I remember the first time I blew a 7805. Sparks, a little puff of smoke, and a dead circuit board. I was trying to power a bunch of LEDs from a 12V adapter, and the single 7805 I had just couldn’t handle the difference. It felt like a personal insult from a tiny plastic component. So, the question on a lot of tinkerers’ minds, especially when they’re staring at a power supply that’s a bit too high, is: can 7805 regulator be in series for higher voltages?
It’s a tempting idea, right? You’ve got a project that needs 5V, but your only power source is, say, 18V or even 24V. Throwing two or three 7805s in a line seems like a straightforward, cheap way to get there. But as I learned the hard way, sometimes the simplest-looking solutions are the ones that bite you. Let’s cut through the noise and see what’s really going on.
Why Chaining 7805s Is Usually a Bad Idea
Look, I get it. You’ve got a power source that’s way higher than the 5V your circuit needs, and you’ve got a fistful of 7805 regulators lying around. It seems like the most logical thing in the world to just connect them in series.
You hook the input of the first one to your high voltage, its output to the input of the second one, and so on, until the last one spits out a nice, clean 5V. Simple. Elegant, even.
Except it’s usually not. My first attempt at this involved three 7805s in a row, trying to step down 24V to 5V.
I was so proud of myself, thinking I was being clever and thrifty. Within about thirty seconds of powering it up, the first two 7805s got so hot I couldn’t touch them.
The third one was merely warm. The output voltage was… well, it wasn’t 5V. It was a sad, fluctuating 7V.
Total failure. And I wasn’t even drawing much current!
The fundamental problem here is power dissipation. A linear regulator like the 7805 works by basically acting like a variable resistor. It drops the excess voltage across itself, and that dropped voltage, multiplied by the current flowing through it, turns into heat. The formula is simple: Power (P) = Voltage Drop (V_in – V_out) * Current (I_out).
When you chain regulators, each one drops a portion of the voltage. If you have an 18V input and need 5V output, and you use two 7805s, the first might drop 13V (if it outputs around 5V, which it won’t, but let’s pretend for a second), and the second drops the remaining voltage to get to 5V.
If you’re drawing even a modest 100mA (0.1A), that first regulator is dissipating 13V * 0.1A = 1.3 Watts. That’s a fair bit of heat for a small TO-220 package without a heatsink. If you’re trying to drop, say, 20V down to 5V with a single regulator, that’s a 15V drop.
At 100mA, that’s 1.5 Watts. Now imagine you split that drop across multiple regulators – each one is still dropping a significant voltage. The total heat generated by all the regulators combined is the same as if one regulator were doing the entire job, but it’s now spread out.
This spread, however, is often not enough to make it practical or safe.
The real kicker is that each 7805 needs a minimum input voltage higher than its output voltage to function correctly – typically around 2V to 3V. So, if you’re trying to get 5V out, the second regulator in your chain needs at least, say, 7V or 8V coming into it.
This means the first regulator can’t just drop almost all the voltage and output 5V. It has to output something higher, like 8V, to give the next regulator something to work with.
This forces the first regulator to dissipate even more power, making the heat problem worse, not better. It’s a vicious cycle that leads to overloaded, overheating components and unreliable voltage. It’s like trying to cool down a hot room by opening two slightly ajar windows instead of one big one – you’re still letting in a lot of heat, just in more places.
The Real Math: Why It Gets Hot
Let’s get down to brass tacks with some numbers. The 7805 is designed to take an input voltage and give you a steady 5V output. The difference between your input voltage and the 5V output is the voltage that gets ‘burned off’ as heat. This is what we call power dissipation. The formula, as I mentioned, is simple: $P_{dissipated} = (V_{in} – V_{out}) imes I_{out}$. Now, when you try to string multiple 7805s together for a large voltage drop, you’re not really dividing the heat load effectively; you’re just creating multiple points of failure and heat generation. Consider this: you need 5V and your source is 15V. That’s a 10V difference.
If you put one 7805 in, it drops 10V. If you’re drawing 100mA, that’s $10V imes 0.1A = 1W$ of heat. That’s usually manageable with a small heatsink or even just good airflow. (See Also: Can Fan Regulator Be Used As Light Dimmer )
Now, let’s say you think, ‘Okay, I’ll use two 7805s to split the load.’ For the second 7805 to output 5V, the first one needs to output at least 5V plus its dropout voltage. Most 78xx series regulators have a dropout voltage of around 2V to 3V. So, the first regulator needs to output at least 7V or 8V.
