I built two units of the bass equalizer kit from elliott sound products that includes an on-board dual power supply.
They are working well except for the following glitch: The units will not work upon power-on. Ground to negative rail reads 15v on the voltimeter but ground to positive rail reads 0. Now, when I briefly shortcircuit + and - they suddenly power up and give a correct +15v. Each board has its own power supply but they share a common AC transformer.
Any hints? I sure don´t want to be jump-starting the units like this everytime.
Thanks
Here´s a link to the project that includes a circuit diagram
Eight Band Sub-Woofer Equaliser
They are working well except for the following glitch: The units will not work upon power-on. Ground to negative rail reads 15v on the voltimeter but ground to positive rail reads 0. Now, when I briefly shortcircuit + and - they suddenly power up and give a correct +15v. Each board has its own power supply but they share a common AC transformer.
Any hints? I sure don´t want to be jump-starting the units like this everytime.
Thanks
Here´s a link to the project that includes a circuit diagram
Eight Band Sub-Woofer Equaliser
Try to put two diodes in backward direction on the regulator outputs (catode to +15V on positive output and catode to GND on negative output).
According to National data sheet for IC voltage regulator it is beneficial expecially when you have a transverse load (not ground referred).
In the power supply you are referring to the problem is exacerbate due the half wave rectifier: the postive and negative rails "rise" with half mains period delay between them
My guess is that due the 1/2 wave design and smallish input filters, the IC input voltage is too low to give the required minimum 2V input to output differential. When you short the outputs (not a great thing to do repeatedly) The ICs go into protection, and allow the input caps (C2 - C3 470uf) to charge up beyond the dropout voltage. I'd guess there's a bit more load on the positive leg, causing it to be the one effected (once one regulator shuts down, the opposite one gets enough input voltage to avoid dropout). One thing that may help is to increase C2 and C3 as much as practical (the author states he used 2200uf in the prototype). This will help keep the ripple valleys from dropping low enough to cause trouble. You can check this easily if you have a scope. The raw DC ripple valleys have to be higher then +-17v - higher would be better. I didn't see a Xfmr secondary rating anywhere, but it's probably a tad low. A better design can be found here LM7800 pdf, LM7800 description, LM7800 datasheets, LM7800 view ::: ALLDATASHEET ::: on the last page of the spec sheet, but that will require new Xfmr(s). Hope it helps.
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Don't guess... find the fault and fix it ! It's a very basic circuit that should work perfectly.
"Ground to Negative rail reads 15volt".... are you sure ? do you mean "reads minus 15 volt" ?
With it in the faulty state, and with the meter negative lead left connected to the "ground" rail measure to each end of C2 and C3 (The inputs to the regs) and tell us what you measure.
If shorting the plus and minus rails makes it work it could be a poor connection somewhere and the "shock" of that causes a small arc and remaking of the connection... or perhaps D1 is intermitent... it happens.
Two minutes with a DVM should isolate the fault for sure. Have you tried it with no load connected to the outputs ? Does the regulator get hot when there is no output ?
Mooly: So it happens I thought it was fixed but it isn´t.I powered it up again today and the problem remains.
The transformer output is 14.6v while connected to the board. 14.75 on the positive and -14,90 on the negative rail (relative to ground).
Later,I will measure C2 and C3 when in faulty condition and post the readings. If I turn it off and on again now it works fine, probably because the capacitors remain charged.
I may try adding two extra diodes (same as D1 and D2 but after the rectifier if I understand correctly) as diy-audio-fo suggested.
I do not have a scope, only a voltemeter
The transformer output is 14.6v while connected to the board. 14.75 on the positive and -14,90 on the negative rail (relative to ground).
Later,I will measure C2 and C3 when in faulty condition and post the readings. If I turn it off and on again now it works fine, probably because the capacitors remain charged.
I may try adding two extra diodes (same as D1 and D2 but after the rectifier if I understand correctly) as diy-audio-fo suggested.
