How can I wire a 3-pin bi-colour LED to show green when a DC fuse is OK and red when it is blown on 5 V, 12 V, and 36 V circuits?
Use a relay in each supply line and drive the bi-colour LED as a common-cathode indicator: choose a relay whose coil voltage matches the line voltage, and use a change-over contact to switch the LED from red to green [#21664814] With power applied and the fuse open, only the red LED lights; with a good fuse, the relay energizes and the contacts switch to green [#21664814] Use these series resistors for the LED side: 150 Ω for 5 V, 200 Ω (220 Ω is OK) for 12 V, and 1.5 kΩ 1 W for 36 V [#21664814]
Can anyone supply me with a circuit for the above using Bi Color LED'a (3 Pin Type - Red & Green) i need to show Green for good condition & Red for blown condition. I will be installing it on 5, 12 & 36 volt max 40 mA DC circuits.
Hi Ray Whats the supply voltage Is the supply voltage variable if so what is Vmin What is Vmax Does the fuse indicator depend on the fused supply to function What is Imax Imin of the circuit itself that is fused What is Imax that the power supply for that circuit assuming Dc only
Is the fuse on the primary side of the power supply i.e ac in Or is the fuse on the secondary side of the powersupply If so where is this fused i.e before bridge rectifier or after bridge rectifier
Have you a circuit of the power supply at all Have you a block diagram of your circuit showing where fuse is situated even a wiring diagram?
What condition does the power to the load need to be when the fuse is blown? Absolutely zero, or is a small current acceptable? How precise does the input voltage need to be?
Attached is a schematic that may or may not work for you, assuming the fuse is on the DC portion of the circuit. Install the red LED in series with a resistor across the fuse, with the cathode facing the load. Install the green LED in series with the fuse itself, with the cathode also facing the load. (not shown on the schematic)
Some things to note: you will will see a small drop in the DC voltage due to the forward bias voltage of the green diode. Additionally, when the fuse is blown, you will still see some power at the load side of your circuit. And, if the load you are connecting to is highly resistive, that may impair the functionality of the circuit as well.
So, like I said, it may or may not work for you.
EDIT:
If the limitations don't do it for you, there are some decent isolated DC current sensors out there based on the Hall effect. We use them at my work as part of our Multimeter kits, but I believe they come in small IC packages as well. Something to look in to if you don't like having the diode affect the circuit. Not as simple as the first solution, but better if you need to meet design constraints, or if you aren't sure what the load will be.
That is the simplest method This is just an observation I know full well that Cody knows his subject, but normally when you fuse a supply involving rectification of some type, you always fuse before the bridge
Reasons being that a fuse is not fail safe, hence in certain circumstances, particularly in power amps for example where the output devices draw extremley high current when they fault If you dont fuse before the bridge then what happens is part of the rectifier circuit becomes either leaky or short circuit across part of the diode network
The final result then leads to blown smoothing bank, forgive my old terminology, and can also simultainiously blow the transformer as well
Quite a common fault on the early Sansui amplifiers and also some other manufacturers
So strictley speaking there should be two fuses One in line with AC input into the Tx and one out of the Tx before the Bridge rec circuit
But Cody's circuit will work but with limitations also if you are ever really going to monitor a fuse its always handy to ascertain if you have power into the circuit in the first instance
I've actually never considered that fuses weren't fail safe... makes complete sense now that I think about it though. I know some stuff, but I've only really been a working EE for about a year now. The insight is much appreciated! Thanks
More than welcome Cody Youd be surprised at how many circuits you see that dont do this Its a common mistake which I make as well
There is actually another reason for this as well and that is snubber capacitors "capacitors in parrellel with each diode" to damp ringing especially when dealing with switchmode power supplies but here is an article worth the read just to gain overall knowledge and is concerened with emi interference but strengthens the aspect of where to fuse quite important when dealing with guitar tube based amps and thier powersupplies
Mark & Cody, Fuse protection sounds dead simple but I see now there is a bit more involved when things are done properly. Can you advise a bit further about fuse types. I know there is some differentiation between fast & slow blow, but is better fuse selection something worth more attention?
Hi just put a circuit together for you using proteus design package You can test this by going to www.labcenter.co.uk and run the design file in the simulator
Theres another very quick schematic ive put together for you which will do the same using an opamp as window comparitor 4558 op amp will do the job quite nicely but you can check design in proteus or simply build this on bread board or similar If you want more this you need to drop me a line and ill put something together for you which would be cpu controlled electronic fuse if you like far superior than ordinary fuse but need time to do this
Thanks for your reply. Firstly, i am new to electronics, so science will definateley baffle me. My question obviously needs clarifying. My circuit is for the control of a CNC milling machine, using a PSU, Breakout Board and Stepper Motor Drivers.
