Battery eliminator for VTVM

[WizardOfBoz]

So I had a battery leak into my Heathkit IM-5228 vtvm. Cleaned it up and replaced the battery but it made me think about a battery eliminator (BE). Implementing such a thing requires some careful planning and design, especially concerning thermal management. Not so much in terms of heating the box up. Nominal power use in the device is 10W, and almost any conceivable BE will dissipate 1W or less. But an LM317 dissipating 1W requires a heat sink. This would only occur when measuring ohms in the Rx1 region, where the 1.5V is applied to the resistor being measured and a 9.1ohm range resistor in the cabinet, and then only when a test is being done. I never have a resistance check that takes more than a few seconds.

I thought a lot about this – way more thought than the application deserves. But in thinking about this and noodling around with designs I learned a lot. In any case, the discussion below is admittedly over the top. But here goes.

Most of the designs are based upon the venerable LM317. Some simple circuits tap the hot lead from the vtvm filament transformer tap (6.3vac). And the BE output is 1.55v. If we are measuring a near-zero-ohm resistor then 1.55v into 9.1 ohms gives a current of 170ma. Dissipation in the range resistor is i^2r, or .26W. Dissipation in the LM317 would be (6.3-1.55) * .17, or 810mW. Assume 1W, so we need a heat sink. Altenately, could we put a 6.8 or 7.5 ohm resistor in series with the power input? With .17ma current, this drops the supply voltage by 1.3v, so that LM317 dissipation is ((6.3-1.3) – 1.55) * .17 = 0.58W, and the resistor dissipates .22W. The idea is that the power dissipation is not in the LM317. This still is borderline as to whether a heatsink is required.

My current design is shown below. Note that in some versions of the IM-5228 manual online there are patently wrong circuits, and other designs where the circuits could operate correctly but are lacking in some elements (e.g. protective diodes). What do you all think?

https://photos.google.com/photo/AF1QipPQvZo_AO6WsuXSF7wXAydkg_NT6C52XlTaohje

Some LM317 questions:
1) Can I put the resistor in series with the LM317 input to offload some of the power out of the LM317?
2) Does the LM317 provide a stable enough, noise-free output?
3) Can I safely tap into the filament tap on the Heathkit transformer or do I need to add a transformer?
4) Some designs use a single diode for half-wave rectification. It seems to me that this puts a higher stress on the transformer filament windings. So would a full bridge be better? It’s only 3 diodes more.

There are a lot of really great resources online. Of course, Mr. Carlson is one of them. Others are:
a) Dave Jones, of the Australian EEVBLOG (highly energetic discourse on diagnositic tools and general topics)
b) Mark Maher, of Mend-it-Mark (Expertly diagnoses and fixes consumer electronics both recent and vintage)
c) Jonn Teel, of Predictable Designs (Chip centric (Teel designed chips for TI) with good advice for product design
d) Hans Rosenberg, more good advice about designs, emphasizing high performance topics (e.g. eighth order filters and resonance loops forming between layers of multi-layer boards).

In one video, Teel points out chips that you shouldn’t use for current designs. One such chip is the LM317. Fine for one-offs, but for products being made on a Grand scale it has liabilities like a large (3V!) drop out voltage. So while Im sure that the LM317 design above (or some tweaked variant) would work, I started looking at other voltage regulator chips, such as TIs 767TLV. In designing a 1.5v supply based upon this I thought that I could take advantage of the lower drop out voltage and possibly use a 115vac to 2.5vac (center-tapped) to lower thermal load on the regulator chip (yes, I realize that a whole transformer is a bit over the top here, but I’m pretty sure it would work really nicely). My candidate design is below. Comments? Criticisms? In addition, some questions.

https://photos.google.com/photo/AF1QipMTk-a-qw-W2EDNmJbQY2w7usUgA8_tpRgbWdcS

5) Is it worth it to use the more modern 767TLV chip? It looks like about the same cost either way.
6) Again, if I want to limit the dissipation in the chip itself, my design has a transformer. Could I instead just use the filament voltage with a bridge (and a series resistor)?

I’m leaning towards using the 767TLV with filament voltage, a full bridge, and a dropping resistor.

[booscrawl]

So I had a battery leak into my Heathkit IM-5228 vtvm. Cleaned it up and replaced the battery but it made me think about a battery eliminator (BE).

