VTVM Battery Eliminator
An Improved Battery Elminator to Replace Leak-Prone, Unreliable, Dry Battery.
Issues and the Reasons for Replacing the Battery
One of the disheartening things about buying an older (…well, they’re ALL older, now…) VTVM is opening it up and finding either an old, leaky battery, or evidence of one having leaked in the past. This usually damages or destroys the battery contacts and sometimes also the circuit board or other components in the VTVM. The battery, usually a standard 1.5V C-cell, is necessary for measuring resistances with the ‘ohms’ scale.
A great improvement has been suggested over the past couple of years to replace the battery entirely, using a modern voltage regulator, drawing power from the filament circuit. A version of this has been available on several Heathkit lists and I’ve used it in my V-7A and IM-18. This replaces the battery and removes the risk of leakage.
Among the benefits of a Battery Eliminator are 1) it never gets weak, 2) it never leaks, 3) resistance measurements will be consistent with no battery voltage decay, and 4) you'll never have to open the cabinet to replace a bad battery just at the time you'd like to make a resistance measurement.
However efficiency issues with this circuit were discussed on the AntiqueRadios.com Forums (ARF), and appeared in an excellent project article in Popular Communications by Peter Bertini.
Efficiency of Half-Wave Rectifier
Rectifier Efficiency is defined as the percentage of input AC Power that is converted to DC Input Power, or
dc power delivered to the load / ac input power from transformer secondary = Pdc / P ac
From the reference below, we can calculate the efficiency of a half-wave rectifier:
- Excessive Ripple (at 60hz)
- Low Rectification Efficiency of about 40.6%
- Transformer Utilization Factor (TUF) is 0.286
- Current flows only one way in the transformer core, increasing loss to saturation.
Efficiency of Full-Wave Rectifier
- Output is higher for a full-wave rectifier (2Im/π).
- Ripple factor is lower (~0.48 compared to ~1.21).
- Efficiency is ~81% compared to ~40.6%.
- Transformer Utilization Factor (TUF) is 0.693 compared to 0.286 for the half-wave.
- Current flows both ways, reducing loss to transformer core saturation.
Although ripple is diminished in a full-wave rectifier, it is at 120hz rather than 60hz. For our purposes, this is a good thing, since (see LM317 Datasheet), ripple rejection is at over 80db when ripple frequency is around 120hz - See Figure: "Ripple Rejection (db) vs Frequency (Hz)", Page 7, LM317 Datasheet.
An Improved Circuit
If, in the lowest range of ‘ohms’ mode - RX1 - the probes are shorted together, the entire 1.55 Volts is placed across a 9.1Ω resistor, resulting in the maximum current draw of (I=V/R), or 1.55/9.1 = 170ma. This doesn’t count diode loss and heat loss in the voltage regulator. So, to reduce the stress on the filament windings on the transformer we really want to use the mose efficient circiut possible, and that would require the more efficient full-wave rectifier.
In addition, we can take an opportunity to provide additional ripple rejection, according to National Semiconductor's LM317 Datasheet and Application Notes. We want to model a standard C- or D-cell as closely as possible to provide a good replacement for the battery used in testing resistance.
The input is from the 6.3 VAC filament circuit. The output goes to the same locations as the original dry-cell battery. A schematic in ExpressPCB CAD is supplied, as well as a BOM of typical components in XLS and Text. There's a problem with this schematic, however, with regard to Heathkit VTVMs which have one side of the 6.3VAC filament supply circuit grounded.
In the case of a Heathkit V-7A, I haven't found a clever way around this, as the ground isn't easily separated from the filament circuit — it's in the PCB. Fortunately, if the less-optimal, half-wave version is used (above), the +1.55 VDC connects to the ‘free end’ of the 9.1 Ω resistor (see a schematic for an Heathkit V-7A at http://www.heathkit.nu/heathkit_nu_V-7A.html). There's no need to supply the DC Ground, as it's already been connected via the Filament circuit.
Addendum: Battery Elminator Current Draw (September 2014)
While converting a Knight-Kit KG-620 for battery-free operation, I was able to measure the current draw. Instead of an LM317 (1.5A) regulator, I substituted an LM317L - 100mz,
current-limited, regulator. Pinout is the same in the TO-92 case: Adj, Out, In. Current was measured at the input to the battery eliminator.
Idle current is measured around 5 ma and peak current at around 95 ma with shorted terminals, and range switch to Rx1. Current limiting would probably curtail measurements on the low-end (below 100 ohms).
A second way to address current draw is as follows: the KG-620 has a Type #47 lamp, drawing 150ma. Replacing this with a LED will recover any current used by the battery eliminator by reducing the lamp current by 10x from 150ma to around 15ma. This would also work for the Heathkit V-7A and IM-18 (and variants) as the pilot lamp is also a 6.3V, Type #47 lamp across the filament supply. Added benefits: a) reduced heat, b) reduced transformer load, c) never have to replace the pilot lamp again.
- Peter Bertini, "Fixing Up A Vintage Heath IM-13 VTVM", Popular Communications, March 2010.
- AntiqueRadios.com, forums - Search "VTVM battery draw..."
- LM117/LM317A/LM317 3-Terminal Adjustable Regulator (Datasheet), National Semiconductor
- 3-Terminal Regulator is Adjustable (Application Note 181), National Semiconductor, Figure 2. Adjustable Regulator with Improved Ripple Rejection.
- The V.T.V.M.: How it Works, How to Use it , Rhys Samuel, Gernsback Library 1956.
- Servicing Radio and Television with a Vacuum-Tube Voltmeter, an excellent, 1951 document from Sylvania Electric Products.
- AD5X's method of turning Type 47 Lamps into LED Lamps, saving heat & current