My mains supply is 240 vac @ 10 amp max, the PSU is supplied with the mains power, built into the PSU is one 5 vdc & 3, 36 vdc outlets, the 5 vdc supply, powers the Breakout Board, the 36 vdc supplies the Stepper Driver Boards. I wish to fuse these circuits, along with the so called Blown Fuse Indicators.
So the MinI / MaxI will be for the LED's only, there are no Bridge rectifiers in the circuit.
Thanks for your reply. Firstly, i am new to electronics, so science will definateley baffle me. My question obviously needs clarifying. My circuit is for the control of a CNC milling machine, using a PSU, Breakout Board and Stepper Motor Drivers.
My mains supply is 240 vac @ 10 amp max, the PSU is supplied with the mains power, built into the PSU is one 5 vdc & 3, 36 vdc outlets, the 5 vdc supply, powers the Breakout Board, the 36 vdc supplies the Stepper Driver Boards. I wish to fuse these circuits, along with the so called Blown Fuse Indicators.
So the MinI / MaxI will be for the LED’s only, there are no Bridge rectifiers in the circuit.
"Reasons being that a fuse is not fail safe, hence in certain circumstances, particularly in power amps for example where the output devices draw extremley high current when they fault If you dont fuse before the bridge then what happens is part of the rectifier circuit becomes either leaky or short circuit across part of the diode network
The final result then leads to blown smoothing bank, forgive my old terminology, and can also simultainiously blow the transformer as well
Quite a common fault on the early Sansui amplifiers and also some other manufacturers
So strictley speaking there should be two fuses One in line with AC input into the Tx and one out of the Tx before the Bridge rec circuit"
Great looking circuit, bit a bit to complex for my liking, i am thinking more of a single LED for each fuse, rather than a rotary switch that manually sets a particular circuit, an LED per circuit will give me an instant visual ID of which fuse is blown.
Apart from all the other considerations, here is perhaps the simplest method of indicating if power is good and if the fuse is open circuit: (Circuit Attached)
Resistors R1 and R2 are 1k5 (1500 Ohms), 1 Watt; Resistors R3 and R4 are 150 Ohms 0.25 Watt.
LEDs 1 and 3 indicate PSU output is present, LEDs 2 and 4 indicate fuses OK. ----- Fairly simply, fast blow fuses (marked F or Q) will be used for delicate electronic equipment, slow blow fuses are for motors and other devices which take a higher than normal current when starting up.
For more detail, read the Wikipedia article: en.wikipedia.org/wiki/Fuse_(electrical)
Hi Raymond the switch is simply for indication tests ie 5v 9 volt, 12volt I did this to ensure overall operation from a test point of view
This was so that you would know for example My supply is 12volts for arguments sake, so therefore I use that resistor value In the second circuit the switch is there to simulate load conditions Its for simulation purposes only
Truthfully speaking if you want good design there is no simple answer to this
What would be ideal is what we term a solid state fuse Ie a relay which disconnects supply in the event of a fault condition with a reset switch
Of course what you are looking for is not associated with Dc so you would need opto couplers across each side of the fuse or an inductive pickup since you are dealing with AC which is then coupled to an op amp configuration of some type and then possibly onto a micro which has its own dc supply derived from your Ac rail inbound, operating some type of relay which might disconnect the mains islolating the unit
Thats the ideal answer and possibly the safest all round and by far the best route since it would meet your requirements in full
Frank, Simple & very easy to understand, I get it. BUT,..... should we not be concerned to avoid the losses in R1 from the necessary large voltage drop, which will be ongoing whilst the piece of equipment is switched on?
And regarding Slow/fast fuses: as I recall the initial recommendation was to fuse either side of the transformer/rectifier bridge. Is this a fast or slow situation, no motors here?
You have lost me with the science bit, i have no idea the effect on RI, the max current is 0.030mA as for slow-fast fuses either side of a bridge rectifier ? i never mentioned these in my initial general inquiry.
Sorry Ray, that was just a quick idea of what I thought was the simplest solution to display the required information.. But now I shall put my thinking cap on ! Back soon !
Yes Peter, but I am basically looking for simplicity, as Ray says he is a beginner at electronics. Obviously a PWM circuit could more efficiently replace the resistor, if you know of a suitable chip or simple circuit please add it.
