In many of the string heater electronics like AA5 radios for example, there really isn't room to put a power transformer. Wouldn't just using a fuse, say 500mA be just as safe as a transformer? Also what would be the best way to add the 3 prong ground since they always ground the chassis directly to the neutral. Would I just disconnect all the chassis grounds from the original circuit and then ground only the chassis to the plug ground?
No a fuse is not going to provide the required isolation to make an AA5 safe. You need an isolation transformer, and it can be external to the radio if need be.
Working on or using such a radio without an isolation transformer is potentially hazardous, and discussion in an instance like this is also outside of forum rules.
Working on or using such a radio without an isolation transformer is potentially hazardous, and discussion in an instance like this is also outside of forum rules.
Thanks, sorry not trying to violate any rules, just wanted to ask before i did something unsafe. I am a little confused on this though, wouldn't a fuse blow before someone was electrocuted? I am not going to try this now that I have been educated, at this point I'm just curious for the explanation of why this wouldn't work.
Typically the current required to hurt someone is just a few mA at line voltage and your fuse is rated at much higher currents and takes a finite amount of time to blow. It provides absolutely no protection against electrocution.
Hi Blaxshep,
A fuse will not keep you safe. A few mA can kill you stone dead. You absolutely must use an (inexpensive) isolation transformer. I use one on my bench even for low voltage, low power things like preamps, all the way up to the larger tube amplifiers.
There is an excellent reason why this type of construction was made illegal. Many consumers died due to the live chassis. I've seen test equipment destroyed by live chassis' and many serious shocks to technicians. It might be inconvenient you might think, but being dead or having a gash on your arm due to shock is somewhat more inconvenient - wouldn't you say?
You cannot connect the common to ground in an AC power connection. For one, outlets have been known to be wired improperly, and you create a ground loop at the very least. Please don't do that.
Go buy an isolation transformer. Either on Ebay, you can get them with a variac built in also (extremely handy). Look for something like a B&K 1655 if you can find one. It's worth it's weight in gold and safely isolates the AC mains for you. I don't know where you are, so I don't know what market you are in. Consider filling in your location information, at least the country.
-Chris
A fuse will not keep you safe. A few mA can kill you stone dead. You absolutely must use an (inexpensive) isolation transformer. I use one on my bench even for low voltage, low power things like preamps, all the way up to the larger tube amplifiers.
There is an excellent reason why this type of construction was made illegal. Many consumers died due to the live chassis. I've seen test equipment destroyed by live chassis' and many serious shocks to technicians. It might be inconvenient you might think, but being dead or having a gash on your arm due to shock is somewhat more inconvenient - wouldn't you say?
You cannot connect the common to ground in an AC power connection. For one, outlets have been known to be wired improperly, and you create a ground loop at the very least. Please don't do that.
Go buy an isolation transformer. Either on Ebay, you can get them with a variac built in also (extremely handy). Look for something like a B&K 1655 if you can find one. It's worth it's weight in gold and safely isolates the AC mains for you. I don't know where you are, so I don't know what market you are in. Consider filling in your location information, at least the country.
-Chris
Hi Blaxshep,
-Chris
Perfect, use them. If this is a permanent installation of an old radio, then you can use it as it was manufactured. I have several radios from the 1920's through to the 1960's and use them. Just make sure that any exposed hardware (screws, knobs, trim) are isolated from the chassis. You should be fine doing it that way.
-Chris
This is a very informative, yet interesting discussion, given that most if not all the people I have watched on restoring old radios simply replace all the caps and then sell them without adding a transformer.
What is the proper way to add the grounded plug? Hot and neutral go to the circuit and the added ground goes to the chassis and nothing else?
What is the proper way to add the grounded plug? Hot and neutral go to the circuit and the added ground goes to the chassis and nothing else?
Ground and Neutral are only joined at the Mains Power Panel (fusebox or circuit breaker box).
The voltage between them is 0V (only 0V all the way back at the mains power panel).
Ground is only there for 2 things:
1. Safety
2. Noise reduction
Ground is never supposed to have to pass power current.
It is there to pass current of unsafe connections, bad equipment, or other failure.
It also assists in reducing some types of noise or EMI.
Because the Hot and Neutral wires pass equal current, and they use the same wire gauge, the voltage drop in each wire is the same.
Suppose the mains power panel has 130V, and the drop due to the load current is 5V on the Hot wire, it is also 5V on the Neutral wire. The load receives 130V - 5V - 5V = 120V.
But that means that at the user device, there is 5V difference between Neutral and Ground at the power outlet (not the Mains power panel).
I have seen as much as 8V difference between Neutral and Ground at the user outlet. Never connect Neutral to Ground.
That also means you must not connect a scope probe ground clip to Neutral.
I have seen that condition, and have seen the scope ground clip wire insulation smoke.
The little 22 gauge scope probe ground clip wire was unable to make 8V from a 12 gauge Neutral wire become 0V by shorting Neutral to Ground.
The voltage between them is 0V (only 0V all the way back at the mains power panel).