Why not just install a switch so you can disconnect the battery when it's not in use?

There are a lot of really great resources online. Of course, Mr. Carlson is one of them. Others are:
a) Dave Jones, of the Australian EEVBLOG (highly energetic discourse on diagnositic tools and general topics)
b) Mark Maher, of Mend-it-Mark (Expertly diagnoses and fixes consumer electronics both recent and vintage)
c) Jonn Teel, of Predictable Designs (Chip centric (Teel designed chips for TI) with good advice for product design
d) Hans Rosenberg, more good advice about designs, emphasizing high performance topics (e.g. eighth order filters and resonance loops forming between layers of multi-layer boards).

What is this section? Who is Mr. Carlson? Do you realize you're on the eevblog forum?

In one video, Teel points out chips that you shouldn’t use for current designs. One such chip is the LM317. Fine for one-offs, but for products being made on a Grand scale it has liabilities like a large (3V!) drop out voltage. So while Im sure that the LM317 design above (or some tweaked variant) would work, I started looking at other voltage regulator chips, such as TIs 767TLV. In designing a 1.5v supply based upon this I thought that I could take advantage of the lower drop out voltage and possibly use a 115vac to 2.5vac (center-tapped) to lower thermal load on the regulator chip (yes, I realize that a whole transformer is a bit over the top here, but I’m pretty sure it would work really nicely). My candidate design is below. Comments? Criticisms? In addition, some questions.
[/quote]

"My car (5 years old) needed an oil change a few weeks back. Is it overkill to spend $20,000 to replace the entire engine with one from a Prius?"

https://photos.google.com/photo/AF1QipMTk-a-qw-W2EDNmJbQY2w7usUgA8_tpRgbWdcS

Link doesn't work.

[studiot]

How about some details of the battery requirements ?

Try here, these guys really know their onions

https://www.vintage-radio.net/forum/forumdisplay.php?f=40


I used to just add a jack for an external mains adapter, but have been playing with rechargeables as they are now economically viable.

Just doing this for a Taylor 44 meter  (1960s )

[WizardOfBoz]

Thanks for the comments.

Why not just install a switch so you can disconnect the battery when it's not in use?

The battery is internal and degrades even when not in use. So the point would be to either create a non-battery, very steady  1.55v battery eliminator inside the VTVM, or to wire in a jack and plug an external battery case into the unit.  Both approaches avoid leaking battery guts on the VTVM internals as the thing sits unused on the bench.  And frequently checking the battery is a PITA:  12 screws hold this thing together!

What is this section? Who is Mr. Carlson? Do you realize you're on the eevblog forum?

Ah, yes.  I was confusing. I have been working on a capacitor tester on Paul Carlson's site, and so reflexively originally posted this on his site but got frustrated with the inability to add figures into the posts (see other post).  Namely, my LM317 and 767TLV schematics.  I was planning to ask here anyway so I cross posted but before doing so I should have edited that part. 

"My car (5 years old) needed an oil change a few weeks back. Is it overkill to spend $20,000 to replace the entire engine with one from a Prius?"

He he.  Well, I get hyperbolic rhetoric.  But in math, there are such things as ultrahyperbolic equations (it's true! look it up!).  I think your quote nails ultrahyperbolicity.   
Even for the most involved BE design, the parts are less than $(yankee)40, and if I use the existing transformer, less than $15 bucks even if I use the newer regulator chip.   Too, recall that I did mention I was doing overkill, mostly to learn stuff.   I'll try to get valid links for the schematics.

https://photos.google.com/photo/AF1QipMTk-a-qw-W2EDNmJbQY2w7usUgA8_tpRgbWdcS
https://photos.google.com/photo/AF1QipPQvZo_AO6WsuXSF7wXAydkg_NT6C52XlTaohje

Link doesn't work.

Hmmm, when I click on these the schematics opens up.  What are others experiencing? Broken links, or the schematics? The png's  are on google photos.  Is this site not linkable here?  I got the link and put it inbetween [URL][\URL]   Not the correct way?

I'll try to figure out where Im going wrong, so you all can see the schematics and the talk can be about electrical topics rather then website technique.

