INTRODUCING: the "Roto Damper"


 
I've been asked a couple times about the mods I did to the little thermocouple amp board that came with the HMv4.1 to make it work as an external TC amp, so I decided to repost the info in this thread for reference....
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Today I finally got a chance to dig in and examine the little thermocouple board that came with my HMv4.1 board. Although this was an unsuccessful prototype thermocouple amp, the circuit structure is pretty similar to the working thermocouple amp that is found on the HMv4.2 boards. Their are quite a few component value changes, and two additional resistors added to the HMv4.2 circuit.

I found the easiest way to modify this board to match the new circuit was to first cut the trace between the 1nf cap and the 10K resistor (where the red dotted line is in the picture below). You have to ignore the component values silk screened on the board and install the values I have added in the pic (values from the HMv4.2.3 circuit*) then add the two extra resistors shown. I chose to use through hole resistors because I found that to be easier. I built and tested this circuit tonight and it is working great.

*Bryan noted that he has changed the SMD 10K resistor shown in the diagram to 1K on the HMv4.2.4, however, I am running the 10K and it is working great. Either value should work fine, this value can be adjusted depending on how much noise you may be picking up on the TC line. I am running the TC amp output through a 50+ft CAT5 cable with the 10K resistor and I've got no noise whatsoever, I never see the noise symbol appear during normal operation and the TC graph is always nice and stable.

Here is a pic of the HMv4.1 thermocouple amp board with the above modifications detailed:
HMv4.1_to_HMv4.2_ThermocoupleAmp.jpg

Note the black X on Pin4 of the thermocouple amp, that pin is not used so you don't have to worry about soldering it. Also note that Pin2-3 are bridged (and are gnd), and Pin5-6 are bridged as well (this is the output), so you don't have to worry about clean solder between those pairs of legs on the IC

And here is a pic of the actual board all built up:
HMv4.1-2ThermoBoard.jpg


So this board could be modified and used as either an external thermocouple amp, which can run outside the HM and allow you to send the thermocouple output through the CAT5 cable with the blower and servo, or it can be used to add thermocouple pit probe support to pre v4.2 Heater Meters, or it could be used to add a second thermocouple input to a HMv4.2. I'm really happy how it worked out.....

To connect an external TC amp to the HM you should lift the end of the resistor in the RC filter for the Pit Probe and insert the output from the external TC amp there. Make sure you lift the end of the resistor that is toward the TC jack, not toward the ATMEGA, as this will leave the RC noise filter in place which is critical to get a clean signal from the TC.

Here is a pic of the resistor lifted with the external TC amp output inserted (through the white wire):
TC_Insert.jpg

Note in the pic I have the standard probe jack installed (rather than the TC jack) because I am not using the on board TC amp. I installed the standard jack simply to test the fit of my sliding back case on an all standard probe HM build. You could install the TC jack and and use the on board TC amp in some capacity if you like. Either install a switch to toggle between internal/external TC amp going to the resistor shown (to use as pit probe), or you could also jumper the output of the onboard TC amp over to a food probe and use the onboard TC amp to add support for a second TC probe. Note that you must remove the pullup resistor for the probe you jumper the onboard TC amp to if you decide to go that route....
 
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I have question about the board Ralph. At the smd 10k on the tc- , where the four pads are, you have a cut line. I cut that line and I was wondering about the 10ksmd component pads seem to be connected. Is that correct?
 
The two pads that have the green box around them (all pads with green boxes) are connected together, they are all ground. The pad with the orange box should NOT be connected to the pad beside it or above it (after you cut the trace). The colored boxes/circles indicate where pads are +3.3v, gnd or output, respectively, for reference...
 
Yeah I figured as much, looks like I'm going to have fun trying to figure out why the two bottom pads seem connected when I don't have anything on the board yet, lol. I made the cut, between the top left pad and the bottom left pad and that broke the connection between the 1 and 2,3 pads of the amp.
 
I figured out the problem, these boards also suffered from bad layout. My bottom two pads were connected via the path that was running on the bottom. So, now I need to reconnect the bottom right pad to ground. And I broke the connection between the bottom left pad and ground. It was very hard to see them connected. Grrrrr.
 
That's really odd, you sure? That pad is the new TC- after the mods, it is a dead pad (after you cut the trace) with only those two resistors connected to it, doesn't have traces going anywhere....
Here is the schematic, R24 is the SDM resistor that goes to gnd and the through hole 10K resistor makes the circuit to the TC amp...
ThermocoupleAmpCircuit.jpg
 
Yeah, I had to cut the trace to break the connection to it, but in doing so I also broke the the pad, that's going to ground. I added a ground wire to that pad and connected to the ground via .1uf
 
Ralph, one more question or actually to make sure, I got this correct.

