"battery eliminator" question

[John]

I am making this for my old Heatkit V-7, to replace the C-cell battery. My question is, at the end of the string, is there anything wrong with putting in a high value cap in place of the .047? I am thinking it would take out *even more* ripple, but would it create problems instead? If mods want to move this to the tools section, that's fine; I just know this is where all the eyeballs go.    And yes, I realize I'll rarely actually use the vtvm to measure ohms, but it is nice for checking pots. And it's just so cool.

[sluckey]

I gotta ask why??? It just doesn't get any neater or more compact than a "C" cell, and the DC is just sooooo smoooooth.

[John]

I had a bet with myself that you'd ask that!!  The reason being is that my "shop" is an old milk house that gets cooolllld in the winter, and steenking hot in the summer, and I'm afraid of leakage when (not if) I forget to change the battery on a regular basis. The insides are so nice and clean, I'd cry if I had a battery leak over stuff. Especially since I just got done re-capping it, replacing drifted resistors, and calibrating.

[scrimpus]

I think that .047 is there to dump HF noise not to smooth AC ripple.

[John]

That would make sense. I have room on my little board, I might add another smoothing C/R before the regulator.

[jjasilli]

I gotta ask why??? It just doesn't get any neater or more compact than a "C" cell, and the DC is just sooooo smoooooth.

It's easy to forget about the batteries which can result in damage by corrosion. (I've acquired some VTVM's in such condition)

[sluckey]

The reason being is that my "shop" is an old milk house that gets cooolllld in the winter, and steenking hot in the summer,

I have the same problem. Well, maybe not as cold but surely hot and humid on the gulf coast. I don't leave my test equipment in my shop. It stays in my house until I need it. But then, I'm just a hobbyist and don't need my TE every day. I still would not do what you are considering doing. I'd just put a sticky note on the calendar. Some people might set a reminder announcement on their smart phone. Lot's of ways to remember it's time to check the battery. But then, I've just retired from a 40 year career where scheduled maintenance is a way of life, batteries included.   

[eleventeen]

I would move one of the 1000 ufs to the output side of the regulator. The diode from output to input is not needed. Make sure you locate those caps very close to the chip. Indeed, the preferred method I saw when I was more involved with this was to solder tantalum caps right onto the leads of the regulator.

Can an LM317 go all the way to zero? I think not, I believe the lowest voltage is 1.2 volts. Probably doesn't matter, but your adjust pot will likely be all the way to one side.

[John]

But then, I'm just a hobbyist and don't need my TE every day

Hah. If you're a hobbyist then I know don't know what to call myself. (I'll ask my wife, she'll know) The milkhouse is attached to the barn, not the house. I am way too lazy to drag my stuff back n forth.

Since I've already got the parts, and decided how to lay it out I'll give it a shot. Gonna test it with the battery first, and then just clip in the power supply to test. I'll follow up, good or bad.

[sluckey]

I do understand. Maybe you should put the sewing room in the milkhouse and bring all that fragile electronics stuff into the house. I know your wife will understand.   

On a side note, I was just thinking. It's been over 50 years since I milked a cow (by hand). Are you still doing that?

[John]

Nope, we sold the herd in 1990. It was a small herd, we had the milk buckets w/ dump station. Only had to milk by hand in order to get mastitis out, that kinda thing. The milkhouse was used for many things after we sold them. I claimed it a couple years ago for this stuff. I ain't givin' it back.

[HotBluePlates]

... My question is, at the end of the string, is there anything wrong with putting in a high value cap in place of the .047? ...

I would move one of the 1000 ufs to the output side of the regulator. The diode from output to input is not needed. Make sure you locate those caps very close to the chip. ...

I would advise against going above 1uF on the output cap. My understanding (which you'll want to double-check) is that a small-value cap helps the regulator operation at high frequency (not for smoothing). I have heard that this cap should be mounted very close to the regulator, as long leads to the cap make the output unstable.

Look at page 7 and beyond in the LM317 data sheet for explanations on parts and sample circuits.