Let’s say it outputs 8V. Then the first regulator dissipates $(15V – 8V) imes 0.1A = 7V imes 0.1A = 0.7W$.
The second regulator then has 8V in and needs to output 5V, so it dissipates $(8V – 5V) imes 0.1A = 3V imes 0.1A = 0.3W$. The total heat is $0.7W + 0.3W = 1W$.
So, in this idealized scenario, the total heat is the same. But here’s the catch: real-world performance isn’t this clean. The regulators might not regulate perfectly, and you might end up with one dropping more voltage than the other, leading to uneven heat distribution and potential failure.
And what if you need 500mA? That first regulator is now dissipating $7V imes 0.5A = 3.5W$. That’s a lot of heat for a bare TO-220 package.
I’ve seen many projects where people tried this, and the regulators just cooked themselves. My own early mistakes taught me this lesson painfully.
I remember one instance where I overloaded a chained setup, and one of the regulators didn’t just overheat; it actually melted the plastic housing slightly before failing. It was a mess, and frankly, a bit scary. The common advice is that you can’t do it, and for good reason.
Furthermore, there’s the issue of stability. Linear regulators can oscillate or become unstable if not properly bypassed with capacitors. When you string them together, these issues can be compounded. The output impedance of one regulator affects the input of the next, and you can get unexpected ringing or voltage fluctuations that can damage your downstream circuitry, even if the average voltage seems okay.
It’s like trying to balance a stack of Jenga blocks on a wobbly table – one small disturbance can bring the whole thing down. The voltage regulator datasheet is your friend here; it’ll tell you the recommended capacitor values for input and output. But when you chain them, those recommendations get murky, and you’re really in uncharted territory.
When ‘series’ Might Seem Like a Good Idea (but Isn’t)
Let’s talk about when people might think putting regulators in series is a good idea. The primary scenario is when you have a very high input voltage and need a stable low voltage, like 5V.
So, you have, say, 30V DC from a power brick, and your project needs 5V. Splitting that 25V difference across multiple regulators seems like a way to keep each one cooler. The thought process is, “If one 7805 drops 25V, it’ll cook. But if I use five 7805s, each only drops 5V, that’s easy!”
This is a classic misunderstanding of how they work. As we’ve seen, each regulator needs a certain voltage above its output to regulate properly.
So, the first regulator can’t just output 5V if it’s supposed to feed another regulator that needs at least 7V input. It has to output higher, say 8V. Then the next one needs at least 8V input to output 5V (which is impossible, it would need ~7V+ input).
You’d need to cascade them so each one has enough headroom.
Here’s a more realistic breakdown of why this “load splitting” idea fails with linear regulators in series. Imagine you have 20V input and need 5V. You decide to use two 7805s.
For the second one to work, it needs its input voltage to be at least, say, 7V (5V output + 2V dropout). So, the first 7805 has to output at least 7V. If it outputs 7V, it’s dropping 20V – 7V = 13V.
If you’re pulling 100mA, that’s 1.3W of heat. The second 7805 takes 7V in and needs to output 5V, dropping 7V – 5V = 2V. (See Also: Can A Dual Lumen Regulator Attach To A Inogen One G5 )
At 100mA, that’s 0.2W of heat. Total heat = 1.5W.
If you used a single 7805, it would drop 20V – 5V = 15V. At 100mA, that’s 1.5W of heat.
So, on paper, the heat is the same! But in reality, the first regulator is now working harder and hotter because it’s running at its upper limit of input voltage and has a larger voltage drop. It’s more prone to overheating and instability.
My own past projects have shown me this isn’t just theoretical. I once tried to power a small motor controller that needed 5V from a 24V supply using two 7805s. I figured each would handle half the load. What happened?
The first regulator got scorching hot, and the output voltage was all over the place, causing the motor controller to glitch. It was a frustrating afternoon spent troubleshooting a problem I’d created myself with a bad design choice.
You’re much better off using a single regulator with a proper heatsink or a different type of power supply.
The other scenario where people get confused is when they see regulators used in parallel. That’s a different beast entirely, and even that has its own set of challenges. But series connection for voltage reduction with basic linear regulators like the 7805 is almost always a red flag. It’s like trying to drink a gallon of water through a single tiny straw – inefficient, frustrating, and likely to spill.
