MachTach Build

[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.

Since I have the document open in front of me, and what may help you interpret the doc, if you look on the second page for LM140A/340A, not at the top for input voltage where they show the performance based on three input voltages: 10, 19, and 23. . The table shows you the expected performance for the three evaluated voltages (10, 19, and 23) but the device can assume any voltage between that range (inclusive) but can be driven all the way up to 35 V. So 12V is clearly an OK input voltage.

 

[davidpbest]

Thanks Blackjack.

Here's a snippet from Mark's detailed response - "The MachTach uses an older style linear voltage regulator which dissipates excess voltage by generating heat. So anything above 9V and the regulator will not last w/o a heat sink (and even with that it generates a lot of heat). "

So my takeaway is that it appears best to feed the device what it wants without sending excess voltage up in heat.

Alan, I asked Henry for a heat sink when I ordered the MachTach kit and he obliged. I power my MachTach with 12V and it has worked just fine off that source using the heat sink.

 

[Blackjackjacques]

Had the chance to run up the MachTach attached to more precise equipment as shown. The voltages and results are as follows:

Volts Current (mA) Temp (degrees F)
7.2 138.3 123.5
9 138.3 125
12 138.3 130.5
15 138.1 142.5
18 137.8 164.5
24 135.6 184.5

I could have taken this all the way up to 35V, but I did not have a heatsink on the device. The temperature was measured using a Fluke IR gun on the regulator tab itself. The tests shown concur with the manufacturer's cut sheet and demonstrates that one could energize the MachTach board using the LM 340 regulator as low as 7.2 V, to at least 24 V, and as high as 35V. As a matter of good practice, I would use the matching heatsink even at the low voltages.