Ground is only there for 2 things:
1. Safety
2. Noise reduction
Ground is never supposed to have to pass power current.
It is there to pass current of unsafe connections, bad equipment, or other failure.
It also assists in reducing some types of noise or EMI.
Because the Hot and Neutral wires pass equal current, and they use the same wire gauge, the voltage drop in each wire is the same.
Suppose the mains power panel has 130V, and the drop due to the load current is 5V on the Hot wire, it is also 5V on the Neutral wire. The load receives 130V - 5V - 5V = 120V.
But that means that at the user device, there is 5V difference between Neutral and Ground at the power outlet (not the Mains power panel).
I have seen as much as 8V difference between Neutral and Ground at the user outlet. Never connect Neutral to Ground.
That also means you must not connect a scope probe ground clip to Neutral.
I have seen that condition, and have seen the scope ground clip wire insulation smoke.
The little 22 gauge scope probe ground clip wire was unable to make 8V from a 12 gauge Neutral wire become 0V by shorting Neutral to Ground.
That is not the purpose of a fuse. A fuse should blow if there is a fault in the equipment which would make it dangerous.
I query the safety ground is useful for noise reduction, can you provide more information please?
So in the 2 pronged old equipment the neutral is attached to the chassis and the ground should never be used and in newer equipment the neutral is connected to the circuitry and NOT attached to the chassis and the ground is attached to the chassis only? Do I have this right? Is this how the ground helps reduce EMI?
Hi Blaxshep,
No. 2 prong equipment is supposed to be double insulated. In three wire equipment, the AC ground is for personal safety and has nothing to do with circuit performance unless done wrong where it causes problems.
Protection ground doesn't protect you from EMI. For old radios such as yours, the common connects to the chassis and also the audio and RF ground. That is what reduces EMI problems. Neutral has zero to do with the safety ground.
-Chris
No. 2 prong equipment is supposed to be double insulated. In three wire equipment, the AC ground is for personal safety and has nothing to do with circuit performance unless done wrong where it causes problems.
Protection ground doesn't protect you from EMI. For old radios such as yours, the common connects to the chassis and also the audio and RF ground. That is what reduces EMI problems. Neutral has zero to do with the safety ground.
-Chris
There are too many types of equipment that use 2 wire power. Some early ones in the US did not have any difference in the blades that plugged into the wall power outlet. That meant depending on which way you inserted the plug, some would have the instrument 'common' (or cap from power wire to chassis) could be connected to Neutral, or could be connected to Hot. Also, even for early radios like my 1938 "Cathedral" Philco, it did have a real power transformer, with B+ and filament windings. But there was no 3 wire power cord, and the 2 plug blades were identical, allowing insertion into the power outlet either way. The primary winding had capacitance to the laminations, and so the chassis was elevated according to the capacitive currents.
Many modern CD players and Tuners with 2 wire plugs often have different blades so the plug can only be inserted one way, but some have 2 wire plugs with identical blades. And if the 2 wire cord is removable at the equipment end, some have mechanical features at the equipment end that only allow the plug to be inserted in the equipment one way; others do not have such mechanical features. Turntables have the same 2 wire plugs, some with different blades, and some with identical blades.
Many of these modern CD players and Tuners have switcher power supplies. The power supply likely (and hopefully) has a switcher transformer that isolates the chassis from Hot and Neutral, or has a captivated power cord or is mechanically polarized at the equipment, and a plug with different blades to only allow one-way insertion into the wall outlet. But making sure that one wire on the power plug is connected to Neutral does not solve the differential voltage between ground and neutral at the wall power outlet.
I test all my house outlets with one of those neon lamp safety testers. I look at the outlets with a DMM just to be sure.
All my tube power amplifiers have 3-wire IEC sockets on the chassis and 3-wire cords that I have personally tested with a DMM (I have found some that were wired wrong, so I cut the heads off at both ends, and sent them off to the garbage).
One thing you should consider is testing all your Hi Fi an radios, etc. and all your wall power outlets, as well as all your household appliances.
Many countries other than the US have different power outlet particulars. But they ought to be tested too.
One thing that can happen is that the ground wire can corrode or come loose at the outlet, or back at the power panel.
A little caution can go a long way for safety. Even if a radio or amplifier is isolated from the power, you still have differential voltages, say from the chassis to B+. Always be careful, and keep one hand behind your back.
EMI is another subject, very complex, and should be covered elsewhere. It is both an art, and a science. But devices that pass EMI regulations and use a 3-wire power cord rely on the ground connection to enable it to get rid of certain EMI modes so that it can pass (i.e. CE and FCC regulations). A desk top computer is a good example.
Many modern CD players and Tuners with 2 wire plugs often have different blades so the plug can only be inserted one way, but some have 2 wire plugs with identical blades. And if the 2 wire cord is removable at the equipment end, some have mechanical features at the equipment end that only allow the plug to be inserted in the equipment one way; others do not have such mechanical features. Turntables have the same 2 wire plugs, some with different blades, and some with identical blades.