[ledtester]

https://photos.google.com/photo/AF1QipMTk-a-qw-W2EDNmJbQY2w7usUgA8_tpRgbWdcS
https://photos.google.com/photo/AF1QipPQvZo_AO6WsuXSF7wXAydkg_NT6C52XlTaohje

Link doesn't work.

Hmmm, when I click on these the schematics opens up. The png's  are on google photos.  Is this site not linkable here?  I got the link and put it inbetween ...   Not the correct way?

I'll try to figure out where Im going wrong, so you all can see the schematics and the talk can be about electrical topics rather then website technique.

You can't share that kind of link - it will only work if you are the owner and are logged into Google.

You have to click on the Share icon and then click on the "Create link" - this will allow anyone with the link to view the photo.

The sharable URL will look like: https://photos.app.goo.gl/{bunch of letters and numbers}

[WizardOfBoz]

I used to just add a jack for an external mains adapter, but have been playing with rechargeables as they are now economically viable.

Thanks.  You've added in a whole other chapter to the book of alternatives!  The vtvm tubes themselves run off of mains power, but for the resistance circuitry a battery or BE work best. I was originally drawn to a battery eliminator because I saw a proposed circuit (which was totally wrong*) in the online pdf of the IM-5228 user manual.  In correcting that schematic, I learned a bit.  So for the vtvm I will likely use a BE.

There's been some discussion on this site of adding a rechargeable battery to a VICI VC8145.  This would make a lot of sense - the power draw is very low and being able to move the whole unit to where it's needed is appealing.  And your point about economic viability applies.

*The 317 circuit shown in the pdf mixes up inputs and outputs.  Its a train wreck.

[WizardOfBoz]

Ahh.  I have to "create links" rather than give you a regular link. Do these work, guys?

https://photos.app.goo.gl/4oVqWahuKfWRc9uC9

https://photos.app.goo.gl/iVSMPECAmgoDi6X39

[robert.rozee]

the simplest (but not necessarily the best) solution would be a small relay powered by the heater transformer, with the relay contacts in series with the battery such that when mains power is off the battery is disconnected.


cheers,
rob   :-)

[ledtester]

Concerning the TLV767...

https://www.ti.com/lit/ds/symlink/tlv767.pdf

The adjustable version only comes in a WSON or HVSSOP package both of which require soldering the underside of the chip to a ground pad on the PCB presumably to help with heat dissipation. This makes it a lot more difficult to use versus a through-hole LM317.

[WizardOfBoz]

Robert, Thanks.  The problem I'm trying to solve here is that batteries leak, even when not in use.  So if the vtvm is not used for months (and it isn't - most modern DVMs have pretty good input impedence), I face the prospect of turning it on, seeing that it has failed, and having to take it apart (seriously Heathkit - were 12 screws necessary?) to remove the corroded guts of a battery, and to clean battery goo off the vtvm internals.   So I want the battery out of the box.  Several solutions would work:
1) install a jack in the vtvm and build a battery holder with a plug.  If the battery goes south it doesn't kill the vtvm and its easily replaced.  Cheap, elegant.  But you have to drill holes in the box.
2) Use a lithium C cell (or Lithium AA cell with adapter).  Less likely to leak.  Lithium cell more than adequate for the task.  But it can leak...
3) The battery eliminator.  No leak, should last longer than the vtvm, I get to tweak a design an build it.  Both educational and enjoyable.

BTW, speaking of learning...   the 371 design I posted above won't work.  I got fancy and added a full-wave bridge to the transformer filament secondary.  Since one of the filament secondary leads is grounded, AND the one lead of the full-wave bridge is grounded, this would be a great design to destroy the transformer.  Modification to be posted shortly...

[WizardOfBoz]

Here is a corrected, updated version of the LM317 battery eliminator, using a single diode to do half-wave rectification.  The previous full-wave design I had uploaded would burn out the transformer!  Please comment.

https://photos.app.goo.gl/nHLknciFXXYy4guL8

[WattsThat]

Heathkit issued a service bulletin for this in the mid-seventies, a simple half wave rectifier/filter and two standard 1N400x diodes in series as a Zener. Done.

I was a bench tech at the local Heathkit store at the time, we modified all the meters in the store along with my personal meters which are still working fine with the mod some fifty plus years later. Sure, it’s crude but more importantly, it’s a great example of classic Heathkit engineering, a solution that is appropriate for the problem. Simple, fast, cheap, effective.