There was considerable chat from the others about fuses, so I added a little of what I knew.
Hi Ray, here is another simple solution using a relay in each line with your bi-colour LED. Modern relays are very small and cheap. For each line, use any relay with a coil voltage equal to the voltage on that line. Most common relays have at least one set of DPST (Double Pole Single Throw) (change-over) contacts.
The circuit assumes the bi-colour LED is common cathode (negative pole) (which is usual). For the 5V line, R1 = 150 Ohms For the 12V line, R1 = 200 Ohms (220 Ohms is OK) For the 36V line, R1 = 1k5 (1500) Ohms, 1 Watt
When power is applied, if the fuse is open circuit, only the red LED is lit. If the fuse is good, the relay is energised, switching over to the green LED.
(I am sure there is also a solution using transistor-diode logic, but I am too many years away from college to remember it.) (Does anyone here remember how to do it that way ?)
Many thanks for this circuit, can you recommend what make / type of relays, this could save me time searching for them, do they need to be PCB mounted or on bases?
Hi Ray, I tend to buy my parts mostly on Ebay, as everything from Hong Kong and China is extremely cheap. (So I usually buy things in tens !) However, goods from far east sellers that don't post airmail can take over a month to arrive. If this is an urgent job, you need a electronic supplier in your own country. (I am in England.)
If you don't have any parts shops in your local area (try yellow pages ?), I would suggest googling "electronic parts suppliers" in your town or ask around any friends you know are into electronics by job or hobby.
You could search online or Ebay for relays by coil voltage, find the smallest and cheapest, as small LEDs only take tiny currents. For building on PCB stripboard (as I mainly do), the PCB type are probably cheapest and easiest to use.
That's the best in general advice I can give you on electronic parts suppliers.
If you are new to electronics, a solderless breadboard is excellent (and cheap) for experimentation and testing before building a circuit. They are often sold as kits with set of terminated leads for speed and convenience, otherwise get 1/0.6mm cable for connecting components (often sold in sets of 1 or 2 metres of eleven different colours for the experimenter).
To obtain a greater understanding of electronics, I recommend the following sites which offer good tutorials for beginners:
www.learnabout-electronics.org/index.php - Descriptions of components and principles, good diagrams, animations, & fault-finding techniques
✨ The discussion centers on designing a bi-color 3-pin LED fuse status indicator circuit for DC voltages of 5V, 12V, and 36V with currents up to 40mA, intended for CNC machine control circuits including power supplies, breakout boards, and stepper motor drivers. The user seeks a simple visual indication using a single bi-color LED (red for blown fuse, green for good fuse). Various contributors suggest methods including placing LEDs in series with the fuse and across it, noting voltage drops and residual currents when fuses blow. The importance of fuse placement relative to rectifiers is discussed, emphasizing fusing before the bridge rectifier to prevent damage to transformers and smoothing capacitors. Relay-based solutions are proposed for clearer indication, using relays with coil voltages matching the supply lines to switch the bi-color LED states. Resistor values for different voltages are recommended to limit LED current. The conversation also touches on fuse types (fast blow vs slow blow) and their appropriate applications. Practical advice includes sourcing small PCB-mount relays and using solderless breadboards for prototyping. Simulation tools like Proteus are mentioned for circuit testing. Overall, the solutions balance simplicity for beginners with considerations for reliability and circuit protection. Generated by the language model.
TL;DR: Need a simple fuse-status indicator? Use a relay-per-rail with a 3‑pin bi‑color LED; e.g., 1k5 Ω/1 W on 36 V. “When power is applied, if the fuse is open circuit, only the red LED is lit.” [Elektroda, Frank Bushnell, post #21664814]
Why it matters: This gives clear green=OK/red=blown feedback on 5 V, 12 V, and 36 V rails without leakage issues.
How do I wire a 3‑pin bi‑color LED to show fuse good vs blown on DC?
Simplest: put the green LED in series with the fuse, cathode toward the load. Put the red LED and a resistor across the fuse, cathode toward the load. Green means current is flowing. Red lights when the fuse opens. Expect a small forward‑drop in normal operation and some residual power at the load when blown. Highly resistive loads can impair indication. [Elektroda, Cody Tappan, post #21664795]
Will the LED‑across‑fuse trick leak current to my load when the fuse blows?