I do not have a scope, only a voltemeter
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A meter is fine... try and find the fault first...
If you have +15 and -15 coming out of the regulators then there isn't a problem with the PSU... or do you mean they read that when working ?
Connect meter to ground line and measure the input to each reg first and then the outputs. In your first post you said there was no +15 volts... so you need to recheck that.
If you have +15 and -15 coming out of the regulators then there isn't a problem with the PSU... or do you mean they read that when working ?
Connect meter to ground line and measure the input to each reg first and then the outputs. In your first post you said there was no +15 volts... so you need to recheck that.
You need more than 14.75 out of the transformer (I think you mean rectifer output to ground though) for the regulator to function at all - they need at least 17vdc in to produce 15 out reliably. The regulators cannot increase the voltage. As for the rectifer configuration, the half-wave gives AC X 1.414=Vout. Thus a 12.6V transformer will give about 17.6Vdc best case. Subtract from that the .6 - .7v foward drop of the rectifer and you get 16.9-17Vdc which is right on the regulator's dropout spec. If you could check the transformer secondary voltage (leg to leg) it may shed some light on the problem. With this minimum of headroom, balance between legs is critical to prevent one side from dropping out. In this design ground floats between the 2 DC rails. Simple component tolerances would be enough to kick it one way or the other. This document from Hammond may help.
http://www.hammondmfg.com/pdf/5c007.pdf
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Sorry for not being too clear on the numbers.
14,6v AC at the transformer secondary ( as scott594 says, seems like I need a higher voltage transformer then. What rating would you recommend?)
+14,75 and -14,90 relative to ground after the rectifiers, when the unit is working, but:
-0,72 (this on the positive rail!!!) and -14,92 on the negative relative to ground when in faulty condition, with 2,200uf capacitors added.
18,06 at positive regulator input and -17,86 at negative regulator input.
14,6v AC at the transformer secondary ( as scott594 says, seems like I need a higher voltage transformer then. What rating would you recommend?)
+14,75 and -14,90 relative to ground after the rectifiers, when the unit is working, but:
-0,72 (this on the positive rail!!!) and -14,92 on the negative relative to ground when in faulty condition, with 2,200uf capacitors added.
18,06 at positive regulator input and -17,86 at negative regulator input.
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14.6Vac from the transformer makes it seem like this is a 12Vac transformer.
Normally a transformer voltage for a regulated supply is chosen to equal the DC output voltage.
eg.
for a 15Vdc 200mA supply choose a 15Vac 800mA transformer. This 12VA transformer may have a regulation of about 20%. Measuring across the secondary when the primary is fed with an accurate mains voltage supply will be ~18Vac.
If this were a 230:15Vac transformer fed with UK mains then expect ~16.9Vac when mains is at 216Vac and expect ~19.9Vac when mains is at 254Vac.
Your regulator MUST be designed to work reliably within that range of supply voltage and over the range of output currents between 0mA and 200mAdc.
Normally a transformer voltage for a regulated supply is chosen to equal the DC output voltage.
eg.
for a 15Vdc 200mA supply choose a 15Vac 800mA transformer. This 12VA transformer may have a regulation of about 20%. Measuring across the secondary when the primary is fed with an accurate mains voltage supply will be ~18Vac.
If this were a 230:15Vac transformer fed with UK mains then expect ~16.9Vac when mains is at 216Vac and expect ~19.9Vac when mains is at 254Vac.
Your regulator MUST be designed to work reliably within that range of supply voltage and over the range of output currents between 0mA and 200mAdc.
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The transformer is 15v secondary.I checked before doing the connections. The measured AC is with it connected to the power supply, which seems to reduce the voltage.
I should have said regulator instead of rectifier. However I was referring to the regulators´input or output as per the circuit diagram on the ESP site which marks them as in, out and ground legs.
