to the inside of the back of the metal case where the air holes and the square 50 mm hole pattern is. All of the 60 mm fans seem to have the 50 mm bolt pattern, and I happened to find one fan that had threaded inserts in the four holes so no nuts were even required. The fan is on the inside of the case blowing the air outwards just like your computer does.
I did this fan experiment on a 2215A I have been working on for some time. With this modified oscilloscope running, the fan is getting about -8.4 volts, which keeps it fairly quiet yet it does move some air, and there is about 60 mV of ripple at the fan connection.
Next I thought I would try making this a full wave fan supply, so I added another identical diode and a piece of wire (all in shrink tubing), to the back of the mainboard. This second diode's cathode connected to pin 22 of the main transformer (the other end of the 8.6 volt winding) and went over to the end of resistor R965 where the first diode CR965 connects. The full wave arrangement delivers about -8.6 volts (up from -8.4) and the same 60 mV ripple so it seemed foolish to bother with the full wave configuration and I removed it. In one of my books it talked about how you need about 0.5 volts of ripple in the first stage of a power supply to make sure both diodes are actually working, and I only had 60 mV.
tom jobe
¡. tom jobe wote
Hello John,
Your mathematical explanation helps makes more sense of this for me.
This morning I was reading in one of my older text books about all of this,
and they gave a rough formula to figure out the approximate capacitor
required for a given ripple on a half wave rectified 60 Hz sine wave
circuit.
C = It/V
where "C" is the capacitor size in Farads, "I" is the load in amps, "t" is
the time in seconds, and "V" is the allowed ripple in volts.
If you plug in .12 for the fan load in amps, times .000050 for the 50 uS
period of 20 kHz, and divide by .05 for 50 mV of ripple, it shows you that
you only need 120 uF
The square wave would have a higher RMS value than the sine wave, so I guess
the capacitor could probably be even less, maybe only 100 uF?... if I
haven't messed this calculation up somewhere!
If there is no real downside to leaving the 1000 uF capacitor in there? I
think I will just leave it alone, and move on.
I've had a good time with this project, and I thank you for your help!
tom jobe...
tom jobe wrote:
Hello John,
Thank you for the additional insight into the capacitor sizing. I'm in
way
over my head, so I can use all of the help I can get to understand this!
One thing I failed to mention was that this transformer runs at 20 kHz,
and
the winding you are connecting the fan to is an approximately 19 volt
Peak to peak square wave with a period of 50 uS. (this square wave meters
18.4
VAC with an RMS DVM)
OK. The same formula applies, but the dT is 25 or 50 uS. With a square,
the cap
will be oversized.
The square wave is fairly symmetrical, and when it was being rectified
full wave, the resulting trace (without a capacitor) was not a perfect
straight line of course, but it wasn't bad. The full wave rectified square wave
trace is made up of approximately straight pieces on the tops and bottoms of
the square waves with little glitches in-between every 25 uS, so the smoothing
probably does not require nearly as much capacitor as a full wave rectified sine wave, or half wave rectified square wave would.
Correct.
I was surprised at how the ripple did not change much when the fan
Supply was changed back to half wave rectification, but I guess the fan uses so little power (0.12 amp?) that the capacitor does not get pulled down very much in the 25 uS between half wave pulses.
The ripple depends on the load current and the cap size. With a square wave, current is supplied
throughout the cycle, hence the lower ripple.
Arthur pointed out in another reply to this subject, that 1000 uF is
probably far too much capacitor for this little fan supply. What made me
start with 1000 uF was two things, one was that a study of the similar
full
wave fan supply in a 2232 has 4700 uF (was Tektronix trying to tell me
something?), and the other thing was the mounting holes for the
capacitor
were on 0.3 inch centers so the smaller capacitors would not sit down on
the
board properly because their lead centers are closer together at 0.2
inch or
less. Also. I could not see that it would hurt anything to have a larger
capacitor if the current was limited through the diode at start up, even
though I am questioning my choice of resistor size at just 3 ohm. (the
2232
uses 2.7 ohms of resistance with a 4700 uF capacitor)
tom jobe....
The charging current is roughly 1/2 x 19 = 9.5 V peak. The resistor is 3
ohms,
so the current is limited to about 3 amps.
I = C dV/dT, soI/C = dV/dTso3 / 0.0047 = 14,100V/sec = 0.014
V/uS
so in 25 uS (1/2 period)it rises 25 x 0.014 = 0.35 V
that's about 1V in 3 cycles or 10 V in 30 cycles, which seems reasonable.
Best,
-John
PS I'm not trying to design anything here, I'm just trying to learn the
basics with small projects like this, and I thank all of you for helping
me.
----- Original Message -----
From: "J Forster" <jfor@...>
To: <TekScopes2@...>
Sent: Saturday, February 02, 2008 7:18 PM
Subject: Re: [TekScopes2] Optional fan for the 2235 or USM-488?
[snip]In one of my books it talked about how you need about .5
volts
of ripple in the first stage of a power supply to make sure both diodes
are
actually working, and I only had 60 mV.
It is good fun learning about electronics with this fine Tektronix
gear!
tom jobe...
No. If you look at the waveform to a capacitor input filter on a 60 Hz
line, you should see an increased voltage bump every 1/120th of a
second. If
you see one every 1/60th of a second, it's a half wave rectification.
The amplitude depends on the C size and load current:
I = CdV/dTso dV = I / C dT
dV is roughly the P-P voltage of the ripple
dT is the interval between charging pulses(1/60 or 1/120 second)
SO:
dVincreases w/ current drawand decreases w/ capacitor size.
Best,
-John
Hello Arthur,
Yes, the diodes I used came from a junk 2235 mainboard, and they were
rectifier diodes from one of the secondary supplies. I checked them on the
curve tracer and they seemed to be identical. You are right about getting
almost peak voltage across the large 1000 uF cap.
Thanks for your help!
tom jobe...
On 8/4/2018 7:47 PM, Roy Morgan wrote:
Tom,
I would want to do this fan modification to my 2215. A modest amount of cooling can help electronics life span a lot.
So I¡¯d be most grateful for the pictures you have.
Thanks in advance.
Roy Morgan
k1lky68@... <mailto:k1lky68@...>
On Aug 4, 2018, at 8:08 PM, tom jobe <tomjobe@... <mailto:tomjobe@...>> wrote:
Hello,
Do not forget that scopes such as the 2215A and the 2235 have the fan drive circuitry you need already printed on the mainboard,...
I have some photos of the above modifications that I can send to anyone directly on request.
