MachTach Build

[Blackjackjacques]

I don't see a conflict with what Mark has to say. The LM 340 generates heat requiring dissipation via a heat sink anyway -- at 9V, at 12 V, at 18V. Even the newer switch mode power supplies generate waste heat requiring dissipation via some means. However, the newer regulators are more efficient and therefore generate less waste per watt than the older linear regulators. But the main thing to take away is that, based on my tests and the product data sheet, that the quiescent current demand, as well as the regulator's temperature, is the same whether it is energized by 9V or 12V. In other words, 12V does not place any additional stresses on the regulator, or such stresses are well within the product's design as detailed in the product data sheet (attached for your ref). Therefore if you have a 12V dc supply on hand, you can connect it to the MachTach without need to reduce the voltage.

 

[Cadillac STS]

Yeah -- I think I will go with the Hall-effect sensor. After all, I already have it on hand. Thanks

When I put the Hall effect sensor together I did it just like the pictures in the instructions. Cut the leads to quarter inch and solder the three wires. With the solder and the wires there was little room between the leads. And then you are supposed to push it up into the channel of the 3D printed housing. I was concerned about the wires shorting out when it was pushed into the channel. What I found helpful was to take the heat shrink tubing that comes with the kit, cut 1/4 inch of it, slit the tubing then work that around the middle lead. Don’t heat shrink it. That way if the leads do shift right away or later in use you have some insulation to prevent failure.

 

[mksj]

My experience with linear regulators and the one in the MachTach differs from your findings. MachTach also supplies a heat sink for individuals wanting to run their tach at 12V. It may be that your wall wart voltage sag or limited current makes it appear to be different than if you use a larger power supply. When I power my MachTach at 9V using my BK regulated power supply, the regulator gets warm to the touch. When I power it at 12V I can barely touch the heat sink, I do not have a thermal gun. Since the regulator draws the same current (106-109 mA) at 9 and 12V, the power difference is given up as heat. Without a heat sink the regulator has very limited thermal dissipation abilities. Switching regulators are more efficient thus run cooler and do not require a heat sink. They can generate more electrical noise which can be an issue for some circuits. I guess I do not have a fondness for linear regulators, they made nice room heaters in the winter time, and then there is my tube amplifiers.

 

[Blackjackjacques]

My experience with linear regulators and the one in the MachTach differs from your findings. MachTach also supplies a heat sink for individuals wanting to run their tach at 12V. It may be that your wall wart voltage sag or limited current makes it appear to be different than if you use a larger power supply. When I power my MachTach at 9V using my BK regulated power supply, the regulator gets warm to the touch. When I power it at 12V I can barely touch the heat sink, I do not have a thermal gun. Since the regulator draws the same current (106-109 mA) at 9 and 12V, the power difference is given up as heat. Without a heat sink the regulator has very limited thermal dissipation abilities. Switching regulators are more efficient thus run cooler and do not require a heat sink. They can generate more electrical noise which can be an issue for some circuits. I guess I do not have a fondness for linear regulators, they made nice room heaters in the winter time, and then there is my tube amplifiers.

That is what they used to say about tube equipment only--how it heats the room. Now linear regulators are lumped in with tubes. I'll take the simplicity and low parts count of a three term regulator any day. While I use switching regulators here and there, I'll look for a passive solution first, heat and all.

 

[Blackjackjacques]

My experience with linear regulators and the one in the MachTach differs from your findings. MachTach also supplies a heat sink for individuals wanting to run their tach at 12V. It may be that your wall wart voltage sag or limited current makes it appear to be different than if you use a larger power supply. When I power my MachTach at 9V using my BK regulated power supply, the regulator gets warm to the touch. When I power it at 12V I can barely touch the heat sink, I do not have a thermal gun. Since the regulator draws the same current (106-109 mA) at 9 and 12V, the power difference is given up as heat. Without a heat sink the regulator has very limited thermal dissipation abilities. Switching regulators are more efficient thus run cooler and do not require a heat sink. They can generate more electrical noise which can be an issue for some circuits. I guess I do not have a fondness for linear regulators, they made nice room heaters in the winter time, and then there is my tube amplifiers.

No voltage drop with my wall wart and no current limit issues. The voltage under no load is 12.1 V and at 140 mA, what the MachTach pulls, it is still 12.1 V. The wall wart is rated for 300 mA, so given the actual test results, the wall wart is performing as specified. Additionally, I am getting the same heat dissipation from the heat sink at either 9V or 12V -- as described in the LM340 spec sheet, and as verified with my Fluke thermal device. Therefore, no surprises on my end and everything appears to be in concert with the LM340 manufacturer. However, if you are burning your fingers as 12V and not at 9V, your heat sink may not be properly coupled to the device. You can fix that by using heat sink compound between the device and the heat sink, or perhaps some other problem. Switching regulators have their place, but such technology would be incredible overkill in this particular application -- in fact, given the low current demand of the MachTach, a simple Zener diode arrangement would have sufficed.

