Sign in to follow this  
joebiodiesel

Electronics 123 Exchanger Working Voltages

7 posts in this topic

I had some exchanger problems on my first video build. I ended up having a bad exchanger board. While attempting to diagnose my problem. with the help of several board members, I ended up doing a lot of testing. A later conversation with TCSCOUT led to some additional testing, after I got my cam fixed.

 

The exchanger is rated for 5V operation. but it doesn't like voltage below that. That is not news to most people already using it. To make sure the exchanger switches reliably, many people are running it on 6V and 9V with no problems, or 12V with the assistance of a 6.8 ohm resistor to keep it from burning up the board. Today I broke out my signal generator and scope to make a few different voltages, and record some voltages and amperages. I found some interesting stuff. I thought I'd share my pics and my opinion. Please keep in mind I'm not trying to portray myself as an expert. Just read my other posts to convince yourself that that is not the case! :) I'm just trying to contribute, for once.

 

Here is a pic of my doctored exchanger board.

 

2012-02-05_10-06-29_145.jpg

 

I built this for testing purposes. I didn't want to burn up my exchanger in case there was a problem, so my initial testing was done with this board. I got brave later and duplicated the tests using the exchanger. The exchanger has 9 ohms of resistance, and the resistor was a 10 ohm. Both my test board and the board in my cam acted the same.

 

I started testing my setup at 5V. With my scope I recorded the amperage to the exchanger (Pin 1 on the three wire connector).

I watched the voltages on both wires to the exchanger from the board. these wires are power and ground, alternately, as the board moves the lens shutter back and forth.

On the bottom is source voltage available on pin 1.

 

Test 1 at 5V. A few things are notable here.

The exchanger is rated at 500mA, but my probe measures it at 250mA. Some of that may be an error in the probe, but not all of it. These draw less current than claimed. That is OK though.

Notice that when the board moves the lens that it puts power on both the power wire and the ground wire. The voltage to move the lens is the difference between these 2 voltages. With 5V to the driver board, the exchanges sees only 1.4V when operated normally. That seems like a strange way to do things, but I imagine it makes for a very closely controlled circuit. Note that whether it is opening or closing the shutter, it does it with 1.4V.

 

1-1.jpg

 

Next I tried 4.8V. It still worked just fine. note that the operating voltage for the exchanger dropped to 1.29V (2.73-1.46)

 

2-1.jpg

 

At 4.5V the exchanger worked intermittently, and here, at 4V it stopped working entirely. You can see it is still trying to work but there is not enough voltage available to do it(1.22V). Testing at 3V yielded the same results, of course.

 

3-1.jpg

 

 

So, we can see that this exchanger works great on 5V, but it sure doesn't like much less than that. (Again, not news to most of you)

 

After that I tried some higher voltages. I was originally going to test it until it failed, but didn't have to. You could see it coming, so there was no need.

 

Here is 6V. Everything still looks OK.

 

6-1.jpg

 

Here is where the trouble begins. 6.5 Volts. It's not terrible at the voltage, but the problem is clear once you look at the higher voltages. Note that when the circuit is being switched 2 things happen. The voltages available to the exchanger rise as the current flow through it drops off. This change in voltage and current in a switched circuit is troubling to me. It signals that the drivers are not able to control the current flow, or maybe the electro-magnet windings are heating up, increasing the resistance and dropping the current(and the board is raising the voltage trying to maintain it).

These are just a couple of guesses, but wither way, it is troubling. And it gets worse.

 

7.jpg

 

7 Volts.

 

8.jpg

 

8 Volts. It gets bad faster here.

 

10.jpg

 

9 Volts

 

11.jpg

 

From repeated testing on my camera I can tell you that above 7.2V the problem of current drop get beyond where I would put mine.

 

So, I went to radio shack and picked up a pile of assorted diodes. I prefer them to a resistor, because they are not dependent on available voltage or current flow to maintain the excat same voltage drop. I put several resistors in series in order to get the voltage to my exchanger board under 6.5V (while using at 12V SLA). At 250mA I don't think it's too important what type of diodes you use, but the only way to know for sure is going to be to use them for a while.

 

Be sure to let me know if you see an error in the way I tested this circuit, or anything else I may have goofed up.

 

Joe

 

 

Share this post


Link to post
Share on other sites
 

Fantastic read. So far I've had no issues with the 6.8ohm resistor, and I would figure that reason it has been durable at the higher voltages, is because it rarely switched more than 3 times in a 24 hour period and the voltage runs to it for very short periods. We'll have to keep an eye on this.

Share this post


Link to post
Share on other sites
 

I agree Joe, it would be better to get the voltage at the exchanger down to at least 6.5 volts. Would probably take 7 or 8 diodes on 12 volts and 3 to 4 on a 9 volt to do this. Maybe post how many diodes and the part # too. I'm not using this exchanger so I'm just taking info and putting it on the drawing. There is also the option to use another set of batteries just for the exchanger, but most of us try to stay away from that. My exchangers are 12 volt and 3 volt, so..... Nice write up, thanks for taking the time to do this.

Share this post


Link to post
Share on other sites
 
 
Would probably take 7 or 8 diodes on 12 volts and 3 to 4 on a 9 volt to do this. Maybe post how many diodes and the part # too. I'm not using this exchanger so I'm just taking info and putting it on the drawing. There is also the option to use another set of batteries just for the exchanger, but most of us try to stay away from that. My exchangers are 12 volt and 3 volt, so..... Nice write up, thanks for taking the time to do this.

 

Unfortunately I bought a bag-o-diodes, so I don't have a way of posting a part number with quantities. It should be easy to pick some out though. Find some that can take at least .5A, and the voltage drops add up to the number you're looking for. Because diodes have the same voltage drop regardless of current flow, it should be simple math.

 

It may also be easier to find a collection of diodes locally than to find a 6.8 ohm resistor, at least it is in my local Radio Shack.

 

I'm glad I got a chance to do this. I wouldn't have thought to do it on my own. It was TCSCOUTs idea.

 

 

Joe

Share this post


Link to post
Share on other sites
 

Joe - is your scope an automotive scope? Probably not an inexpensive item for a poor home hobbyist....but I'd like to have one!

Share this post


Link to post
Share on other sites
 

The one I used for that test is. It is a Verus, by Snap-on. I like it because it records easily, and turns screen shots into .jpg images with a single button push. And yes, it would be prohibitively expensive for a hobbyist, but it makes a great scanner/scope if you're fixing cars.

 

I'm a scope junkie, like we have cam junkies here. I have more of them than I need, but they all get used.

 

Joe

Share this post


Link to post
Share on other sites
 

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this