Many of these modern CD players and Tuners have switcher power supplies. The power supply likely (and hopefully) has a switcher transformer that isolates the chassis from Hot and Neutral, or has a captivated power cord or is mechanically polarized at the equipment, and a plug with different blades to only allow one-way insertion into the wall outlet. But making sure that one wire on the power plug is connected to Neutral does not solve the differential voltage between ground and neutral at the wall power outlet.
I test all my house outlets with one of those neon lamp safety testers. I look at the outlets with a DMM just to be sure.
All my tube power amplifiers have 3-wire IEC sockets on the chassis and 3-wire cords that I have personally tested with a DMM (I have found some that were wired wrong, so I cut the heads off at both ends, and sent them off to the garbage).
One thing you should consider is testing all your Hi Fi an radios, etc. and all your wall power outlets, as well as all your household appliances.
Many countries other than the US have different power outlet particulars. But they ought to be tested too.
One thing that can happen is that the ground wire can corrode or come loose at the outlet, or back at the power panel.
A little caution can go a long way for safety. Even if a radio or amplifier is isolated from the power, you still have differential voltages, say from the chassis to B+. Always be careful, and keep one hand behind your back.
EMI is another subject, very complex, and should be covered elsewhere. It is both an art, and a science. But devices that pass EMI regulations and use a 3-wire power cord rely on the ground connection to enable it to get rid of certain EMI modes so that it can pass (i.e. CE and FCC regulations). A desk top computer is a good example.
Hi 6A3sUMMER,
Your left hand goes in your pocket. This is to prevent current from going through your heart. If your hand is in your pocket, it is more difficult to pull it out to lean one something, often the chassis of the unit you are working on.
New two wire products (like power drills for example) are "double insulated". That means you can't come in contact with a metallic part of the appliance in the event that the insulation fails in a live part. Normal 3 wire products absolutely must have any electrically conductive surface you can come into casual contact with connected to the safety ground pin directly. This connection is often specified to be close to the power entry point and be bolted/screwed to the chassis. The chassis must be solidly connected to ground so that it is unlikely for it to fail, so if it is a loose push on connector, you had better make certain it is a sound connection when you are done.
Ground loop control is normally done between the circuit ground point (let's call it circuit common to avoid confusion) using a resistor. Typically this resistor is 100 ohms but I have seen as low as 10 ohms. It can be easily burned out by contact with any potential and chassis ground. If a speaker common wire is disconnected and touches the chassis, it may quickly burn out that resistor. This can result in a DC offset across the speaker output terminals, or other odd problems. A close lightning strike will typically take these out too, along with a number of common (ground remember?) traces.
Don't mess around with the electrical code and connections between the mains and your equipment. The ground is there to save your life.
Old equipment with 2 wire cords often have a capacitor from the chassis to one side of the AC supply. These can easily short or leak enough to kill someone. The best thing to do is replace them with a capacitor rated for this position. I can't remember if that would be an X or Y rated part. I'm sure someone can clear that up. Anyway, that capacitor only exists in equipment with a power transformer. Appliances with a live chassis are already connected directly to one side of the AC line.
Some very early equipment has a screw base instead of a plug. This was meant to be screwed into a lamp socket (! really) for power. They existed because early houses were wired for lights and did not have the familiar wall outlet. Cool eh?
-Chris
Your left hand goes in your pocket. This is to prevent current from going through your heart. If your hand is in your pocket, it is more difficult to pull it out to lean one something, often the chassis of the unit you are working on.
New two wire products (like power drills for example) are "double insulated". That means you can't come in contact with a metallic part of the appliance in the event that the insulation fails in a live part. Normal 3 wire products absolutely must have any electrically conductive surface you can come into casual contact with connected to the safety ground pin directly. This connection is often specified to be close to the power entry point and be bolted/screwed to the chassis. The chassis must be solidly connected to ground so that it is unlikely for it to fail, so if it is a loose push on connector, you had better make certain it is a sound connection when you are done.
Ground loop control is normally done between the circuit ground point (let's call it circuit common to avoid confusion) using a resistor. Typically this resistor is 100 ohms but I have seen as low as 10 ohms. It can be easily burned out by contact with any potential and chassis ground. If a speaker common wire is disconnected and touches the chassis, it may quickly burn out that resistor. This can result in a DC offset across the speaker output terminals, or other odd problems. A close lightning strike will typically take these out too, along with a number of common (ground remember?) traces.
Don't mess around with the electrical code and connections between the mains and your equipment. The ground is there to save your life.
Old equipment with 2 wire cords often have a capacitor from the chassis to one side of the AC supply. These can easily short or leak enough to kill someone. The best thing to do is replace them with a capacitor rated for this position. I can't remember if that would be an X or Y rated part. I'm sure someone can clear that up. Anyway, that capacitor only exists in equipment with a power transformer. Appliances with a live chassis are already connected directly to one side of the AC line.
Some very early equipment has a screw base instead of a plug. This was meant to be screwed into a lamp socket (! really) for power. They existed because early houses were wired for lights and did not have the familiar wall outlet. Cool eh?
-Chris
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