EEVBlog Bryman786 Differences in measuring AC vs DC Current

[NY2KW]

I was measuring the current through a vintage radio antenna relay coil (8 ohm per manual) with my EEVBlog BM786.  The circuit is a 24vac transfomer (60Hz) into a bridge rectifier without any filtering caps or inductors.  The BM786 reads the rectifier hot lead as no DC voltage wrt GND but shows 12vac wrt GND.  Similarly placing the BM786 in series with the circuit, it reads no DC current but shows 0.7A AC current.    Without filter caps I asssumed the meter sees the half-waves as AC but wrt current, I thought for most DMMs, DC or AC current was measured as the voltage drop aross an internal shunt resistance.  Shouldnt the meter in either AC or DC current postions give me a similar reading with low freq AC such as 120Hz?

Thank in advance

Jerry

[Solder_Junkie]

Rectified AC without a smoothing capacitor is going to be a problem as it is neither smooth DC or the original AC sine wave. An analogue meter may give a better reading than a digital multi-meter.

SJ

[ejeffrey]

Yes it should be reporting the average voltage / current on DC operating modes.  For fill wave rectified 24 VAC, the DC voltage should be about 20 VDC I think.  If it doesn't that's a problem with the meter or the measurement setup.

[floobydust]

Your circuit is 24VAC -> bridge rectifier -> 8Ω relay coil for around 3A.  What kind of diodes? Silicon, selenium etc.?
It kinda looks like your rectifier is pooched, shorted or damaged diodes 

With no load (and no filter cap), the rectifier diodes' leakage current can cause ghost AC voltage to be present.
Are you using LowZ to measure that? Oh wait 786 does not have that feature maybe. Or use a 10k resistor load at the bridge output.

Pulsing DC is not AC.
I don't know if the BM786 is AC-coupled or DC-coupled on ACV function. Fluke 87 and others are AC-coupled (have a series blocking capacitor) so it ignores any DC component that could be present. Other multimeters are DC-coupled, which can saturate the front-end and give bogus AC readings.
I think AC A is DC-coupled. You have to know your multimeter. I hope this is not too confusing.

Usually these circuits will have RF filtering a small film cap say 0.22uF to moderate voltage spikes and protect the rectifier diodes and lessen mains hash/EMI that can end up close to antenna circuits.

[joeqsmith]

I was measuring the current through a vintage radio antenna relay coil (8 ohm per manual) with my EEVBlog BM786.  The circuit is a 24vac transfomer (60Hz) into a bridge rectifier without any filtering caps or inductors.  The BM786 reads the rectifier hot lead as no DC voltage wrt GND but shows 12vac wrt GND.  Similarly placing the BM786 in series with the circuit, it reads no DC current but shows 0.7A AC current.    Without filter caps I asssumed the meter sees the half-waves as AC but wrt current, I thought for most DMMs, DC or AC current was measured as the voltage drop aross an internal shunt resistance.  Shouldnt the meter in either AC or DC current postions give me a similar reading with low freq AC such as 120Hz?

Thank in advance

Jerry

No

Does "wrt" mean With Respect To?  So in the drawing, what you call hot lead is the positive side of the bridge and GND is the negative side?
If that seems correct, do you understand that your meter can read AC, DC and AC+DC and what these mean? 

If I assume the drawing matches your setup and you measure 24V at the secondary of the transformer,   I would expect the 786 will read roughly the following at the output of the bridge:

20.3 volts DC
9.8 volts AC
22.6 volts AC+DC

If you place your 786 in series with the 8 ohm load to measure the current as shown, I expect it will read roughly

2.5 amps DC
1.2 amps AC
2.8 amps AC+DC

Of course, my math may be wrong.   

[joeqsmith]

A quick sanity check with the BM789.   Your 786 should behave the same. 

Load_30_12_8p6ohm:  I did not have an 8 ohm power resistor.  Rather I used a 30 // 12 which should get us 8.6 ohms.  Measure 8.75.

Setup_AC_Secondary:  Basic setup.  Auto transformer used to trim the 24V secondary with load attached.  Measured 23.9V.   
Large black block is a three phase full wave bridge, one input floating.

Setup_DCV:  Setup to measure the output voltage of the bridge, across the load.   

Volts...:  Measured 22.6V AC+DC, 10.5V AC and 20V DC

Setup_DCI: Setup to measure the current through the load.

Current...: Measured 2.7A AC+DC, 1.2A AC and 2.4A DC

A bit of a swag but in the ballpark.   Getting that ham license all those years ago and learning ohms law really paid off....

[NY2KW]

Does "wrt" mean With Respect To?  So in the drawing, what you call hot lead is the positive side of the bridge and GND is the negative side?
If that seems correct, do you understand that your meter can read AC, DC and AC+DC and what these mean? 

