Using the same power supply for negative and positive voltages

Thread Starter

NewLeaf

Joined Mar 30, 2018
11
Hi all,
I'm new to the forum, and excited to be a part of the community.
I'm having a perplexing issue(to me) that I wonder if someone may have some insight into. I'm using a bridge rectifier to provide negative voltage to a bias/heater circuit in a tube amp. I'm also using the same supply to power a relay through a buck DC/DC converter. Its working, but with some issues. I've connected the +/- leads to power the DC/DC converter, but it isn't reducing the output of the rectifier, in other words, its acting as a pass through. My thought is this due to it not having reference to ground. I don't want to mix grounds, as the positive lead is grounded on the rectifier to provide the negative voltage for the bias . I'm thinking of removing the (-) lead to the DC/DC converter from the rectifier and routing it to ground instead to complete that part of the circuit. Does this sound sound?

Thanks!
Mike
 

sghioto

Joined Dec 31, 2017
1,048
Hello and welcome. Can you post a schematic? Why can't you connect the + input of the converter to ground if that is the positive side of the rectifier?
SG
 

Thread Starter

NewLeaf

Joined Mar 30, 2018
11
Hi and thanks for the response. I don't have a schematic, as this is an idea I implemented in an existing circuit. The basic idea is if I lose bias, it kills the voltage to the output tubes via the relay. So essentially, its function is to save the output tubes if the bridge rectifier goes down, which I've had happen before. As I stated, the positive is grounded off of the rectifier to provide negative voltage to the bias/(preamp) heater circuit. I'm not sure about grounding the positive on the DC/DC converter; it isn't designed to be operated from a negative voltage supply, or at least it doesn't mention that in the datasheet. Its my understanding that you can have a negative and positive voltage source from the same supply. I bread boarded a similar setup with bad results. In that I grounded the negative from the DC/DC converter to the same ground from the mock bias supply, shorting the bridge rectifier.
Mike
 

ebp

Joined Feb 8, 2018
2,332
You can connect the buck converter directly across the bridge IF:
- you can tolerate the input capacitance of the converter across the bridge
- the output of the buck (and any metal bits) remains completely isolated from any of the other circuitry powered by the bridge
A buck regulator draws input current as pulses, filtered only by capacitance. It will typically be at some high frequency but some converters will go into cycle-skipping mode if the load is light and that could bring it down into upper audio spectrum. If it is a very small converter that shouldn't be a problem.

What is the voltage from the bridge rectifier?
Can you get a satisfactory relay that will run directly from that voltage, perhaps with just a dropping resistor if necessary?
 

MrChips

Joined Oct 2, 2009
21,326
Welcome to AAC.

You will discover that a circuit diagram is a universal language for communicating about electrical and electronic circuits. Please take the time to draw a circuit diagram. It will go a long way in communicating what it is that you are trying to do.
 

Thread Starter

NewLeaf

Joined Mar 30, 2018
11
You can connect the buck converter directly across the bridge IF:
- you can tolerate the input capacitance of the converter across the bridge
- the output of the buck (and any metal bits) remains completely isolated from any of the other circuitry powered by the bridge
A buck regulator draws input current as pulses, filtered only by capacitance. It will typically be at some high frequency but some converters will go into cycle-skipping mode if the load is light and that could bring it down into upper audio spectrum. If it is a very small converter that shouldn't be a problem.

What is the voltage from the bridge rectifier?
Can you get a satisfactory relay that will run directly from that voltage, perhaps with just a dropping resistor if necessary?
Hi and Thank you....I used a 3.3uf across the input to output as per the datasheet, as the voltage in is above 60v. It also has 1uf internally. It is completely isolated as its a plastic case, not requiring a heatsink, then on to the relay which also has a plastic case. It is a very small converter. Its (supposed) to take 70v from the bridge and buck it to 12v to power the relay. So far, as I stated its not actually dropping it to that voltage.
 

ebp

Joined Feb 8, 2018
2,332
Part number for the converter?

How is it connected and how are you measuring the voltage (i.e. where are meter leads connected)? If a non-isolated buck converter fails with the switch (probably a FET but possibly a bipolar transistor) shorted, the input is essentially shorted to the output with an inductor and a diode between. If you have a meter with a diode test function you could check this by putting the positive lead to the input positive terminal and the negative lead to the output positive terminal. Also check with normal resistance function.
 

Thread Starter

NewLeaf

Joined Mar 30, 2018
11
Ok, from input to output
The diode check gives a reading of .265v
The resistance reading is 520k
Any thoughts?
Mike
 

Thread Starter

NewLeaf

Joined Mar 30, 2018
11
I suppose the question would be, if I remove the negative lead from the bridge feeding the DC/DC converter and then connect a lead from the gnd pin of the converter to the system gnd, would that cause any problems? In other words, would it screw up the bridge, as its supplying negative voltage to the bias circuit?
 

sghioto

Joined Dec 31, 2017
1,048
..I used a 3.3uf across the input to output as per the datasheet, as the voltage in is above 60v.
The capacitor should be connected from the + input of the converter to ground not from input to output. The ground or common lead on the converter is the negative input.
SG
EEE Bias to DC converter.png
 
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Thread Starter

NewLeaf

Joined Mar 30, 2018
11
I checked on that after I wrote that... I did install it correctly(whew). Its been a bit since I had looked at the datasheet. Thanks for catching that though!
 

sghioto

Joined Dec 31, 2017
1,048
Do you have the converter wired as shown in post #12?
EDIT: before you go any further have you tried testing the converter with a battery or some other power supply with the relay connected?
SG
 
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sghioto

Joined Dec 31, 2017
1,048
I would suggest you try that again and double check to make sure there is 12 volts on the output of the converter. Just to confirm the converter is still working correctly.
SG
 
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