Yes. The red LED path supplies a small current to the load side after the fuse opens. This produces a non‑zero voltage that can confuse high‑impedance loads. "It may or may not work" if your load is very resistive. Consider an isolated method if that is unacceptable. [Elektroda, Cody Tappan, post #21664795]
How can I avoid voltage drop and leakage while still using a bi‑color LED?
Use a small relay per rail. Drive the relay coil from the rail before the fuse. Wire the relay’s changeover contacts to select red (fuse open) or green (fuse OK) on a common‑cathode bi‑color LED. This isolates the indicator from the load path and eliminates leakage into the load. [Elektroda, Frank Bushnell, post #21664814]
What resistor values should I use with the bi‑color LED at 5 V, 12 V, and 36 V?
For a common‑cathode LED: use 150 Ω on 5 V, 220 Ω on 12 V, and 1k5 (1 W) on 36 V. These values come from a proven relay‑indicator scheme and keep LED current modest. Adjust slightly if your LED is unusually bright or dim. [Elektroda, Frank Bushnell, post #21664814]
Fast‑blow or slow‑blow fuse—what’s right for stepper drivers and logic supplies?
Use fast‑blow (F) for delicate logic like breakout boards. Use slow‑blow (T) for stepper drivers and motors because they draw higher start‑up current. This balances protection with nuisance‑trip avoidance during inrush. [Elektroda, Frank Bushnell, post #21664807]
Where should I place fuses around a PSU or rectifier?
Place one fuse at the AC input and another before the bridge rectifier on the transformer secondary. This reduces collateral damage if rectifier diodes fail and helps protect the transformer and smoothing caps. "A fuse is not fail safe." [Elektroda, Mark Harrington, post #21664796]
Is a fuse truly fail‑safe?
No. Fuses can fail to clear properly, leaving other parts to absorb fault energy. That is why designers protect both primary and secondary sides in rectified supplies. As Mark notes, “a fuse is not fail safe.” [Elektroda, Mark Harrington, post #21664796]
Can I simulate the indicator circuit before building it?
Yes. Mark shared a Proteus design you can run in Labcenter’s simulator. Simulation helps verify LED polarity, resistor sizing, and relay logic before soldering. It also lets you test at 5 V, 12 V, and 36 V rails safely. [Elektroda, Mark Harrington, post #21664801]
Is there a solid‑state alternative to fuses and relays?
Yes. You can build a CPU‑controlled electronic fuse using an op‑amp window comparator, opto‑isolation, and a microcontroller driving a relay or MOSFET. It offers fast cutoff and reset features, but adds complexity and design time. [Elektroda, Mark Harrington, post #21664802]
How do I size indicator current within my rail budget?
Keep indicator draw small relative to each branch’s limit. The original brief targets up to 40 mA per DC branch. Using 2–5 mA per LED leaves ample margin for the load and keeps heat low. [Elektroda, Raymond Smith, post #21664793]
Will large dropper resistors waste power continuously?
Yes, simple resistor drops dissipate heat while the rail is on. Peter raised this loss concern. Frank’s view: favor simplicity for beginners, but a PWM or smarter driver can improve efficiency if desired. [Elektroda, Frank Bushnell, post #21664813]
What’s a safe, visual method for beginners to implement?
Adopt the relay‑per‑rail LED indicator. It is robust, easy to debug, and uses clear red/green states. Sample values: 5 V→150 Ω, 12 V→220 Ω, 36 V→1k5/1 W on a common‑cathode LED. This avoids confusing partial‑power conditions. [Elektroda, Frank Bushnell, post #21664814]
How do I build the relay‑indicator in three steps?
Feed each relay coil from its rail before the fuse.
Put the relay’s common on LED cathode; NO→green, NC→red via resistors.
Any tips for sourcing relays and prototyping this?
Frank suggests small PCB relays; they are cheap and easy to mount on stripboard. Buy from reputable suppliers or eBay if time permits. A solderless breadboard helps you prototype and confirm LED polarity and coil wiring before final assembly. [Elektroda, Frank Bushnell, post #21664816]
What if my system monitors AC rather than DC rails?
Use opto‑couplers or inductive pickups to sense AC safely, then feed a comparator or microcontroller. Drive a relay or MOSFET to isolate and signal faults. This approach also supports a master relay to disconnect mains on error. [Elektroda, Mark Harrington, post #21664809]
What mains and PSU context did the original build use?
Ray’s CNC controller used 240 VAC at 10 A into a PSU providing one 5 V rail for logic and three 36 V rails for stepper drivers. He wanted per‑fuse indicators with simple red/green status. [Elektroda, Raymond Smith, post #21664803]