Practical Alternatives: What Actually Works
So, if chaining 7805s is a bad idea, what should you do when you have a voltage that’s too high? This is where my experience really kicks in, because I’ve been down so many dead ends.
The simplest, most reliable solution for significant voltage drops is a switching regulator, often called a buck converter. These are incredibly efficient – they don’t just burn off excess voltage as heat. Instead, they use a high-frequency switch, an inductor, and a capacitor to chop up the input voltage and reconstruct it at the desired lower voltage. They can be significantly more efficient, meaning less wasted energy and less heat.
You can buy pre-built buck converter modules for just a few bucks online, often for less than the cost of a few 7805s and a decent heatsink. These modules are usually adjustable, so you can dial in exactly the voltage you need. I’ve used these countless times, and they’re fantastic.
They’re small, cheap, and can handle substantial currents without breaking a sweat.
For example, if you need 5V from 24V, a buck converter module is the way to go. You plug your 24V in, twist a tiny potentiometer, and you get a stable 5V out. The amount of heat generated is minimal, and they can often deliver amps of current. I built a portable battery-powered device recently that needed 5V for its microcontroller and sensors, but I was using a 12V battery pack for the main power.
A single 7805 would have been overloaded trying to drop 7V at 500mA. Instead, I used a small adjustable buck converter module.
It’s tiny, it’s efficient, and I barely have to think about it. It cost me about $3, and it’s been running flawlessly for months.
Another option, if your voltage drop isn’t too massive, is to use a linear regulator with a much higher current rating and a substantial heatsink. Sometimes, a single, beefy regulator is better than multiple smaller ones. For instance, if you need 5V from 9V and are drawing 500mA, a single 7805 would struggle (4V drop * 0.5A = 2W). However, a regulator like an LM338 (adjustable, but can be set to 5V) or even a higher-current 78xx series (like a 7812 followed by a 7805, which is still not ideal but better than multiple 7805s) with a proper finned heatsink and possibly even forced airflow (a small fan) can handle it.
But honestly, even then, a buck converter is usually the more elegant and efficient solution. For my hobby projects, efficiency and not having to worry about thermal runaway are huge priorities. I’ve spent enough time chasing down heat-related failures to know that a little upfront investment in the right component saves a ton of headaches later.
Here’s a quick rundown of my go-to solutions: (See Also: Can A Faulty Fuel Pressure Regulator Cause Rough Idle )
| Scenario | Recommended Solution | Why it Works | My Verdict |
|---|---|---|---|
| Large Voltage Drop (e.g., 12V+ to 5V) | Buck Converter Module | High efficiency, low heat, adjustable output. | Best Option – Cheap, reliable, easy. |
| Moderate Voltage Drop (e.g., 9V to 5V) with Moderate Current | Linear Regulator (e.g., LM317, LM338) with Heatsink | Simpler circuit than buck, good for less demanding needs. | Good if you’re comfortable with heatsinking, but buck is often better. |
| Very Small Voltage Drop (e.g., 7V to 5V) with Low Current | Standard 7805 with small heatsink | Simple, classic, works for low power. | Fine for hobbyist projects with minimal current draw. |
| Trying to cascade multiple 7805s for large drops | Any of the above! | Cascading 7805s is asking for trouble. | Avoid at all costs. |
The key takeaway is to match the solution to the problem. Don’t try to force a square peg into a round hole, especially when dealing with power regulation. The 7805 is a fantastic component for what it’s designed for: small voltage drops at moderate currents. Trying to push it beyond its limits, like in a series configuration for high voltage reduction, is a recipe for frustration and failure.
Common Mistakes and How to Avoid Them
People mess up power regulation in a few common ways, and understanding these will save you a lot of grief. The most obvious one, which we’ve hammered home, is trying to use a 7805 (or any linear regulator of similar class) for a voltage drop that’s too large.
You see the input voltage is high, and the output voltage is low, and you think, “I’ll just put two or three of these in a row!” This is a trap. The heat generated by the voltage drop multiplied by the current is the killer.
If your input is 18V and you need 5V, that’s a 13V drop. If you’re pulling 100mA, that’s 1.3W.
Without a heatsink, the 7805 will likely overheat. If you try to split this across two regulators, the first one still has a large voltage drop to handle to make sure the second one has enough voltage to work.
It doesn’t magically become less heat.
