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 Post subject: LM307 Op Amp Power Voltage?
PostPosted: Nov Mon 13, 2017 7:11 pm 
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Recently I got a 1970's Bounty Hunter metal detector that has an LM307N as part of the circuit and the IC power pins are connected directly to the PC board power input. As found, the machine had a single 9V battery hand-spliced into the only two power wires and the original battery holder(??) is missing. Online references to these old machines talk about AA batteries, battery holders, single and double 9V batteries.

The Data Sheet for a LM307 shows under "Absolute Maximum Ratings" a Supply Voltage of +/-18V. Given that I know the power wires run directly to the IC does this mean it should have two 9V batteries in series?


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 Post subject: Re: LM307 Op Amp Power Voltage?
PostPosted: Nov Mon 13, 2017 7:22 pm 
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It can operate with just one 9V battery. Its common to have a voltage divider in the circuitry, to create a "ground" (circuit common) midway, so it looks like + & - 4.5V. Also, it need not be symetrical. Could be +6 and -3, or any combination summing up to the available battery.

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 Post subject: Re: LM307 Op Amp Power Voltage?
PostPosted: Nov Mon 13, 2017 7:47 pm 
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threeneurons wrote:
It can operate with just one 9V battery.

So you think it may be wired to work OK as is?

Also, is +/-18V really an "Absolute Maximum Rating" or was this just a convenient spot in the data sheet to list the supply voltage?


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 Post subject: Re: LM307 Op Amp Power Voltage?
PostPosted: Nov Mon 13, 2017 10:22 pm 
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Most likely will work. These metal detectors work with a sensor coil which part of an LC tuned circuit. A fairly simple circuit. I've not used the LM307, myself, but used op amps, in general quite often. You really want to stay clear of those "Absolute Maximum Values". They are what mean. Run the the part with a decent margin under that. The primary concern is that the circuit has enough "head room", for the intended signal to swing. For most op amps, that means the signal most positive and negative excursions (both input and output), should stay at least 2 volts away from the power supply rails. For a + & - 4.5v supply that means a signal that stays within +2.5 to -2.5v.

As time progressed, new parts came along. In the late 70's the "single supply" op amps came out. The two most known are the LM358 and LM324. "Single Supply" is salesman hoopla. The op amps can be used just like any other op amp. Of course within their specified limits. The real technical difference with these types is that the low side input and output can come within millivolts of the negative supply. Be that supply -12V or ground ( 0 volts). Its salesman jibberish because its application that determines the supply limits.

Sometime in the 80s, real "rail-to-rail" op amps came out. These can truly operate across their supply range. BUT most are low supply ratings, so no +/-15V supplies. They are handy though within their limits.

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 Post subject: Re: LM307 Op Amp Power Voltage?
PostPosted: Nov Mon 13, 2017 11:11 pm 
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The original power source could have been just about any combination of batteries that would fit in the body of the detector.

Hook it up to a variable power supply and monitor the +Vcc and -Vcc supply pins going to the LM307, and see what the voltages are as you are bringing it up. 15 Volts is about the max for most op amps. If the device has a sensitivity control, turn it up to about midrange when you do this.

If the unit works, you should be able to find a "sweet spot" minimum voltage where the device exhibits the desired sensitivity to metallic objects without being so crazy sensitive that it picks up cyclone fences 25 feet away. That is the voltage to use (or close enough) for regular usage.


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 Post subject: Re: LM307 Op Amp Power Voltage?
PostPosted: Nov Tue 28, 2017 7:58 pm 
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Location: Lexington Kentucky
Thanks for the information.

Now lookee what I found! This is essentially the same unit. Turns out it used 8 AA batteries in a holder that accepts a 9V battery clip. I rigged up a 9V + 2 AA's to get 12V and it works! A similar battery holder is now on order.

https://www.ebay.com/itm/VINTAGE-BOUNTY ... 7675.l2557

Also exactly what I have with two replacement battery packs

https://www.ebay.com/itm/Vintage-Bounty ... Swm2xZl3nl


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