Also, there's very, very little current draw from the C battery (or this battery eliminator) making big caps after the regulator unnecessary. Even the battery lasts so long that in my old RCA VoltOhmyst, there aren't battery clips but wires soldered directly to the + and - terminals.

[John]

I would advise against going above 1uF on the output cap. My understanding (which you'll want to double-check) is that a small-value cap helps the regulator operation at high frequency (not for smoothing). I have heard that this cap should be mounted very close to the regulator, as long leads to the cap make the output unstable.

More good info, thanks!
I will be working on this over the weekend. I'm using veroboard, so everything is nice and tight. This will be my first experience with that too, come to think of it. If it doesn't turn out too ugly, I'll take a couple pics.

[Heinz]

Now please don't laugh...what about using an external battery box? If the battery leaks you disconnect it and throw it away.

Or, to add to the cool-factor, use a little solar panel and a 1.6V zener diode...

[John]

Not laughing. Well, except at the little solar panel. I ain't smart enough to rig one up!

[John]

Okay, it's done, tested, and installed. Attached are the pictures. Can't attach, will try later.  Naturally, the top looks wayyy better than the underneath. Thankfully, I thought to look up the pin-out numbers on the LM (and naturally, only AFTER I had everything soldered in) so re-did that. It works really well. I tested the meter with the battery installed first, so I'd have a bench mark. With both battery and the BE, it's not accurate at 10 ohms and under, it's off by 50% or more. However, anything over that and it's right on the money, from 47 ohms on up. The needle is rock solid, no sign of ripple. The only thing different than the schematic is the 270 ohm resistor is a 470.

This was my first go with the vero board. I *thought* I knew how to solder before I did this project. I ended up plugging my iron into an AC supply to drop the volts, and then things went nicer (I didn't lift any more pads off the board). Thank goodness (and Dad) for my magnifying goggles.

To sum up: it ain't so purty, but it works well, and I'm glad I took the time to do it. I enjoy re-furbing old test equipment, it's just satisfying making something that's my age like new again.

[John]

Top.

[John]

Bottom (cause I got no pride lol)

[PRR]

> take out *even more* ripple

The Ohms is the DC Volts mode (plus C-cell and resistor). Ripple is filtered into the DCV mode (R-C at tube grid) and also by the needle. Ripple alone is NOT a problem.

> it's not accurate at 10 ohms and under

It really flippin should be. My reggie was solid down to "zero" ohms.

Ah. You do NOT want that 10 Ohm in the pi filter. And you probably want more than 1,000uFd.

The load on the 10-Ohm mid-scale range can be up to 1.5V/10= 0.150 Amps. You might think that 1uFd per mA would do. But you use a half-wave rectifier, so at least double it. And there's resistance in the heater winding. And other losses.

Use your DMM to probe the voltage across the LM317 input pins while reading a short on the lowest-R range. Taking 1.56V out and 2V dropout in the '317 you *must* have 3.5V into the '317.... at the dip of the ripple wave! If dip goes lower than that, the '317 outputs 1.56V when it can, and less when it must (because the input is dipped). The VDC mode reads the "average", which becomes lower than 1.56V when the load is high (low-R readings).

Read a short for a few minutes and then touch the '317. If you say "ouch", it needs more heatsinking. (If it reads OK at first and then drops to zero, it NEEDS more sinking because it is cutting-out from the heat.)

I recall going to a single 4,700u input cap, though that may have been larger than realy needed.

The 1N914 is small for a 0.05u output cap and MUCH too small if you (pointlessly) use a large output cap. True, in some sense it may not be needed; you need it on broeadboard or in testing when the Input may get shorted and a large Output cap would be drained through the '317 *backwards*. Sealed inside a Heath box that may be unlikely.

[John]

Printed out, thanks!

I'll jump that 10 ohm resistor, and also test the LM for how hot it gets, and voltage. If I see it needs more attention, I may just build another one with bigger caps before the LM, heat sink if needed. It will give me some practice with vero board anyway.

Now that the V7 is working at 99%, I find I'm using it for most everything. I need a couple more!