Another mistake is neglecting input and output capacitors. The datasheet for the 7805 recommends specific capacitor values (usually 0.33uF on the input and 0.1uF on the output) for stability. These capacitors help filter out noise and prevent oscillations. If you skip them, you can get unpredictable behavior. I once had a project where I’d forgotten the output capacitor, and the 5V supply was just slightly rippling. It was just enough to cause a microcontroller to reset randomly. It took me hours to figure out that the ‘unstable’ reset behavior was due to a missing capacitor, not a faulty chip or code. The fix was a tiny $0.10 capacitor. Learn from my pain: always include the recommended capacitors!
Overlooking current draw is another big one. The 7805 is typically rated for about 1A, but that’s under ideal conditions with a good heatsink. In reality, if you’re trying to pull close to 1A with a significant voltage drop, you’ll need a substantial heatsink.
Many hobbyists underestimate how much current their components actually draw. A simple circuit might draw 50mA, but if you add a few more LEDs, a motor, or a wireless module, that number can climb quickly.
Always check the datasheets for your components to estimate your total current draw. If it’s pushing past 500mA for a 7805, start thinking about alternatives or a serious heatsink. For example, powering a few LEDs is fine, but powering a small servo motor or a USB device from a 7805 is often a bad idea without careful calculation and adequate cooling.
My friend once tried to power a small fan directly from a 7805, and it lasted about ten minutes before it died in a puff of smoke. He thought the fan was low power, but combined with the voltage drop, it was too much. It’s a good lesson in component limitations.
Finally, and this is a contrarian opinion that might rub some people the wrong way: sometimes, the advice to just use a dedicated 5V power supply and avoid linear regulators altogether is overkill. For small, low-power projects where the voltage drop is minimal (like going from 7V to 5V) and the current is low (under 100mA), a 7805 with its recommended capacitors and maybe a small clip-on heatsink is perfectly adequate and much simpler than a buck converter. The issue arises when people try to apply it to situations where it’s clearly not suited, like trying to power a whole computer from a 7805. The 7805 is a workhorse for its intended job. Don’t dismiss it, but understand its limits and don’t try to make it do things it wasn’t designed for, like chaining them for higher voltages.
People Also Ask:
Can You Connect Two 7805 Regulators in Series?
No, connecting two 7805 regulators in series to achieve a higher voltage output is generally not recommended and is considered a poor design practice. Each regulator dissipates a significant amount of power as heat due to the voltage drop across it. Chaining them for large voltage reductions leads to excessive heat, instability, and potential component failure rather than a reliable voltage output.
What Happens If You Connect Regulators in Series?
When linear regulators like the 7805 are connected in series, each regulator must drop a portion of the total voltage. This leads to significant power dissipation as heat in each regulator. The first regulator needs to output a voltage higher than the input requirement of the subsequent regulator, exacerbating the heat problem and often resulting in instability or component failure due to overheating.
Can I Use a 7805 for 12v Input?
Yes, you can use a 7805 for a 12V input, but you will need to make sure it has adequate cooling. A 12V input to a 5V output means a 7V voltage drop. If you’re drawing significant current (e.g., over 100mA), the 7805 will dissipate $7V imes I_{out}$ Watts of heat, which requires a heatsink to prevent overheating and failure.
Can a 7805 Handle 24v?
A 7805 regulator can technically accept a 24V input, but it will get extremely hot and likely fail rapidly if significant current is drawn. The voltage drop would be 19V, and even at a modest 100mA, it would dissipate $19V imes 0.1A = 1.9W$ of heat, which is too much for a bare TO-220 package without a substantial heatsink. For a 24V input, using a switching regulator (buck converter) is highly recommended.
Final Verdict
So, to put it bluntly: no, you generally should not connect 7805 regulators in series for higher voltages. It’s a shortcut that leads to a dead end, usually paved with burnt components and frustration. The heat generated is the primary culprit, and the way these regulators work means splitting the load doesn’t actually make things easier for them; it often makes them work harder.
My advice? If you have a voltage that’s too high, reach for a buck converter module. They’re cheap, efficient, and designed for exactly this kind of situation. They cost a few dollars, but they save you hours of troubleshooting and potentially replacing burnt-out parts. You’ll get a stable voltage, less heat, and a much more reliable circuit.
Learn from my mistakes and the mistakes of countless others. The humble 7805 is a workhorse for its intended purpose, but trying to make it do something it’s not built for, like a series voltage reduction for higher voltages, is a recipe for disaster. Stick to the right tool for the job, and your projects will thank you.