I should have said regulator instead of rectifier. However I was referring to the regulators´input or output as per the circuit diagram on the ESP site which marks them as in, out and ground legs.
a measured 14.6Vac from the loaded secondary does not indicate you are using a 15Vac transformer.
From that example I quoted, the worst case low voltage will be ~20.5Vdc and worst case high voltage will be ~27.4Vdc at the input to the regulators.
This is not what you are measuring.
Your voltages are very much lower, indicating you have selected the wrong transformer.
From that example I quoted, the worst case low voltage will be ~20.5Vdc and worst case high voltage will be ~27.4Vdc at the input to the regulators.
This is not what you are measuring.
Your voltages are very much lower, indicating you have selected the wrong transformer.
You have something odd happening to cause the start up issue you have... and I'm sure Andrew will agree that whether the tranny is right or wrong... there is something else going on. If nothing gets hot when it's faulty, such as the regs, diodes, caps etc then it seems more like an intermitent wiring error.
Could you post a picture of it all
Could you post a picture of it all
I have no camera so can´t post a photo. However, I´ll get another transformer tomorrow and see if it fixes the problem.
The fact is I don´t know my transformer´s amperage.
AndrewT mentioned 15v 800mA. Should I try an 18v or so for a safer bet? Let´s see what´s available at my electronic store.
BTW. I´ve noticed no high temperature at the regulators or anywhere else for that matter.
The fact is I don´t know my transformer´s amperage.
AndrewT mentioned 15v 800mA. Should I try an 18v or so for a safer bet? Let´s see what´s available at my electronic store.
BTW. I´ve noticed no high temperature at the regulators or anywhere else for that matter.
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It won't be the transformer... but try it to "prove" that to yourself.
Do this and copy and paste this with your results.
Connect your meter negative to the left hand ground in Rods circuit (the transformer) and set the meter on DC volts. You don't move that negative lead now, leave it where it is
Now measure these these voltages in this order and write them down and copy and paste the results here.
1. Cathode (stripe end) of D1 ...... Volts
2. Anode (non stripe end) of D2 ...... Volts
3. Pin 1 of the 7815 ...... Volts
4. Pin 2 of the 7815 ...... Volts
5. Pin 3 of the 7815 ...... Volts
6. Pin 1 of the 7915 ...... Volts
7. Pin 2 of the 7915 ...... Volts
8. Pin 3 of the 7915 ...... Volts
9. Positive end C2 ...... Volts
6. Negative end C3 ...... Volts
Now switch the meter to AC volts and measure on the anode/cathode junction of D1 and D2 ...... Volts AC
Do this and copy and paste this with your results.
Connect your meter negative to the left hand ground in Rods circuit (the transformer) and set the meter on DC volts. You don't move that negative lead now, leave it where it is
Now measure these these voltages in this order and write them down and copy and paste the results here.
1. Cathode (stripe end) of D1 ...... Volts
2. Anode (non stripe end) of D2 ...... Volts
3. Pin 1 of the 7815 ...... Volts
4. Pin 2 of the 7815 ...... Volts
5. Pin 3 of the 7815 ...... Volts
6. Pin 1 of the 7915 ...... Volts
7. Pin 2 of the 7915 ...... Volts
8. Pin 3 of the 7915 ...... Volts
9. Positive end C2 ...... Volts
6. Negative end C3 ...... Volts
Now switch the meter to AC volts and measure on the anode/cathode junction of D1 and D2 ...... Volts AC
Mooly, the units powers on correctly now, so it´s a finicky fault whatever it is.
The thing is both boards show the same condition always. That pretty much rules out a bad solder joint.
Thanks for the voltage check-list.As soon as the fault reappears I will measure the values on your table and post.
I won´t change the transformer for now.
The thing is both boards show the same condition always. That pretty much rules out a bad solder joint.
Thanks for the voltage check-list.As soon as the fault reappears I will measure the values on your table and post.
I won´t change the transformer for now.
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