Here’s a YouTube with the schematic at 5:30. Can’t get it to position, the start time is in the link, it just doesn’t want to work today…

[WizardOfBoz]

@WattsThat  Thanks.  This is brilliant.  I drew this up (link below).  In analyzing the design I found that a 33 ohms resistor seemed to fit the system better - is this possible?
I looked at the current vs voltage for 1N4001 (essentially the Shockley equation) and got the current for 0.75v drop.  One design case is with the vtvm set to low ohmage, measuring a very small resistance (highest current draw).  This is shown on the schematic.  Using nominal values I get 33 ohms as the best common resistor value, giving about 1.5vdc delivered.  For this, I am getting 40ma diode current, about 170ma through the range resitor, and about 1.4W dissipated in the 2W resistor.  The other design case is super high ohmage, so essentially an open test circuit and all current goes through the diodes.  For this I get 0.85v drop in each diode, about 20ma current, and the resulting voltage delivered is about 1.7V.

Given your experience (very cool!) and the fact that this was a factory approved change we know it works.  Another upside is simplicity and cost.  The downside is that the voltage varies a bit (between 1.5 and 1.7 vdc, depending upon range.  Practically speaking, the only change in measurement would be moving the ohms pot a bit more than with a battery.

Interesting that this solution is not more widely shared. Very cool and elegant.  Adding a switch to allow a battery to be used came to mind buy I'm not sure why one would need that.

I'm going to order parts. Probably get a few resistor selections. Here's the drawing:

https://photos.app.goo.gl/aF8Te9e8ih98xYnEA

[WizardOfBoz]

In looking at the sensitivity of the above circuit I see that in the lowest ohms setting the measured resistor will affect the voltage delivered by the circuit.  I'll have to do some calculatin' to determine how much.   The point being that if I zero the ohms indication with shorted probes, the circuit is applying the supply voltage to 33 (the BE circuit) + 9.1 ohms (the range resistor).  The current delivered is through both the range resistor and the diodes.   If I am measuring a 2, or 10 ohm resistor this changes the current and the voltage drop through the 33ohm resistor.  So I suspect that this expedient works, but may give a little wonky values for different low-ohms resistors.
Not sure how often my life will be impacted by what a vtvm (primarily useful as a very high impedance device) tells me about a 3.74 ohm resistor though.

[booscrawl]

The key question is, does the stability of the battery voltage even matter?

[WizardOfBoz]

The key question is, does the stability of the battery voltage even matter?

The technical points made online and in articles suggest that if its just a battery winding down (1.4v instead of 1.55), one can adjust for this using the ohms adjust.  But the question you ask is different.  What if the voltage delivered by the battery (or the eliminator) changes because the resistor you are measuring affects the current.    From the interweb:
"Load Variations: On lower resistance ranges (like RX1), the current draw can spike up to 150 mA. Simple, unregulated battery eliminators may sag under this heavy load, causing the voltage to drop and leading to non-linear or shifting scale accuracy across different multiplier ranges."
One could (if one had a precision resistor of value similar to that you are measuring) use that to set the ohms adjust.  That is,  say I am measuring a 4.3 ohm resistor.  If I had a precision 3.9ohm resistor I could put it under test and use ohms adjust to read 3.9.  Then I could trust the result a bit more than if I had zeroed the vtvm with shorted test probes.

My take-home on this is that if you want a cheap, easy, reliable battery eliminator, the Heath solution is fine.  The only thing would be some uncertainty in the Rx1 range.
If you want accuracy in that range, an LM317 circuit probably gives one more confidence in the low ohms results.
Given that we are probably using a DMM for low resistance values, and the VTVM for very high impedance measurements, I suspect that the heath solution is fine for most of us.

[WizardOfBoz]

Here is the result (LM317 per the schematic, except I used 3 dropping diodes, not 4).  A key point is that the back of the TO-220 package is aluminum and is connected to the ADJUST tab on the LM-317.   So it is NOT grounded and should not be.  So the heat sink shown is attached using a silpad and an isolation bushing. 

I didn't want to remove the battery cup, but a youtube video by Jeff Tranter showed me that this was the cleanest approach.  Jeff's video link is below.

Pic below

https://photos.app.goo.gl/sWRDPYWnW7Ayg3tE7