If I assume the drawing matches your setup and you measure 24V at the secondary of the transformer,   I would expect the 786 will read roughly the following at the output of the bridge:

20.3 volts DC
9.8 volts AC
22.6 volts AC+DC

Your diagram is correct as well as where my meter test points are located.  My meter measures  no DC voltage in "DC" mode and about 22 vac in AC or AC+DC.  Thje DC meter seems to work fine for other circuit measurements.

[joeqsmith]

Does "wrt" mean With Respect To?  So in the drawing, what you call hot lead is the positive side of the bridge and GND is the negative side?
If that seems correct, do you understand that your meter can read AC, DC and AC+DC and what these mean? 

If I assume the drawing matches your setup and you measure 24V at the secondary of the transformer,   I would expect the 786 will read roughly the following at the output of the bridge:

20.3 volts DC
9.8 volts AC
22.6 volts AC+DC

Your diagram is correct as well as where my meter test points are located.  My meter measures  no DC voltage in "DC" mode and about 22 vac in AC or AC+DC.  Thje DC meter seems to work fine for other circuit measurements.

Your original post never states if there is a problem with the circuit you are looking at, and that is why you are probing it, or are you trying to understand how to use the meter to read voltage and current.   

Assuming you are trying to troubleshoot the circuit, just disconnect the bridge.   Once removed, you could measure each diode but I assume the bridge is encapsulated.  I wouldn't suggest trying to do anything more outside of replacing it.

[NY2KW]

My apologies, I'm usually criticized for giving too much information when asking questions so I've been working on not being verbose.  In doing so I didnt describe it correctly.  The circuit is part of my Collins R-390 (nonA=-A) receiver.  The 24vac transfomer supplies the osc oven and a center tap provides 12vac to the bridge rectifier.  The manual states : "  The low-voltage secondary (terminals 8 and 10) of T801 supplies 25.2 volts ac to all filament, dial lamps, and oven-heater circuits. A tap (terminal 9) on this winding provides 6 volts dc to antenna relay K101 and break-in relay K602."   Not stated here but shown in the schematic is the center tap feeds a bridge rectifier which feeds the parallel relay coils.  The manual further states that the resistance across the coils in parallel are about 8 ohms.  These relays are the antenna relay and break-in relay.  A previous owner replaced the selenium bridge with a 'modern' sealed bridge.  I found these relays were not working.  The easiest test without a major disassembly of the radio sub-assemblies was an easy direct access to the antenna relay coil tabs.  The coil read about 6 ohms but no DC voltage across it with break-in activated and only could read about 20VAC. Hence my question about the Bryman 786 capabilities in reading a voltage that wasnt quite AC or continuous DC.   Regardless, I will need to take remove some assemblies to replace the bridge.... the radio weighs 80lbs and I'm getting too old to lug it around :-)   

[joeqsmith]

My apologies, I'm usually criticized for giving too much information when asking questions so I've been working on not being verbose.  In doing so I didnt describe it correctly.  The circuit is part of my Collins R-390 (nonA=-A) receiver.  The 24vac transfomer supplies the osc oven and a center tap provides 12vac to the bridge rectifier.  The manual states : "  The low-voltage secondary (terminals 8 and 10) of T801 supplies 25.2 volts ac to all filament, dial lamps, and oven-heater circuits. A tap (terminal 9) on this winding provides 6 volts dc to antenna relay K101 and break-in relay K602."  Not stated here but shown in the schematic is the center tap feeds a bridge rectifier which feeds the parallel relay coils.  The manual further states that the resistance across the coils in parallel are about 8 ohms.  These relays are the antenna relay and break-in relay.  A previous owner replaced the selenium bridge with a 'modern' sealed bridge.  I found these relays were not working.  The easiest test without a major disassembly of the radio sub-assemblies was an easy direct access to the antenna relay coil tabs.  The coil read about 6 ohms but no DC voltage across it with break-in activated and only could read about 20VAC. Hence my question about the Bryman 786 capabilities in reading a voltage that wasnt quite AC or continuous DC.   Regardless, I will need to take remove some assemblies to replace the bridge.... the radio weighs 80lbs and I'm getting too old to lug it around :-)

That makes much more sense with your first post about the measured voltage and current.   Ballparkish, I would expect about 0.6A AC through the coil.   My guess is two of the diodes are now shorted and the other two are open.   Once you pull the bridge, see if it has any part identifiers.   Maybe the original person attempting to do the repairs made a poor selection.   Do these relays have any sort of flyback diodes, or does the bridge need to handle that back EMF?     

I know what you mean about that heavy equipment and getting old.     

[NY2KW]

No diodes across the coil.  The bridge takes the brunt.  Pretty typical 1950, 1960's open style.  Here's a typical picture