The white wire you have connected to the resistor on the HM board is going to the cat5 #7, is that correct. Where did you attach the ground wire and 3.3?
 
Yah, TC amp output on Pin7, the diagram on the previous page is accurate, follow the table that says HMv4.2+ w/ TC

EDIT: in case it isn't clear from the diagram, the TC amp ground is shared with the PROBE ground, not the blower/servo ground...
 
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Ralph, basically you can only have one tc and one probe on the RD, if you have the Tc board in the RD. If not then 3 probes may be placed in the Rd. If that's the case then no need to add the other two probe jacks, one is fine.
 
Yah, I did try to share a ground between the blower/servo/probes/TC but the TC readings went all wacky when I did, so I had to move back to a separate ground between the probes and blower/servo. So, with the TC amp out at the RD you can only run the TC and one food probe through the CAT5 cable because of the 3.3v required by the TC amp. I had thought about putting a 3.3v regulator in the RD on the 5v line to create 3.3v there without using the extra wire, but that could also cause a potential difference to ground for the TC and might throw the TC readings off, though I haven't had a chance to give that a try to say yes or on that....
Are you saying that you haven't received the package yet, cause USPS shows it delivered Wed? At any rate, I put 3 probe jacks in the package for you so just hang on to the spares for any future endeavors....
 
I'm not sure if the TC amp needs to be run at 3.3v to match the ATMega or not? I know Bryan noted that putting a 3.3v regulator at the damper end might cause a ground potential issue and didn't suggest at the time that I just power the TC amp with 5V, but perhaps the both of us overlooked that option. If you want to give it a try and report back that would be great, 'cause I don't plan on ripping my RD apart any time soon....
 
The TC amp will run on 5V but when nothing is plugged in it will put out ~4.9V into the ADC of the ATmega, which is technically over spec. The ADC input voltage max is Vref (3.3V). It might work ok though because the low-pass filter provides such a high impedance input, that the current through the ADC diode clamps will be minimal.
 
Is there another spot on the HM that I can connect a switch for isolating each amp(on HM or Rd)? I'm getting odd results with both amps and using the resistor as a connection point and a switch.

When the tc is connected to the HM, all is OK 70°

When I connect the RD and I switch the switch to the RDs tc amp, I get jump to 300+ then it drops to 85° with no tc

I then connect a tc to the amp on the RD and it jumps up to 130° and slowly drops back to 85°

If I blow on the HM Tc amp the temperature drops, so its interfering with the other tc amp in the RD. This leads me to believe that I need to isolate the amp when I switch over to the amp in the RD.
 
If you look at the schematic, when you lift that resistor you isolate the output of the onboard TC amp, you cannot connect BOTH amps at the same time however, so that resistor must be unsoldered from the HM board on the one side (nearest the TC jack). I have done this on 3 HM boards and it works the same way on each, which is to say, perfectly. To be clear, I leave the onboard TC amp powered and everything, but lift that resistor which cuts off the output of the onboard TC amp before it reached the ATMega, and then I insert the external TC amp output on the lifted end of the resistor.
 
yeah, I have it lifted one side. I thought you have not had a TC on the HM when you have had a TC board in your Damper. I thought you said that you have a pit probe jack connected.

Some reason im still getting interference with the onboard TC when I try to use the TC in the Rotodamper.

Anyways, it could also be my soldering of the TC board itself so I'm going to try a Adafruit board, to see if it helps

I also Tried using the 5v and no difference, lol. but I as Bryan said it does work though.
 
I have done 3 boards this way, the first two had the complete TC amp and connector on board, the last one I put a standard probe jack on instead (to test fit in my sliding back case), but I do have the complete TC amp circuit in place on this one as well (other than the TC connector).

Like I said, if you look at the schematic, if you lift the end of the 100k resistor (that is away from the ATMega) the input to the ATMega for that probe is completely isolated (note that the 10K pullup resistor shown on the schematic is omitted when using the TC Pit Probe). So I am really confused how the onboard amp would have any effect on the pit probe with that resistor lifted, being that there is no electrical connection from the amp to the ATMega.

One side note, I have noticed when you lift that resistor if you do not connect an external TC amp to that resistor (or have one connected that is not powered) the HM will initially look like a TC is connected, but the reported temp will rise and rise at a fairly rapid rate up to about 1000F and then will report "No Pit Probe". As soon as you connect a TC amp (or power the amp that is connected) the TC will start to register properly.
 
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