 

[Blackjackjacques]

When I put the Hall effect sensor together I did it just like the pictures in the instructions. Cut the leads to quarter inch and solder the three wires. With the solder and the wires there was little room between the leads. And then you are supposed to push it up into the channel of the 3D printed housing. I was concerned about the wires shorting out when it was pushed into the channel. What I found helpful was to take the heat shrink tubing that comes with the kit, cut 1/4 inch of it, slit the tubing then work that around the middle lead. Don’t heat shrink it. That way if the leads do shift right away or later in use you have some insulation to prevent failure.

Thanks - sounds simple enough - will do. Looks like the trick now is how I am going to penetrate the gearbox, but I see several options that I just need to comb out.

 

[Alan H.]

No voltage drop with my wall wart and no current limit issues. The voltage under no load is 12.1 V and at 140 mA, what the MachTach pulls, it is still 12.1 V. The wall wart is rated for 300 mA, so given the actual test results, the wall wart is performing as specified. Additionally, I am getting the same heat dissipation from the heat sink at either 9V or 12V -- as described in the LM340 spec sheet, and as verified with my Fluke thermal device. Therefore, no surprises on my end and everything appears to be in concert with the LM340 manufacturer. However, if you are burning your fingers as 12V and not at 9V, your heat sink may not be properly coupled to the device. You can fix that by using heat sink compound between the device and the heat sink, or perhaps some other problem. Switching regulators have their place, but such technology would be incredible overkill in this particular application -- in fact, given the low current demand of the MachTach, a simple Zener diode arrangement would have sufficed.

Can you share the specific LM340 spec sheet you are looking at? How are you confirming the same heat dissipation?

BTW, I am sure anyone who's read this far into this thread knows about heat sink compound.

 

[Blackjackjacques]

Can you share the specific LM340 spec sheet you are looking at? How are you confirming the same heat dissipation?

BTW, I am sure anyone who's read this far into this thread knows about heat sink compound.

I have already attached it earlier to this tread, but will do so again here as DS00781.pdf. I'm sure you will understand it. Given the broadband expertise amongst members and similar applications in most other arenas, I don't believe I was being presumptuous in my identifying heat sink compound. For those not familiar, heat sink compound is a special grease-like material used between the semiconductor and the heat sink. It beds the semiconductor device to the heatsink and ensures maximum thermal coupling. I identified earlier how I was making thermal measurements: I am using a Fluke 62 mini IR thermometer.

 

[Eddyde]

The electrical noise issue was not the enclosure, but the shielded cable between the hall sensor and the MachTach. Convention would be to ground the shield at the source (not both ends), but any grounding of the shield (either end or at both ends) made the MachTach go wild. So if you use shielded cable, my recommendation is float it on both ends. Grounding is always one of those funny things, doesn't always work the way it is suppose to.

BINGO! I had the exact same problem but with the optical sensor (I felt magnets and steel dust might be an issue). When I rough installed the MachTach on my Bandsaw project I used a scrap of unshielded Cat5, it worked perfectly. Then, when I wired it for keeps, I used shielded cable grounding it only at the MT end, the readings were wildly erratic. Snipped the ground and it was fine.

I also went the three turn potentiometer route, better resolution that way.

 

[Blackjackjacques]

I have already attached it earlier to this tread, but will do so again here as DS00781.pdf. I'm sure you will understand it. Given the broadband expertise amongst members and similar applications in most other arenas, I don't believe I was being presumptuous in my identifying heat sink compound. For those not familiar, heat sink compound is a special grease-like material used between the semiconductor and the heat sink. It beds the semiconductor device to the heatsink and ensures maximum thermal coupling. I identified earlier how I was making thermal measurements: I am using a Fluke 62 mini IR thermometer.

You know..if for whatever reason you do not like connecting your MachTach to a 12 V supply and really want to energize it from a 9V source, there are a whole bunch of other traditional methods of reducing voltage - including a resistor divider or even multiple series rectifiers and take advantage of the inherent voltage drop across silicone diodes. At 140 mA load, if you connect a 65 ohm resistor to the 12V supply = 9V. The resistor should be 2 watt and the load demand placed upon the 12V supply is an additional 185 mA. voltage dividers have been around ever since the creation of electronics and is the most used method of creating a voltage gradient in circuits. If your 12V supply can handle the extra 185 mA demand, and it most likely can, it provides you another option - probably the simplest/cheapest. Yes, the resistor will dissipate heat - but that is what they do.